Category Archives: Keeping Bonsai Healthy

Zeolite for Bonsai Mediums

My last post covered Cation Exchange Capacity or CEC. Research for that article surfaced zeolite as the highest cation exchange capacity of any common bonsai substrate by a wide margin. So what is zeolite, and is it worth adding to your mix?

What is zeolite?

Zeolite is a microporous aluminosilicate mineral, which means that its crystal structure is built from a 3D framework of silicon-oxygen and aluminium-oxygen tetrahedra (the same basic units that make up most rock-forming minerals), but with tiny channels running through it. The channels are around 3–10 angstroms across — small enough that the mineral effectively acts as a molecular sieve, with water and dissolved ions able to enter and exit through the structure rather than just interacting with the outside surface.^ref

Zeolite forms naturally when volcanic ash settles into alkaline lakes and slowly recrystallises over millions of years. Most commercial deposits being mined today are 10-30 million years old, with significant sources in the western United States, Cuba, Turkey, Slovakia, Bulgaria and Australia.^ref

There are over 40 naturally occurring zeolites and ~150 synthetic ones. The one used horticulturally is almost always clinoptilolite, the most abundant and chemically stable natural zeolite.^ref

Why does clinoptilolite have such a high CEC?

The high CEC comes from two characteristics of the crystalline structure of zeolite. The first is that the presence of Aluminium creates a persistent negative charge in the crystal which isn’t affected by pH. This is in contrast to akadama which has lower CEC as pH goes lower.

The second is that the tiny channels mentioned above massively increase the available surface area available for cation exchange since it can take place within the particle itself. In mediums without these channels, only the surface of the particle is available.

The theoretical CEC of zeolite is 220–260 meq/100g, which real-world samples typically come out at 75–94% of this or 150–220 meq/100g.^ref This is unmatched in any other commonly used bonsai medium component.

What does clinoptilolite (zeolite) do that other mediums don’t?

1. Pre-charging with ammonium for slow-release nitrogen

Clinoptilolite happens to be very good at holding onto the key macronutrient of Nitrogen, in the form of Ammonium (NH4+). This is because the tiny channels running through the crystal are around 3 angstroms across which is almost exactly the same size as a hydrated NH4+ ion. This slips into the ‘cages’ within the clinoptilolite and is held by hydrogen bonds to the surrounding framework oxygens, like a puzzle piece fitting into place.^ref

If the zeolite is pre-soaked in an ammonium-rich solution before potting, nitrogen is absorbed and then released slowly to roots over weeks to months as it’s displaced. This is important for bonsai because nitrogen is usually quickly leached out of soils by watering. Some data: in sand-based container substrates, pre-charged clinoptilolite at 5–10% reduced nitrogen leaching by 77–89% and phosphorus leaching by 90–96% without compromising plant growth.^ref

The original trials use ammonium sulphate solution at a few grams per litre, soaked overnight. Ammonium sulphate is available as a horticultural fertiliser but a practical alternative more in keeping with an organic feeding regime is a compost or comfrey tea. Both are NH4+-rich leachates and would charge the exchange sites with the same ion you’d be supplying through normal fertilisation. Including some organic matter in the substrate would also then resupply the zeolite as it broke down.

2. Urease inhibition – extending nitrogen availability

Urease is the enzyme that breaks urea down into ammonium, and it’s one of the fastest-acting enzymes known. It can break urea down into ammonium very rapidly and typically within a few days. The problem with this fast action is that the resulting ammonium either gasses off (10–40% of applied nitrogen is lost this way in agricultural studies), or gets converted by soil bacteria to nitrate which then leaches out of the pot with the next watering.

Zeolite slows this whole cascade down by trapping ammonium on its exchange sites as fast as urease produces it. The ammonium isn’t lost to the atmosphere because it’s pulled out of solution and it isn’t converted to nitrate because the nitrifying bacteria are starved of substrate. It stays plant-available for weeks rather than days.^ref This matters for bonsai because urea (or urea-like intermediates) is the major nitrogen pathway in every animal-derived organic fertiliser (manure, blood meal, fish emulsion) so the addition of zeolite will assist with retaining more of the nitrogen in the fertiliser for your tree.

3. Particle stability

Unlike akadama, clinoptilolite is a crystalline silicate and it does not break down under environmental stresses. The particles are resistant to damage by freeze-thaw cycling, repeated wetting/drying, or root pressure and last a lot longer than akadama whilst retaining their beneficial properties. Industrial uses for water treatment and radionuclide capture have demonstrated the same particles functioning for decades.^ref

4. Water retention via internal porosity

The channels inside clinoptilolite particles help hold onto water, so zeolite can improve the water retention qualities of your substrate. Horticultural studies have reported water-holding up to ~60% by weight, and substituting 30% clinoptilolite into peat-based potting substrate increased water-holding capacity 2.6× and total porosity 8%.^ref

What does zeolite NOT do well?

pH. For various reasons, zeolite holds the substrate close to neutral. For ericaceous species being deliberately kept acidic (such as those potted in kanuma at pH 5.5), zeolite would slowly push pH up toward 6–7, against the goal for those trees. So zeolite is not recommended for acid-requiring species.

Variable product quality. Commercial clinoptilolite ranges from ~50% pure (mixed with feldspars, smectites, opaline silica) to >90% pure and bag labelling rarely specifies the deposit source, purity or measured CEC. The CEC of what you actually buy could be anywhere from 80 to 200 meq/100g.^ref Probably it’s best to source from a bonsai supplier who will be motivated to find a product which works for bonsai.

How to use zeolite in a bonsai mix

I personally add some zeolite to all my substrates except for azaleas and ericaceous species (as well as biochar). If you want to do this too, here are the pointers:

Buy clinoptilolite specifically — other types of zeolite don’t have the same evidence base.
Source from a bonsai supplier where possible — they’re motivated to stock a product that works for bonsai.
Match particle size to your substrate — 2-5mm for typical mixes, larger only for very coarse mixes.
Mix at 10-20% by volume of total substrate — higher proportions don’t add proportional benefit.
Pair with organic fertiliser — these supply nitrogen as ammonium, which zeolite holds strongly.
Pre-charge (optional) — soak in dilute ammonium sulphate solution or a strong organic fertiliser leachate for 24 hours before potting to load the exchange sites with ammonium for slow release.

I hope this is useful and happy soil mixing!

Full reference list for this article

Mumpton, 1999 — La roca magica: uses of natural zeolites in agriculture and industry. PNAS. https://doi.org/10.1073/pnas.96.7.3463
Inglezakis et al., 2022 — Ion Exchange in Natural Clinoptilolite. Minerals. https://doi.org/10.3390/min12121628
Cappelletti et al., 2002 — Methods of determining cation exchange capacities for clinoptilolite-rich rocks. Clays and Clay Minerals. https://doi.org/10.1346/000986002761002739
Eroglu et al., 2017 — Applications of Natural Zeolites on Agriculture and Food Production. Agronomy. https://www.mdpi.com/2073-4395/11/3/448
Williams & Nelson, 1997 — Nitrogen leaching reduction in container substrates amended with clinoptilolite. HortScience. https://journals.ashs.org/hortsci/view/journals/hortsci/32/4/article-p615.xml
Tzanakakis et al., 2021 — Clinoptilolite zeolite influence on nitrogen dynamics. Journal of Soil Science and Plant Nutrition. https://doi.org/10.1007/s42729-021-00566-1
Ostrowska & Porębska, 2021 — Long-term stability of zeolite in soil. Minerals. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8156034/
Gholamhoseini et al., 2018 — Effect of clinoptilolite zeolite on water-holding and porosity of potting substrates. Acta Universitaria. https://www.redalyc.org/journal/2033/203359541002/html/

Cation Exchange Capacity in Bonsai Soils

Bonsai trees take up nutrients through their roots from the ‘soil solution’ within the pot. The soil solution is the water held in the spaces between substrate particles, together with everything dissolved in it, like dissolved ions (the nutrients themselves), along with gases and organic compounds. Given that bonsai are usually kept well-drained and water generally runs through in a few minutes, any nutrients dissolved in that water leave the pot with it so there’s a limited window for roots to take nutrients up directly from solution. This is where the Cation Exchange Capacity (“CEC”) of the substrate in the pot becomes very important.

What is CEC?

Cation Exchange Capacity is literally the capacity of a substrate to hold positively charged ions (cations) which can be accessed by roots. Why do we care about positively charged ions? Because this is the form taken by many plant nutrients, including four of the six macronutrients — ammonium (NH4+, one of the two nitrogen sources), potassium (K+), calcium (Ca2+) and magnesium (Mg2+) — and five micronutrients: iron (Fe2+/Fe3+), manganese (Mn2+), zinc (Zn2+), copper (Cu2+) and nickel (Ni2+).

The other macronutrients — phosphorus, sulphur and the nitrate form of nitrogen — exist as negatively charged anions in soil solution and aren’t held by CEC at all. They have to be supplied by regular feeding because the substrate can’t store them between waterings.

The reason that substrate particles are able to attract and hold cations is because they themselves are negatively charged. This arises due to imperfections in their mineral structure (where a lower-charged atom sits where a higher-charged one should) and because of acidic hydroxyl (-OH) groups on their surfaces that release their H+ at typical soil pH, leaving the surface negatively charged. The strength of the cation attraction for a particular substrate is its CEC measurement.^ref

How do roots get the ions back from the substrate?

Root cells actively pump out Hydrogen ions (H+) using membrane-bound proton pumps which are powered from cell respiration. You can learn more about these proton pumps and their role in plants in this video. When these ions exit the roots they create a space for another positively charged ion to enter – in this case the nutrient cation. So the plant swaps the hydrogen ion for the cation, bringing that nutrient in.^ref

It should be noted that different substrates bind cations in different priorities. For clinoptilolite (zeolite): Potassium (K+) > Ammonium (NH4+) > Sodium (Na+) > Calcium (Ca2+) > Magnesium (Mg2+) which means a potassium-heavy fertiliser will outcompete ammonium for available sites, effectively reducing the substrate’s nitrogen-holding capacity.^ref

So What?

Hopefully you can see that a substrate with higher CEC will attract some nutrients as they flow through during the watering process and hold onto them so they are available to roots over a period of time. Let’s take a look at the evidence available for CEC levels in different bonsai substrates, where we are looking for the highest numbers:

Component	CEC (meq/100g)	Source
Clinoptilolite (zeolite)	150–220 (theoretical 220–260)	Peer-reviewed [Cappelletti et al., 2002]; [Inglezakis et al., 2022]
Sphagnum peat	100–180	Standard horticultural
Composted pine bark	50–100	Peer-reviewed [Jackson et al., 2014]
Kanuma	62	Hobbyist journal [J. American Bonsai Society Vol 43 #4, 2009]
Turface	33	Hobbyist journal [J. American Bonsai Society Vol 43 #4, 2009]
Akadama	21–31	Hobbyist journal [J. American Bonsai Society Vol 43 #4, 2009]; mineralogy peer-reviewed [Asaoka & Aono, 2006]
Allophanic andisols (akadama parent soil)	10–60, pH-dependent	Peer-reviewed [Harsh et al., 2002]
Kaolin clay	~28	Peer-reviewed [Ma & Eggleton, 1999]
Pumice	~10 (variable: 9.9–73 depending on source)	Peer-reviewed [Guler & Sarioglu, 2014]; higher-end values for zeolitically altered pumice [Kantiranis et al., 2011]
Kiryu (river sand)	11.7	Hobbyist journal [J. American Bonsai Society Vol 43 #4, 2009]
Perlite	3–4	Peer-reviewed [Kantiranis et al., 2011]
Coarse sand	~0	Standard mineralogical

You can see straight away that zeolite has by far the highest CEC of any bonsai medium. In fact its CEC properties mean it is also used in the nuclear energy industry as a way of capturing dangerous radioactive ions such as Cesium-137.^ref

Caveats – pH

Although CEC measures how many cations a substrate can hold, it doesn’t directly translate to the cations accessible to the plant, because pH also affects soil solution properties. Above roughly pH 7 (neutral), several essential nutrients precipitate out of solution and become unavailable regardless of how much exchange capacity is present, including iron, manganese, zinc, copper, boron and phosphorus.^ref

In the UK, tap water in southern and eastern England is ‘hard’ — rich in Ca2+ and bicarbonate (HCO3-) (see my post on Water hardness, pH and bonsai). Over time this causes two related problems in a bonsai pot: calcium builds up on CEC sites in preference to other cations (displacing the K+, Mg2+ and NH4+ you actually fertilised with), and the bicarbonate gradually raises substrate pH into the range where micronutrient availability drops. A high-CEC substrate watered with hard tap water therefore slowly alkalinises and Ca-saturates over months, even if it started at a sensible pH.

There are a few ways to mitigate this. The simplest is to use rainwater where possible, particularly for ericaceous species like azaleas and camellias which need sustained low pH. Reverse osmosis (RO) systems also produce near-neutral water with effectively zero buffering, but they’re a more involved option costing hundreds of pounds with annual filter replacements and a membrane change every couple of years, plus they waste 2–4 litres of mains water per litre of pure water produced. RO probably only earns its place if you already have a system for drinking water or have a small collection of acid-loving species that genuinely warrants the investment.

Where neither rainwater nor RO is practical, some growers acidify hard tap water before feeding using household vinegar (5% acetic acid) at roughly 1 tablespoon per 4.5 litres, which drops typical UK hard tap water from around pH 7.5 to pH 6. This works but with caveats: acetic acid is a weak acid so the effect is short-lived once the water hits the substrate’s buffering capacity, and any sustained pH shift requires consistent dosing at every watering. Commercial nurseries use stronger acids (sulphuric, phosphoric) for the same purpose, but these require careful handling and aren’t really hobbyist-appropriate.^ref

A more sustained option is to add elemental sulfur chips to the substrate. Sulfur isn’t itself acidic — it’s an inert yellow solid — but soil bacteria (primarily Thiobacillus and Acidithiobacillus species) oxidise it to sulfuric acid over weeks to months, releasing H+ ions that lower substrate pH.^ref This is the standard commercial method for acidifying soil around ericaceous shrubs and works in bonsai pots too. The trade-off is that it’s a biological process, so it stalls below about 10°C and finer particle sizes oxidise faster than chips.^ref For a bonsai, chips give a slow, controlled effect that doesn’t require daily dosing, with the bonus of supplying sulfur as a macronutrient.

What does this mean in a bonsai pot?

The implication from this post is that to maximise your fertiliser efficiency and tree health you should probably add a high CEC component to your substrate whilst also taking steps to manage pH if you’re in a hard water area. Practically this means adding zeolite to your mix at around 10–20% by volume, in 2–5mm particle size to match the rest of the substrate.^ref If you don’t have access to rain or RO water, also mix elemental sulfur chips into the substrate at repotting — chip-grade sulfur (not powder) works best as it oxidises slowly over the growing season. Also recognise that a substrate with entirely inorganic components such as pumice, sand or perlite will dramatically reduce how much of your fertiliser makes it into your trees.^ref

You should also aim to use an organic fertiliser because in high CEC substrates ammonium is retained in favour of nitrate. Organic fertilisers tend to provide their nitrogen sources as ammonium whereas chemical fertilisers provide it as nitrates.^ref If you’re growing in a low-CEC mix (pure pumice/lava/perlite), it makes little difference which form you use as both wash through, so frequent weak feeding is the more effective strategy regardless of feed type.

CEC matters more for trees in refinement than in development. A development tree in a large training pot has plenty of substrate volume and is usually being fed hard anyway — the substrate’s ability to retain nutrients between feeds matters less when you’re delivering nutrients faster than they leach. For a refined tree in a shallow pot being fed carefully to control vigour, CEC is more relevant since there is less substrate.

For acid-loving species such as azaleas and camellias which are in low pH substrates such as kanuma, CEC is reduced versus the CEC which would be in place at neutral pH. So ericaceous species need feeding little and often rather than relying on the substrate to buffer between feeds.

Top 10 ways to help your bonsai thrive

Creating a healthy bonsai depends on managing many different variables, from the physical environment through to feeding regimen, styling procedures & timing. I wanted to get a sense of which factors should take priority. In the end I have two tiers for you – 1/ must-haves which are necessary to keep a tree alive and 2/ should-haves which have a significant impact on tree vigour. While these apply to most trees, each species will have different needs across these factors so it’s important to work out the right level of inputs for the species you have (there isn’t really a one-size-fits-all answer for every tree).

Six Must-Have Factors for Healthy Bonsai

#1. Adequate Watering

It may seem obvious, but underwatering a tree can kill it through hydraulic failure. When there isn’t enough water available for the tree to maintain transpiration it suffers the plant equivalent of an embolism, where a bubble interrupts the water stream and stops water from moving from roots to leaves.^ref (More information in my watering post). Underwatering can be avoided by checking the medium regularly and watering when it begins to dry (in summer at least once a day in the UK and sometimes twice if hot).

Similarly, overwatering can kill a tree by drowning the roots. Roots need access to oxygen, without it they cannot respire and the root tips die, then opportunistic pathogens (Phytophthora, Pythium) move into the damaged tissue.^ref Overwatering can be avoided by ensuring good drainage of both medium and pot, and by simply watering less.

#2. Correct Temperature Range

Extreme temperatures kill trees – both too cold and too hot. The main cold-related problem is frost, which can cause a range of damage to your tree – there is more information (including suggestions for avoiding frost damage) in my post: What does frost do to bonsai trees?. By contrast a 30-minute exposure to 45–57 °C is irreversibly damaging to plants while indirect injury (reduced photosynthesis, impaired water/nutrient uptake, increased disease susceptibility) begins at 38–40 °C.^ref A lack of humidity can worsen the damage from heat, more information is in my post Transpiration.

The correct temperature range can also include a certain amount of cumulative cold (usually 0-7°C) to release endodormancy and bud break properly^ref. This is species-dependent and can usually be determined by working out the natural conditions of the tree’s origin or natural range.

#3. Suitable Substrate

The planting substrate, or bonsai medium, is a critical factor for keeping bonsai trees alive and healthy. As noted in #1, roots need oxygen to respire as well as physical space to extend and to allow for gas exchange to remove CO₂ and ethylene. A key concept here is Air-filled porosity (AFP) or the proportion of substrate volume occupied by air after free drainage. A more aerated substrate ensures oxygen availability (avoiding root drowning), enables gas exchange and produces denser, finer root systems, which is what you want for a bonsai.^ref The substrate also plays a significant role in nutrient storage & uptake, water accessibility and improving the microbial environment in the pot. I have lots of information about bonsai medium in this post: Bonsai growing medium.

#4. Limiting Lethal Pathogens

Whilst a healthy, well-managed tree can often deal with pests in its stride, there are some pathogens which can be catastrophic across species, and others with similar impact for particular species.

Oomycete root rots are the primary must-avoid pathogens as they devastate a tree very quickly. They are water moulds which require water in the substrate to release zoospores, so creating conditions where excess water is not available is the best way to avoid them. This includes not overwatering, and ensuring good AFP in the substrate. A particularly devastating group of oomycete root rots is Phytophthora cinnamomi and Phytophthora plurivora which infect a documented host range of over 3,000 plant species^ref, including common bonsai species such as Rhododendron, Pieris, Taxus, Chamaecyparis, Quercus, Castanea and Pinus ^ref. The wider Phytophthora genus extends this to most temperate woody genera. There are fungicides which have some effectiveness against Phytophthora (mefenoxam and phosphonates) but they suppress rather than cure, resistance is widespread, and most are restricted to professional use. For a hobbyist, prevention is the only realistic option.^ref

Other seriously damaging pathogens include some soilborne fungi like Fusarium oxysporum/Fusarium verticillioides which is a vascular wilt pathogen^ref and Verticillium dahliae another vascular wilt fungus which is particularly dangerous to Acer. These do not have any reliable fungicidal cure and the main approach for limitation is to avoid introducing contaminated plant material or substrate into your environment and to maintain strong tool disinfecting hygiene.

#5. Enough Light

Plants need light to photosynthesize, how much depends on the species and where they evolved. I wrote a detailed post about Photosynthesis, but the main takeaway is that bonsai trees need enough light, ideally sunlight. For example the photochemistry of acers is impaired at 20% or below of full sun, with optimum being 55–75%^ref.

The challenge for bonsai practitioners usually arises when the tree they have isn’t native to their geography, which means accommodations are needed to enable the tree to receive the temperature and light combination they require. Many tropical species can’t be left outdoors outside of the subtropics due to the risk of frost, but even these may need dedicated lighting (see Artificial light for bonsai), a heated greenhouse or a well-lit position near a window.

A classic mistake made by beginners is to situate a bonsai tree in the wrong place – for example putting them indoors on a window ledge when the tree is an outdoor species like Juniper, Pine, Oak or any other tree you would see in a local forest or woodland. One interesting aspect of photosynthesis is how plants react when they get too much light – you can read about this in: Why do some conifer leaves go bronze in winter? In general it’s not going to be too much light which kills a bonsai tree, it is more likely to be too little.

#6. Critical Nutrients

Six macronutrients are required in large quantities for trees to survive and these are a structural and metabolic backbone. Nitrogen (N) for amino acids and chlorophyll, Phosphorus (P) for ATP and nucleic acids, Potassium (K) for osmotic regulation, Calcium (Ca) for cell walls, Magnesium (Mg) for the chlorophyll molecule itself, Sulphur (S) for amino acids. Deficiency in any of these is genuinely lethal because they cannot be substituted. Read more in What each nutrient does (x17). Providing these is usually achieved by the application of fertiliser and soil improvements of various forms. Standard chemical fertiliser will usually provide some ratio of NPK so the other elements need to come from elsewhere. A great source of these for bonsai is organic matter such as manure, chicken pellets, compost, and liquid seaweed.

Four Should-Have Factors for Healthy Bonsai

So assuming you have the top six sorted, there are a few others which are still important but won’t necessarily threaten the life of your tree in the short to intermediate term.

#7. Room to Grow

Obviously a big part of bonsai is keeping the tree constrained in a small pot, that’s the whole point! But we cannot ignore the fact that constraining a tree in a small pot also restricts its growth significantly, with studies showing that doubling pot size increases biomass by an average of 43%.^ref The pot constraint will slow down growth, this is why we should develop trees in larger containers to get the trunk size needed before moving to a bonsai pot for refinement.

But once in a smaller pot, there are things which need to be done to make the best of that situation which mainly revolve around root pruning and encouraging root ramification. Root pruning prevents the roots from filling and crowding the pot, and allows the root tips to grow. Root ramification helps to maximise the root surface area given the constraint of the pot, and there are many different ways to do this – see Ramification of Roots (lateral root development).

Failing to root prune or encourage root ramification will end up penalising the tree and reducing its vigour over time, which also reduces its tolerance to other stressors like pathogens, or excesses in temperature or water.

#8. A Balanced Diet

Aside from the six critical nutrients in #6 above, there are eight additional elements also needed. These are the micronutrients including iron, manganese, zinc, boron, copper, molybdenum, chlorine and nickel. Since they are only required in tiny quantities and are usually present in trace amounts in substrate components and irrigation water they are less critical to replace with a fertilising regime. Deficiencies of these nutrients do occur and can damage your trees (for example iron chlorosis on alkaline-irrigated trees), but they cause disorders rather than death in the short to intermediate term. Dealing with #6 above usually tends to cover the elements in #8.

#9. Well-Timed & Sized Pruning & Repotting

Pruning and repotting both impose a carbohydrate cost on the tree as they remove tissue that contains stored non-structural carbohydrates (NSC), and the tree has to fund regrowth from what’s left. The conventional bonsai advice says repot just before bud break in spring, but the underlying NSC data suggests this may not be the optimal time for root work. Root carbohydrates are used for leaf growth, so removing roots just before bud break deprives the tree of those reserves at the moment it needs them most (unless you want small leaves, which you may want but acknowledging this will constrain leaf surface area for photosynthesis).

The chart I worked through in root food storage and pruning timing shows root NSC pools are lowest in late summer / early autumn for most deciduous species, which is also when peak root growth occurs naturally.^ref So for root pruning, August is a defensible choice and arguably better than spring. The risk is frost damage to newly grown roots if you leave it too late, but since this is when most root growth happens anyway, the risk is smaller than it sounds. But this is definitely one for the endless bonsai debates!

Branch pruning is the inverse since branch NSC reserves are lowest just after bud break in spring, having funded the new leaves, so spring is the cheaper time to remove branches. The “amount removed matters more than exact timing” rule still applies so keep moderate pruning under about a third of foliage in any one session^ref, less on a weak tree. More detail on both in pruning and repotting tips.

The practical override is that within reason, timing matters less than the literature implies. A maxim I have is: the right time to do something is when you have time to do it. Unless you’re being aggressive (heavy root reduction, hard branch cutback), most temperate species tolerate work outside the theoretically optimal window if you compensate by reducing transpiration if you’ve removed a lot of roots, with foliage thinning, shade, or a humidity bag.

#10. A Healthy Microbiome

A bonsai tree has a microbiome, which is a community of fungi and bacteria living on the leaves, in the roots, and inside the plant tissue itself, likely started from when it was a seed. A tree and its microbiome together form what’s now called a holobiont. I’ve written more on the underlying biology in the microbiome and symbiotic microbes.

For bonsai practice, three things actually matter:

#1 Don’t kill what’s there. Systemic fungicides and insecticides which you might turn to for treating a pathogen or pest problem may have the unwanted side effect of damaging the microbial community in the substrate. The implication isn’t never to treat, but to weigh the cost since a healthy tree shouldn’t need routine chemical intervention, and prevention via the other Tier 1 factors (#1–6) avoids the choice altogether.

#2 Replenish after disturbance. When repotting, collecting Yamadori, seed-growing or air layering there may be minimal microbial community remaining in the substrate. Commercial dry mycorrhiza and bacterial inoculants could help here, though the evidence on whether they establish at useful densities is mixed and species-matching matters (different products contain different types of mycorrhiza which are relevant for different tree species). A handful of humus from a healthy, chemical-free woodland with similar species, keeping some of the previous substrate and limiting root pruning to retain enough microbes in the rootball are ways to provide a replenishment source. Also eventually the microbiome will reestablish by picking up local inoculum on its own.

#3 Create conditions microbes prefer. Good substrate structure with nooks for microbe colonisation (such as Biochar), reasonable moisture, drainage, light, stable temperatures, and not too much root disturbance.

It’s important to say that not all microbes are friendly! Improperly composted manure can carry Salmonella and E. coli. Legionella (the bacterium behind Legionnaires’ disease) is present in many composts made from wood, bark, green waste and peat^ref. So wash your hands and tools after working with substrate, and mix bonsai soils outside or under cover rather than indoors in a way that sends dust into the air. This won’t negatively impact the holobiont of your trees but will reduce the risk of you catching anything nasty.

Basalt as a soil improver

Wandering around a bonsai show a while ago I noticed one of the traders selling basalt as a soil improver. I wanted to understand what exactly it improves, how, and when it could be useful.

What is Basalt?

Basalt is a fine-grained extrusive igneous (volcanic) rock formed from the rapid cooling of low-viscosity lava exposed at the surface of a rocky planet (eg. Earth).^ref More than 90% of all volcanic rock on Earth is basalt, and this comprises about 13.5% of the Earth’s current land surface.^ref The product available to gardeners is a powder produced as a by-product of quarrying basalt for aggregate used in roads, concrete and railway ballast. Basalt quarries are found worldwide, wherever volcanic rock is present.

There are three types of basalt. Tholeiitic basalt is rich in iron and poor in alkali metals (such as sodium & potassium), and includes most basalts of the ocean floor, large oceanic islands, and continental flood basalts such as the Columbia River Plateau. Alkali basalt is relatively rich in alkali metals and is characteristic of continental rifting and hotspot volcanism. High-alumina basalt has greater than 17% alumina (Al₂O₃) and is intermediate in composition between tholeiitic basalt and alkali basalt.^ref

How Does Basalt Act as a Soil Improver?

Around half of the composition of basalt is SiO₂ (Silicon Dioxide, the same compound that makes up quartz and sand, but here chemically bound in silicate minerals). Basalt also contains other elements including aluminium (Al), iron (Fe), magnesium (Mg), potassium (K), titanium (Ti), and calcium (Ca) with the relative amounts of these depending on where the basalt was quarried. As you may remember from Nutrients for Trees, most of act as nutrients for our bonsai so there is the potential for basalt to be a useful additive. Research suggests that the main role basalt can play is as a slow-release multinutrient remineraliser and a mild liming agent (increasing pH).^ref

The micronutrient effect is because nutrients and trace elements are released as the basalt powder weathers over time. Which ones are released will depend on the basalt source and composition.

The liming effect has three causes: 1/ when the silicates are dissolved they react with hydrogen ions and remove them from the soil (hydrogen ions determine acidity). 2/ bicarbonate is formed from CO₂ & water reacting with released Ca²⁺/Mg²⁺, which buffers the soil against further acidification and 3/ aluminium in the soil (another source of acidity) is displaced and precipitated out into an insoluble form.

The released Ca²⁺, Mg²⁺ and K⁺ (nutrient ions beneficial for trees) are then held on the substrate’s exchange sites (its cation exchange capacity) rather than leaching away with watering — basalt’s pH effect actually helps with this retention too. This makes these nutrients more available to roots over a period of time instead of washing away.

Which Trees Might Benefit from Basalt Addition?

Although most of the relevant research has been done on crops rather than trees, the implication is that species which would benefit from basalt supplementation are those on acidic, leached, or highly weathered soils where Ca/Mg/K depletion and Al³⁺ toxicity limit growth. It might be considered a good addition if the substrate hasn’t been changed in a long while with the nutrients having been depleted through constant watering. Although best practice here would be to repot versus add anything.

It might also be considered for species which appreciate or tolerate higher pH and require nutrient supplementation. To find out which species these are (for European species), you can look at this spreadsheet – the Reaction column lists the plant’s soil pH preference.

Among species commonly used in bonsai, lime-lovers are a minority. Some species are happy enough to be in moderately alkaline soils since they evolved in higher pH environments such as the Mediterranean basin, Crimea and Caucasus. These include European Ash (Fraxinus excelsior), Field Maple (Acer campestre), Field & English Elms (Ulmus minor & procera), Oriental Hornbeam (Carpinus orientalis), European Spindle (Euonymus europaeus), Box (Buxus sempervirens), Olive (Olea europaea), Mastic (Pistacia lentiscus) and Italian Cypress (Cupressus sempervirens).

For Mediterranean species in particular the benefit isn’t only the pH effect but also that the slow release of silicon and trace minerals matches the mineral-rich, free-draining substrate these species evolved on.

Which Trees Would NOT Benefit?

Basalt supplementation will increase the pH of the bonsai medium, so should not be used for any tree which prefers acidic soil, such as azaleas, camellias or any species originating in Japan where surface soils are generally acidic (pH 4.5 to 5.6).^ref Most plant species tend to prefer mild acidity or neutral pH, so adding basalt should only be considered for trees which you know actively prefer or tolerate alkalinity.

In conclusion, basalt is a niche addition rather than a general-purpose supplement. Repotting is the better answer for tired soil. Micronutrients are better supplied via a balanced fertiliser or seaweed extract, as this acts on a much shorter timescale and contains a wider range of nutrients. I personally also add biochar which provides better nutrient ion retention capacity, encourages beneficial microorganisms and assists with water retention.

SB Invigorator

Another product which pops up as a recommended one in the bonsai world is this one – SB Invigorator. This product is for pest control and claims to control “Whitefly, Aphid, Spider Mite, Mealybug, Scale and Psyllid.”^ref As I have recently added a lot of indoor plants to my collection, these pests are becoming rather annoying, so I have been looking for ways to get rid of them without using toxic chemicals. Would SB Invigorator work?

The main claim for this product is that is uses a “physical mode of action”. However the manufacturer fails to explain what this actually means, so it sort of floats in the ether as a claim without any rationale. A physical mode of action is basically one which physically affects the pests in question. Scraping a pest off a leaf or squashing it with your fingernail would be a physical mode of action. Horticultural oil such as neem also uses a physical mode of action by altering the leaf surface characteristics.^ref This method does not rely on poisons, instead it disrupts pests’ ability to move around and/or eat your plants.

What is the physical mode of action in SM Invigorator? Well, there are a couple of clues in the company’s safety data sheet and more in the company’s product manual for commercial users.

The main hazardous component (ie. the one which must be identified on the safety data sheet) is Sodium Lauryl Ether Sulphate (1-3% by volume), also known as SLES. SLES is an ‘ionic surfactant’, basically a detergent and foaming agent. A surfactant is a substance which reduces the surface tension of water of a liquid – on a plant this can make the surface slippery to insects and harder for them to gain purchase on a leaf or stem. In fact plants themselves make surfactants, known as saponins, below is an image of the saponins created by the fruit of Sapindus makorossi in a research study into the subject.^ref You can see the foam in the tube, which has been shaken – this is due to the surfactants making it easier for air bubbles to be created.^ref

Side note – the study identifies a range of plants^ref which produce high quantities of saponins, including chickpeas. The saponins in chickpeas result in the sticky liquid left behind when you strain a can of chickpeas – also known as aquafaba. The surfactant properties of aquafaba are used to create meringues and other dishes which require air bubbles, without the need to use eggs.^ref

So one of the main ingredients in SB Invigorator is detergent, the likes of which can be found in many consumer detergents. How does this affect pests? According to their product manual, which is published for commercial users, “two separate modes of action have been observed: (1) adult whitefly have been observed to stick by the wings to any surface they make contact with and aphids, juvenile whitefly and spider mite if directly hit are trapped by its wetness. (2) On mealybug an initial application removed the protective wax and a second application controlled them.”

This is why they also promote one of the features of the product being “plant wash for a cleaner, shiny appearance”!

I was interested that the biological control company ‘Dovebugs‘ had contributed to the product safety data sheet. I thought perhaps there were microbes in the product as well. But instead I believe they must have been consulted about the effect of SB Invigorator on beneficial microbes. The company’s information^ref states “Studies so far have shown SB PLANT INVIGORATOR to be compatible within an integrated pest management programme where beneficial insects are used.”

On other websites selling this product there are several additional claims which are not listed on the company’s website including:

“SB Plant Invigorator contains naturally elements, such as seaweed”^ref [this would act as a fertiliser, particularly good at providing micronutrients]
“improves plant health due to the inclusion of chelated iron and nitrogen fertilisers.”^ref [more standard fertiliser]
“Active ingredient: Carbonic acid diamide/urea”^ref [source of nitrogen = fertiliser]
“based on a blend of natural ingredients, including surfactants, amino acids, and plant extracts.”^ref [as above]
“is a foliar feed that can be used on an extensive range of ornamental and edible plants. The spray contains a wide range of nutrients and micro nutrients that encourage growth and improve the condition and health of the plants when sprayed on the leaves.”^ref [foliar fertiliser]
“Consisting of blends of surfactants and nutrients or fatty acids and algae extracts”^ref

So if the above are true, in addition to the detergent component, SB Invigorator may also contain liquid seaweed and some fertiliser. Since the product is sprayed on the leaves, it could be acting a a foliar feed (see my article on the effectiveness of these here) as well as a general fertiliser since any runoff would end up in the soil.

On Amazon 500ml of this product is currently £13.45. Assuming their product data sheet reflects the diluted product, with 1-3% of SLES, it’s pretty similar to my eCover washing detergent (with 5-15% surfactants undiluted) which is worth 70p for an equivalent concentration and volume. Let’s say it also has 10% or 50ml of liquid seaweed – based on my Shropshire seaweed purchase recently this would be worth 67p – or to be generous 100ml, which is £1.34. Add to that 50g of Chempak 3 fertiliser (probably way too much since 800g makes 1600L) – worth 63p and you have a grand total of £2.67 for a DIY version.

Now one big caveat here is that the actual proportions of these components may be important, and this company appears to have tested their product – although they have not made their tests publicly available. Since the company is based in Guernsey their financial reports aren’t publicly available either, so it’s not possible to read about their company in much detail. So maybe there is a magic formula which they have perfected and of course there are the costs of management, marketing, packaging, distribution etc.

But, if you can’t afford SB Invigorator, and you wanted to try something similar as a do-it-yourself version, you could do worse than start with the recipe for insect deterrent provided by Jerry Coleby-Williams (a botanist, presenter on Gardening Australia and environmentalist). He says his grandad used to use ‘white oil’ for controlling scale. This recipe suggests mixing half a cup of dishwashing detergent mixed with two cups of sunflower oil, and then using one teaspoon of concentrate mixed into a litre of water. If you wanted to, you could add some seaweed extract and/or fertiliser as well.

Note – I tried a detergent solution to get rid of aphids on some succulents in my indoor plant collection (actually Portulacaria afra) and it made the leaves drop off! I think the solution was nowhere near diluted enough (it was before I read Jerry’s recipe). So do a test leaf before you spray everywhere.

Water hardness, pH and bonsai

I live in London, a city sitting on a giant chalk deposit which formed in the Cretaceous period and stretches all the way to France (via the Eurotunnel)^ref Chalk is a form of limestone made up of the shells of marine organisms, and is comprised mostly of Calcium Carbonate (CaCO₃).^ref According to my water supplier (Thames Water) “When your drinking water seeps through this rock, it collects traces of minerals like magnesium, calcium and potassium. This is what makes it hard.”^ref

As you can see the water in my area is towards the harder end of hard. But there are plenty of places in Europe with hard water as well, as you can see in this map which comes from a study measuring groundwater in 7,577 sites across the region – most areas in fact are hard with exceptions in Scandinavia, Scotland and northwestern Spain (where igneous/volcanic bedrock dominates)^ref:

https://essd.copernicus.org/articles/13/1089/2021/#&gid=1&pid=1

What is also interesting from this research paper is the corresponding map of groundwater pH (see below). Groundwater pH determines your tap water pH if that’s where your drinking water comes from. Some areas source their drinking water from surface water as well, such as lakes and running watercourses – for example in Sweden it’s 50/50.^ref

pH is closely associated with water hardness, with higher levels of calcium carbonate leading to increased pH (in the world of agriculture a common practice to raise the pH of acidic soils is ‘liming’ – or adding calcium carbonate)^ref. Look at the areas in Southern Spain and France below which are pH 8 and above – their groundwater is also hard as shown in the map above.

The water in my taps is pH 7.75, so getting close to 8 which is relatively high. Not only that, but continued watering and drying of a bonsai medium with calcium-carbonate-rich water could increase the concentration of calcium carbonate in the pot and potentially make the pH even higher. But is this a bad thing?

To answer that question we need to take a detour into pH and what it actually means. At this point you can be thankful that usually I wait for a couple of days before posting, because otherwise you’d be deep in the weeds of ions, acids & bases and cursing my lack of editing skills! The (relatively) simple version is that pH is a measure of the concentration of hydronium (H₃O⁺) ions relative to hydroxide (OH^–) ions in water. In a neutral solution like pure water, they are at equilibrium and there is the same amount of each. The chart below shows the different ratios of hydronium to hydroxide ions at each pH. You will notice that in the red section there are more hydronium than hydroxide – this is acidic. In the blue section there are more hydroxide and less hydronium – this is alkaline (aka basic).

https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch17/ph.php#ph

pH is mainly a useful way of describing a chemical environment, as it helps to explain how other chemicals will react in that environment. For example, when a low pH (acidic) solution reacts with many metals, hydrogen gas and a metal salt are created.

pH is one of the fundamental attributes that affects living things – including plants. In living cells a difference in pH across the cell membrane is harnessed to drive some of the most fundamental processes for life itself – photosynthesis and respiration.^{ref1, ref2} Living things are generally very good at managing the pH inside their cells and have feedback processes to adjust it up or down according to their needs and the environment (called homeostasis). Studies have shown that pH within plant cells is maintained at a small range of 7.1–7.5.^ref

It’s when plant cells interface with the outside world, such as when taking in nutrients from the soil, that pH can make a difference to the efficiency (or not) of these reactions. Nutrients are taken up by plants as ions – ie. dissolved in water. This means that they need to be in solution for root hairs to take them up, and that solution can be acidic, alkaline or neutral.

Dissolved substances in the soil water (which change its pH) can also change the availability of nutrients – for example calcium ions will react with phosphorus ions to make calcium phosphate, so the phosphorus is unavailable for plants.^ref But plants adjust their uptake according to these changes, so when they detect pH levels which reduce nutrient availability, in many cases they adjust their uptake to compensate, and these forces work in opposite directions.^ref The overall effects of pH on the availability of nutrients to plants are a combination of the effects of pH on absorption by soils and the effects of pH on plant uptake.

Below is a chart showing the absorption of different nutrients by soil (in this case geothite, an iron rich soil). You can see that due to their different chemical makeup, each nutrient has a different absorption rate – the higher the absorption, the less available for plants.

https://media.springernature.com/lw685/springer-static/image/art%3A10.1007%2Fs11104-023-05960-5/MediaObjects/11104_2023_5960_Fig2_HTML.png

Negatively charged metals (‘anions’) have a more consistent soil absorption profile – and most are absorbed by the soil eventually when the pH is 6 or above. But uptake by plants is significantly increased as pH rises.

So far it seems like acidic soils might provide more nutrients – but also more toxins (eg. cadmium, lead & aluminium). But the release of organic matter, including nitrogen, sulphur and the activity of microbes which perform this breakdown, is increased at higher pH, and the uptake of metals is increased.^ref So it’s really a conundrum to work out the net effect of all these interactions! What do we actually know? Some findings include:^ref

Phosphate fertiliser is least effective near pH 7; it is necessary to apply more of it to achieve the same yield as at lower pH. It is most effective near pH 5
Boron uptake is consistent between pH 4.7 and pH 6.3, but a 2.5-fold decrease occurs at pH 7.4
Molybdenum uptake is eight time higher at pH 6.6 compared to pH <4.5^ref
Uptake of metal ions from solution by plants is increased by increasing pH – but their availability is decreased. This applies to toxins as well as nutrients. Magnesium and potassium are two important nutrients to which this applies.
Sulphate’s absorption by soil decreases markedly with increasing pH but plant uptake also decreases – the net effect has not been determined.

There is actually a fantastic diagram which shows the best soil pH range for each plant nutrient – you can see this all over the internet and it looks so useful! But unfortunately this diagram, which was created in the 1940s, is incorrect and has no real numbers behind it.^ref In reality “nutrients interact and different plants respond differently to a change in pH” as described above so there is no one-size-fits-all diagram.^ref

While I’m in mythbusting mode, there isn’t any such thing as ‘soil pH’ either! As noted in this excellent study from March 2023, pH can only be measured in a liquid. Unless you are over-watering, it’s likely your soil is not a liquid, therefore the soil itself does not have a pH. The pH that is being measured when ‘soil pH’ is measured is actually the pH when the soil is mixed with water – whilst this is indicative of the pH that might be present on individual soil particles, there is probably a range of pH instead across different particles. The pH of the water on a soil particle and the pH of the water on a root hair combine to create the true pH environment for a particular nutrient on a particular root. This is obviously not very easy to measure! See the end of this article for my bonsai media pH experiment.

The study mentioned above basically claims that most studies on pH and soils have failed to take into account the interplay between availability in the soil and plant uptake of a nutrient, which often work in opposite directions and so pH should not be taken to be the main factor in nutrient uptake except in specific circumstances. But looking at all of the above, it does seem like slightly acidic conditions should optimise all of the different reactions taking place – between 6 and 7 pH.

To bring it back to my bonsai, in my London garden with hard tap water of pH near 8, on the surface it would appear that this has the potential to cause a phosphorus deficiency in my plants, and perhaps affect their boron, molybdenum and metallic ion levels (we care about magnesium particularly which is used for photosynthesis – magnesium uptake increases at high pH but availability in the soil decreases).

But tap water is not the only thing affecting pH in the water in my bonsai soil. It’s also affected by the pH of my rainwater, which was 5.89 on the last measurement^ref, as well as the medium in my pots. I use composted bark, biochar and molar clay. Composted bark has organic components so is acidic, biochar is slightly alkaline and molar clay appears to be acidic – and this pH will become evident when particles of these components dissolve into the water. So the actual pH of the solution in my bonsai soil is anyone’s guess! All I can conclude from this is that a long summer without rain might cause my soil to increase in pH due to the removal of one acidic component – the rainwater.

The other thing to consider is that you can obviously adjust the availability of nutrients by adding them to your soil. So even if uptake is reduced by a particular pH, making more nutrients available could compensate for this. Hence the importance of regular fertilising for our bonsai, and using a range of different fertilisers which provide different nutrients.

Finally if you want to test the pH of your bonsai medium, a good approximation can be made by using a red cabbage and some distilled water (don’t use tap water, as this will affect the outcome if it’s not neutral to start with). Simply boil up a bit of red cabbage in (distilled) water, let it cool and while you are doing that put a representative piece of your bonsai medium into some water (also distilled). Allow them to soak for a while. Remove the cabbage from the cabbage water, strain the medium out of the bonsai medium water, and pour some of the cabbage water into the bonsai medium water. It should change colour according to the pH as follows (you can read more instructions here):

https://i0.wp.com/www.compoundchem.com/wp-content/uploads/2017/05/Making-a-Red-Cabbage-pH-Indicator.png?ssl=1

I performed this experiment on different bonsai mediums I had sitting around in my shed by soaking them in filtered water for 1 hour, then adding the cabbage indicator. The results were interesting! I was expecting the Kanuma to be acidic but it was actually neutral, as was my bonsai mix (which included some molar clay, bark, biochar, pumice and compost), and the pumice was surprisingly slightly alkaline. A rather small amount of biochar caused the indicator to go dark blue, which definitely tells me it needs to be used in moderation (although other mechanisms in biochar make nutrients available to plants, which you can read about in my biochar post).

What I conclude from all this is that my use of composted pine bark in my bonsai mix is probably a good thing as it will counteract the alkalinity from the tap water. This was a suggestion I learned from Harry Harrington’s website – although he recommends it for water retention, it would appear to balance a high pH medium or water as well. It also has the added benefit of being organic matter, which is a fertiliser in itself, creating more nutrient availability even if the calcium carbonate in my water locks some away. The need for applying fertiliser regularly is also apparent, as you just don’t know how nutrients are behaving in your particular bonsai soil and you need to give each tree every chance they have to access the nutrients they need. But overall other than causing annoying limescale marks on pots, my bonsai seem completely fine with hard water.

Mulch – relevant for bonsai?

If you’re a fan of Gardening Australia as I *massively* am, you will have noticed they are always going on about mulch. Mulch (often in the form of bark or woodchips) gets added religiously to everything they plant whether in a pot or in the ground. This got me wondering whether mulch could be beneficial for my bonsai.

What is mulch? Well back to my expert source Gardening Australia in their article Mulch, mulch, mulch, it is a layer of materials such as compost, bark and woodchip products, and/or various grades of pebbles and gravels which are placed on the soil. The benefits they claim for mulch include water retention, weed control, protection from extreme hot or cold, reducing erosion, delivering organic matter and nutrients into the soil, and even – that it looks good!

Actually I want all of these things for my bonsai, so what does the science say about the effects of mulch?

The main benefit most studies seem to agree on is that mulch reduces weeds, and the thicker the mulch the more weed reduction.^ref In one study on container-grown Thuja plicata it was as effective as chemical weed control.^ref This finding is repeated across many other studies as well.

How about reducing hot root temperatures? Potted tomatoes with grass mulch showed a direct relationship between mulch depth, soil moisture and soil temperature (see the chart below.^ref Moisture was increased and temperature decreased with additional depth of grass mulch. I don’t think it’s realistic to add 10cm of mulch to a bonsai pot though!

In a winter study, chopped newspaper as well as other mulches moderated cold temperatures.^ref The Thuja plicata study by contrast found no soil temperature improvement by using mulch, and they blamed the colour of the pots (black) for this.^ref So it looks like there might be a positive effect on root temperature but not if you have black pots – and only if you put a decent amount of mulch on the soil.

What about water retention? A study using plastic mulch (ugh) on Japanese privet plants found that the water that needed to be applied was 92% less in mulched pots^ref but the Thuja plicata study stated that no change in water retention resulted. The researchers proposed that transpiration was the main driver of water use (and since this happens at the leaf surface mulch will not impact it).^ref An intriguing study in South Africa found that only a mulch of white pebbles was useful for water retention in the hot summer, but mulches of other organic types (bark & leaves) were also effective at reducing evaporation during the colder winter period. They were pretty brutal with their research subjects – potted Polygala myrtifolia – which only got a watering once at the beginning of the trial and then had to tough it out for 6 weeks without any more water being added! In the summer period of the trial only 7% of survived, and 50% of these had white pebble mulch. During the winter trial 92% of plants survived and in these circumstances mulch of any kind provided a 20% improvement in soil water content relative to no mulch.^ref So it looks like mulch provides some improvement in water soil content as long as it’s not a drought scenario (and you don’t have black plastic pots).

One thing I have noticed is that a layer of Melcourt propagating bark (2-7mm) on my bonsai seems to ‘suck’ the water into the pot in when I am watering. Several studies have found that a layer of mulch on soil increases water infiltration rates.^{ref1, ref2} This may be because the pieces of mulch are “able to absorb the kinetic energy of rainfall…[or watering]…and maintain soil aggregates longer” and result in “an increase in the tortuosity of water pathways due to the higher roughness”. A study on Holm oaks found that rock fragments were a good mulch for shallower root systems and improved soil moisture.^ref A rough-textured mulch might be useful if water is bouncing off the surface of your planting medium.

So should you use some form of mulch on your bonsai? If you want weed control, probably. If you have trees which are particularly prone to drying out or succumbing to the elements – for example they have very shallow or small pots, or are potted in medium without some form of water retention (such as coconut coir, vermiculite, bark or sphagnum moss) it might be worthwhile. It may also act like a form of insulation (as discussed in the post on frost) to protect roots from the cold. Finally if your medium doesn’t want to cooperate with the watering can or hose, and water bounces or flows off it, mulch might be a way to reduce overflow and improve infiltration.

What options are there for bonsai mulch? There are quite a few different types of mulch described in this article but not all of these would be practical for a bonsai pot, and many you wouldn’t use for aesthetic or ‘aromatic’ reasons. Only a mulch with a relatively small component size would be feasible – this could include a small-sized bark mulch, or even a layer of smaller medium such as akadama, pumice or molar clay. I’d love to be able to use seaweed but I don’t think it would smell good, and it’s not that easy to find in suburban London. Organic mulches will break down over time and add organic matter to your soil – which you may or may not want to do. So – maybe this is a practice you might want to consider.

Nitrogen-fixing and bonsai

You’ve probably heard the term ‘nitrogen-fixing’ – it means extracting nitrogen from the air. Which doesn’t seem like it should be too difficult, since nitrogen makes up 78% of air^ref, but in reality plants can’t use gaseous nitrogen.

In nature (ie. where nitrogen is not added artificially as fertiliser) plants mostly rely on microorganisms to help them get nitrogen – they access it in dissolved inorganic forms as ammonium (NH₄⁺) and nitrate (NO_3-). This is the nitrogen cycle, where organic nitrogen from dead organic matter is converted back to inorganic nitrogen as ammonia (NH₃), then ammonium, then nitrate.^ref Although this is performed by a range of different bacteria and fungi, this is NOT nitrogen-fixing, it’s ammonification followed by nitrification.

Nitrogen-fixing is the specific act of extracting nitrogen from the air, and it’s also performed by a range of different bacteria, known as diazotrophic bacteria. Certain plants create symbiotic relationships with these bacteria, with the most effective being root nodule symbiosis. These plants have evolved to provide a safe home for nitrogen-fixing bacteria in their roots, in small nodules where the bacteria live. The bacteria get food from the plant and protection from the outside world, and in return the plant gets nitrogen. Plants which can do this all belong to the ‘FaFaCuRo’ group – Fabales, Fagales, Curcubitales, and Rosales – they are all flowering plants (angiosperms).^ref You can download a database of all 825 known species with root symbiotic nitrogen fixation here – they include green manure such as clover and legumes, as well as some trees – Acacia (wattle), Casuarina (sheoak), Albizia (Persian silk tree), Robinia (locust), Wisteria, Alnus (alder), Elaeagnus (oleaster) and Hippophae (sea buckthorn).

The initial question behind this article was me wondering whether planting clover or similar nitrogen-fixing plants in my bonsai pots would achieve anything – like somehow supplying my tree with a free source of nitrogen. After looking into it further I concluded that the answer is no! Nitrogen-fixing plants have a great system – for themselves. The reason why they are used as green manure, or as rotational crops, is because they don’t require (or require less) supplemental nitrogen, so the land where they are planted gets a break from fertilizer. When they are harvested they can be ploughed back into the ground for bacteria to break down via ammonification/nitrification, so the next crop can benefit from a nitrogen source which hasn’t come from fertiliser. Basically it’s a way of making natural fertiliser – effectively compost – which hasn’t had added fertiliser as an input.

You could benefit from nitrogen-fixing plants such as clover for your bonsai practice – if you composted it and used it as organic matter in your soil mix. In fact it has been found that nitrogen-fixing trees in a tropical forest inhibit their neighbours (possibly due to their stronger growth rates), so you definitely don’t want your trees to share a pot with these species while they are alive.^ref

There is also what’s known as ‘associative nitrogen fixation’ – this is when a nitrogen-fixing bacteria ‘associates’ with a species of plant without actually taking up home in root nodules. They are found on the roots, in the rhizosphere, and sometimes within plant tissues as endophytes.^ref It has been suggested that up to 24% of nitrogen supply to cereal crops such as maize, rice and wheat is actually supplied in this way and that ‘mucilage’ (sugar exudates from roots) may be responsible for attracting the responsible bacteria.^ref Although interestingly it may not actually be that the bacteria provide nitrogen directly, but instead they influence the plant to be able to access more nitrogen in the soil, for example by increasing root hair surface area.^ref This is the mechanism by which biochar improves nutrient acquisition as well – by increasing the plant’s Nitrogen Use Efficiency or ‘NUE’.^ref1,ref2

Which unfortunately brings us back to needing a source of nitrogen in the soil in the first place. What I have concluded is that unless a bonsai tree is a nitrogen-fixing species itself, the only way for it to obtain nitrogen is from the soil via the nitrification of dead organic matter, or by adding chemical fertiliser. And from a sustainability point of view, using at least some dead nitrogen-fixing organic matter (such as legume plants) for composting may be best as this is net-positive for nitrogen, bringing previously inaccessible air-borne nitrogen into the soil (so – go forth and compost your legumes!)

The main impact you can have as a bonsai tree custodian (aside from providing a nitrogen source) is to improve your tree’s nitrogen use efficiency so it can gain the most from the nitrogen which is present. There are a few ways to do this. Adding beneficial bacteria to the soil provides the associative nitrogen fixing effects explained above, and keeping the pot at the requisite temperature, pH, aeration and soil water level that is attractive to these microbes is also a factor – although it’s hard to know exactly what these conditions are! Avoiding extremes is probably the best approach. Adding biochar to the soil is known to improve nitrogen use efficiency.^ref Encouraging a high root surface area through root pruning and encouraging root ramification is another contributor. Finally, do not overfertilise, as this has the opposite effect on root ramification since nutrients are easy to find and roots do not need to increase their surface area.^ref

Transpiration

I’ve talked about transpiration in quite a few different posts on this site, but a recent thread on http://bonsainut.com caused me to think maybe I should have a post dedicated to it, so here goes…

Transpiration is the evaporation of water from the leaves of a tree. It’s actually a critical process for trees, because excess transpiration is one of the few ways in which a tree can die; so-called ‘hydraulic failure’ has been identified as the most prominent cause of tree death.^ref Hydraulic failure – the failure to access enough water to replace water lost mainly through evaporation – causes cell death, xylem failure and a fatal reduction in photosynthesis. So it’s really important for bonsai practitioners to understand this process.

The main driver of transpiration is not – as you might think – to cool the leaves (although this is one reason for it). In fact transpiration is a by-product, or ‘cost’, of photosynthesis, and it happens because of the way that leaves obtain carbon dioxide. You may already know that plants have small pores called ‘stomata’ which open up to let air – and CO₂ – inside the leaf. But you might not have known that gaseous CO₂ in air needs to be dissolved in water before it can be accessed by chloroplasts and used for photosynthesis (this is explained in Vogel (Chapter 5 – ‘Leaking Water’). This means that water needs to be available on the surfaces inside the leaf – which means that when stomata open up, this water is subject to evaporation.

Vogel says that “only if the relative humidity is 100 percent will water not be lost…[and]…if the leaf’s temperature is above that of the surrounding air, then water can be lost even at that humidity.” He also says that for every gram of CO₂ used by a leaf for photosynthesis, it’s estimated that 125 grams of water is lost.

Smith et al (Chapter 4.10 Movement of Water & Minerals) explain that this evaporation causes a constant flow of water known as the ‘transpiration stream’. As water evaporates from the leaf cells, pressure in those cells is reduced, and this negative pressure causes water from the xylem to move into the space, due to strong mutual attraction between water molecules. That in turn pulls more water behind it and so on. This hydraulic mechanism is responsible for pulling water all the way up the tree from the roots. Actually this process is fundamental to the health of the tree, maintaining cell turgor (stiffness), transporting nutrients, metabolites & growth substances synthesised in the roots throughout the tree, and providing a source of water for the phloem stream which flows in the opposite direction providing energy to the tree. When there is enough water available, all of this works perfectly – when there isn’t, problems arise.

The extent of evaporation from the leaves of a tree is determined by several different factors, which can be divided into environmental, tree-specific physical factors and tree-specific response factors.

The main environmental factor which drives transpiration is the ‘vapour pressure deficit’ (“VPD”) – this is the “difference between the amount of moisture in the air and how much moisture the air could potentially hold when it’s saturated.”^ref VPD is a function of both heat and humidity, and provides a measure of how powerful the evaporative force of the air is with any combination of these.^ref

Occasionally while writing articles for this blog, I end up in the world of cannabis cultivation. Maybe because they are very motivated to keep their crops vigorous, cannabis growers and their equipment suppliers sometimes have the best data and charts out there! This is just such an occasion, see below for an excellent chart from ‘Ceres Greenhouse Solutions’ showing the VPD for a given temperature and humidity (you can download a copy here). The VPD is low in the blue section and high in the red.

https://ceresgs.com/wp-content/uploads/2018/10/VPD-Bioengineering-Chart.pdf

What you will notice is that the relationship between humidity and temperature isn’t exactly linear. Also, VPD increases with higher temperature and lower humidity. Since a higher vapour pressure deficit means there is more ‘pull’ on the water in leaves, increasing temperature and decreasing humidity both increase transpiration – and they reinforce each other, so dry and hot is a high transpiration combination.

Another environmental factor is wind. One study found that wind actually improves water use efficiency, because whilst it does increase transpiration, it also increases CO₂ uptake, and the net effect is greater water use efficiency and not less.^ref But for the purpose of this article, wind does increase transpiration.

Coming onto tree-specific physical factors – these are all the attributes that relate to the size, shape, position and structure of the tree. In general the more foliage a tree has, the more it will transpire – so a large broadleaf tree will transpire significant amounts on a hot day – in one study they found a large canopy tree in the tropics (Eperua purpurea) transpired up to 1180 litres per day!^ref By comparison in the same study, smaller (presumably more shaded) trees transpired a lot less. Thomas (Chapter 2: Leaves the food producers) gives the following figures: “<100L/day in conifers, 20-400 L/day in eucalypts and temperate trees such as oaks, reaching perhaps 500 L/day in a well-watered palm and as high as 1200 litres per day in specimens of Eperua purpurea growing out of the top of the Amazonian rainforest canopy.”

The chart below shows the daily transpiration rate during the growing season for a sessile oak tree in Turkey which measured 18.5m x 34.5m – this maxed out at 160 kg/day (effectively 160L).

https://www.ewra.net/ew/pdf/EW_2017_59_34.pdf

As well as the volume of foliage, trees have different stomatal size and density (number of stomata in a given area) which are determined by genetics as well as environmental factors (such as intensity of light and VPD to which they are exposed when developing).^ref1 ^ref2 Low stomatal area (ie. density x size) will result in lower transpiration when compared to a tree with higher stomatal area. These researchers measured stomatal area for 737 plant species across 9 forests and at the lower end of the spectrum conifers such as Cunninghamia lanceolata (0.2%) and Picea koraiensis (0.4%) had 100 times less stomatal area than angiosperms such as Viburnum betulifolium (23.77%) or Quercus serrata (21.74%). You can download all their data here. Basically the more stomatal area which is open to the air, the more transpiration there will be.

Many trees have wax plugs in their stomata which reduce their efficiency, and transpiration at the same time. To copy a piece from my article on needle leaves, wax deposits in Sitka spruce stomata reduce transpiration by two thirds but photosynthesis by only one third.^ref One study found that 81% of the species they looked at contained such plugs and that wax plugs are particularly numerous in conifers.^ref

Another factor is the level of transpiration via bark. This isn’t due to stomatal opening but simply due to partial permeability of bark to air – also genetically determined and due to the presence of ‘lenticels’ – small channels which allow passage of water and air for the metabolism of living cells in the bark. One study on Pinus halupensis found that “Bark transpiration was estimated to account for 64–78% of total water loss in drought-stressed trees, but only for 6–11% of the irrigated trees.”^ref This is because bark transpiration is passive and unmanaged, unlike leaf transpiration which can be somewhat controlled by the tree (see below).

Also relevant for individual trees is their position relative to other trees and the sun. Shade will reduce the temperature at the leaf surface and reduce transpiration, a mass of trees together along with undergrowth may increase humidity, also reducing transpiration. A tree standing alone or above others will be exposed to higher temperatures and lower humidity, thus increasing transpiration. Different areas on a single tree will be exposed to different combinations of these factors as well, so rates of transpiration will differ even from leaf to leaf on a given tree.

The final category of attributes which determine transpiration relates to the trees’ ‘behaviour’. That is, how they react to different environmental conditions. As we all know trees may be sessile but they are also incredibly dynamic and can adjust a wide range of parameters of their own biology. The main issue they need to address in this case is losing too much water, which could lead to death. As a result, they change their physiology to manage evaporation as well as water intake at the start of the transpiration stream.

To manage evaporation, trees adjust their stomata based on water availability, changing their ‘stomatal conductance’ to reduce transpiration if not enough water is available.

They do this in a couple of different ways – ‘passively’ and ‘actively’.^ref The passive mechanism is where lower water pressure within leaves causes guard cells around the stomata to lose their stiffness, which has the effect of reducing the stomatal aperture. The active mechanism relies on the tree producing abscisic acid (ABA) – this “triggers efflux of anions and potassium via guard cell plasma membrane ion channels, resulting in decrease of turgor pressure in guard cells and stomatal closure”.^ref

A study on Metasequoia glyptostroboides found that in most conditions of water availability the passive mechanism was in play, and it wasn’t until prolonged or severe water stress was experienced that the active ABA-mediated mechanism came into play.^ref The article explains that different gymnosperm species use different combinations of these passive and active processes to manage a lack of water availability by reducing transpiration. Angiosperms by contrast use a more sophisticated and more recently evolved version of the active process, mediated by ABA.^ref

Thomas says that stomata usually close when it is “too cold or dark for photosynthesis” or when the leaves are in danger of losing too much water and wilting”. The consequence of stomatal closing is an associated reduction in photosynthesis – so when a tree is drought stressed, it won’t be generating energy at the same rate as when it was healthy. A study measuring photosynthesis versus stomatal conductance for Pinus radiata (see in the chart below) found there was a roughly linear relationship, as the stomatal conductance increased, so did photosynthesis.

https://www.researchgate.net/publication/8690701_Variation_in_foliar_13C_values_within_the_crowns_of_Pinus_radiata_trees

There are several other ways that trees manage their transpiration – by adjusting their root conductance (ability to draw in water), changing their leaf expansion so that there are fewer/more leaves which are smaller/larger in area, pointing exposed leaves downwards during hot periods of the day, changing the root/shoot ratio to match water source to water use and by operating a daily cycle of metabolism which optimises transpiration (eg. increasing their root hydraulic conductance at night when there is lower evaporation, and ‘filling up’ to deal with higher transpiration during the day).^ref So they are very much active participants in responding to and controlled their transpiration rate.

But what does it all mean for bonsai? The first thing is, if your tree has plenty of water availability, transpiration should not become a problem, but you need to remember that up to 95% of water use is evaporation^ref so trees need a lot more water than you might expect. The best way to avoid issues associated with excess transpiration is to supply your trees with all the water they need. This is achieved by regular and sufficient watering, and by using a medium which has some water retention to avoid drought stress – but is also well-draining. A well-draining medium allows you to water more often without the risk of waterlogging roots or creating conditions for pathogens to take hold.

Also – a tree’s ability to handle water loss varies widely depending on the species – Thomas gives the examples of eucalypts and alder as species which cannot control transpiration effectively, and some oaks as species which can. So each tree in your collection will be different.

But let’s consider all the factors explained above that increase transpiration: high vapour pressure deficit (high temperature and/or low humidity), wind, lots of foliage, high stomatal area, clean (unwaxed) stomata, passive bark & leaf evaporation, a sunny/solitary/high position, and a lack of water availability to the roots which activate stomatal closure.

Some of these are adjustable for bonsai. If it’s going to be a hot, dry, windy day then your trees are going to transpire a lot more than normal and if their roots can’t keep up, you need to improve their environment; newly collected and recently root-pruned trees or trees in particularly small or shallow pots will be most affected. You can help them by providing shade (reducing the temperature), increasing humidity, and moving them out of the wind – and obviously by watering. For a temporary period, on a very hot day, it might even make sense to sit pots in water (do not do this for an extended period).

Transpiration can also be a problem in the winter as trees do continue to transpire, albeit at lower levels, even if they are deciduous. As such, they do need water to be available which means you need to keep an eye on moisture levels in pots. If they get dry, water them. If the medium is frozen, this will lock up water and can have a dehydrating effect so in this case you need to also water, ideally when it’s above freezing. Mulch is suggested to avoid hydraulic failure for trees in the ground^ref, a similar approach can be used for bonsai in pots, to reduce freezing and make more water available to roots. Even at night it is not the case that transpiration completely stops – typically it is 5% – 15% of daytime rates.^ref

Balancing the amount of foliage with the roots when repotting or pruning is another important way to help your trees manage their transpiration rates, so that there is enough root mass to meet transpiration demands. Root pruning in the heat of summer should be avoided unless a comparable foliage reduction takes place. If you’ve gone a bit far with the root pruning, use the approaches above – provide some shade, increase the humidity and maintain a watering regime. This is where the bagging method for collected trees comes from – it reduces transpiration by increasing humidity and can be used for trees struggling to recover from a severe root prune.

Anti-transpirant is a product that some bonsai aficionados use. This does what it says on the tin – it is a “film-forming complex of polyethylenes and polyterpenes that when applied to foliage will reduce the moisture vapor transmission rate”^ref The active substance is derived from conifer resins. In reducing transpiration these products also reduces photosynthesis, which is a consideration. I’m personally not a fan of disrupting a plant’s natural processes in this way, and successful use of the product depends on the individual tree and product selected (read more here).

Hopefully you can see from all of the above that transpiration is an extremely important concept to understand as a bonsai geek, but one which can be managed, as long as you are aware of the factors at play. Here’s to helping our trees avoid hydraulic failure!

What does frost do to bonsai trees?

Living in London means that even though spring has blossomed forth, there is still a chance of frost all the way through to the end of May. And many of us will have learned the hard way that frost and/or sub-zero temperatures can seriously damage our trees.

One of the key risks of frost and extreme cold is the problem of ice. Ice formation pulls water from plant cells, causing dehydration, which can be just as lethal as dehydration from underwatering.^ref Ice masses which form and thaw can deform and damage cell membranes, as well as buds and other plant organs, and ice in trees’ xylem vessels can cause embolisms (air bubbles) to form, cutting off the water flow above the bubble.

Another problem is that low temperatures make photosynthesis dangerous due to an excess of energy which ends up as damaging reactive oxygen species (“ROS”). This is why some trees convert green chloroplasts to bronze chromoplasts in cold weather – read more here, and others shed their leaves altogether to eliminate the problem.

Plants which are ‘hardy’ or ‘frost-hardy’ have mechanisms to resist the effect of ice – either by ‘avoidance’ such as supercooling (lowering a liquid to below freezing point without freezing), or ‘tolerance’ – using biochemical changes and physical adaptations allowing the tolerance of ice in their tissues.^ref Some pines have evolved specific mesophyll cells which allow the ice into the intracellular spaces without deforming the key structures within the needles – a physical adaptation. Oaks and ring porous trees regrow their conducting xylem every year, since embolisms during winter make last seasons’ xylem ineffective.

Crucially, since many cold-hardiness mechanisms rely on biochemical changes within the plant, they require time, so that the relevant proteins, enzymes and other metabolites can be synthesised in sufficient quantities to have the desired effect in plant cells. This process is called ‘cold acclimation’. Smith et al (2010) give the example of rye. 50% of non-acclimated rye plants will die at -6^oC, but after spending 2 days at 4^oC, they can go down to -21^oC before 50% of plants will die. The cold acclimation process is what’s known as an ‘epigenetic’ process – where environmental triggers such as shortening days and reducing temperatures turn on genes – in this case known as COR (cold-regulated genes).^ref These then produce the proteins which are used to create the cold-hardy biochemical changes.

There is an equal and opposite process known as cold deacclimation. Smith et al (2010) say that above a temperature of 10^oC cold hardiness is rapidly lost, which is why a spring frost at the end of May can be so damaging.

To complicate matters, shoot hardiness is not the same thing as root hardiness.^ref Studies have shown that roots are less hardy than shoots, even when exposed to identical temperature acclimation treatments.^ref For example, the stems of Pyracantha are hardy to -25.6^oC, whereas mature roots are hardy to -18.8^oC and young roots are hardy to -6.1^oC.^ref This is an important insight for bonsai because the scale of our trees and the fact they are suspended in pots means their young roots are particularly vulnerable. A symptom of winter damage to young roots is when a tree flushes later than normal, and has retarded growth during the season.^ref It may not kill the tree, but it will certainly give it a handicap for the next season.

So what does it all mean for bonsai?

For me the main risk boils down to root damage. Those lovely fine roots we work hard to encourage during the growing season are more vulnerable to frost than any other part of the tree, and unfortunately due to the size, shape and positioning of most bonsai pots, they are very exposed to cold temperatures. In one study it was found that container-grown trees were subjected to temperatures down to -15^oC when the night-time temperature reached -30^oC, whereas the soil only went down to -6^oC.^ref And since surfaces cool faster than air when it’s cold, resting up against a cold pot surface is the last place a young root wants to be! When the temperature gets really cold, mature roots can be damaged as well, which could be fatal to the tree.

Sure – leaves can be damaged by frost as well – mainly when deciduous trees have leafed out expecting above-zero temperatures and then a frost comes along. Most will deal with this and should regrow their leaves – there will be an energy penalty which will reduce the overall energy they have to devote to the growing season, and they won’t look great in the meantime, but it shouldn’t be fatal if the stems and roots are still healthy. But certainly to keep your trees looking and growing their best, you want to avoid deciduous trees leaves being exposed to frost if at all possible (see below for some strategies).

To be able to manage and prevent frost damage, we need to know when a frost will happen. This starts with monitoring the temperature during winter. Importantly, when you hear or see a temperature forecast for a location, it is a forecast for the air temperature. The temperature of the ground is often several degrees lower. The UK Met Office says “As a general rule of thumb, if the air temperature is forecast to fall between 0 °C and 4 °C on a night with little or no cloud and light winds, then you need to bear in mind there may be a frost outside in the morning. The closer it is to zero, the greater the chance of seeing frost. If the air temperature is forecast to be below zero, then the risk of seeing frost is much higher.”^ref This goes for surfaces like pots as well. Wind and cloud cover reduce the chance of frost at a given temperature.

The actual temperature which will damage or kill roots is species-specific, so there’s no hard and fast rule but knowing where a tree comes from should give you an indication. In general the species common in the boreal forests such as many Pinaceae and Betula pendula will cope best with freezing temperatures. Some ornamental tree ‘killing temperatures’ are provided in this list.

So what should you do to help your trees defend themselves against frost damage?

The first thing to do is to let them be exposed to the cold over time, before it freezes. Give them time to activate their COR genes and establish cold acclimation. This includes not putting them in a polytunnel or shed or wrapping them up until they’ve had some time in colder temperatures (but not freezing).

It’s also useful to have actual temperature data from the location where your pots are going to be over winter. This can be achieved using a digital thermometer/hygrometer which records the temperature and humidity at regular intervals – there are many wireless/bluetooth-enabled options now which are very reasonably priced (I just bought 3 for £35, one for my garden bonsai bench, one for my allotment bed and one for my allotment greenhouse). This will enable you to monitor what the temperature does relative to the forecast so you can better predict the forecast in your actual location – and you can see which locations might provide better winter protection. If you see the temperature approaching zero you can act to protect your trees.

Then, adopting a form of overwintering system could be beneficial. Somewhat counter-intuitively, this involves watering everything well, without overwatering.^ref The reason for this is to avoid dehydration. Then providing a physical barrier to the cold, effectively a ‘tree duvet’. This can move the frost surface away from the pot edges. You could use an insulating blanket of some kind, but do some research – one study found that ‘microfoam thermoblanket’ made a difference, but clear, black or white poly did not.^ref The covering would need to have insulating properties, and not get saturated with water such that it would then freeze anyway. You could also move your trees into a polytunnel, garden shed or greenhouse, with the walls and air inside acting as a form of insulation. If you use a digital thermometer you can monitor these to ensure they aren’t getting too cold – note that polytunnels have been found to get cold enough to damage roots.^ref

The absolute best is to provide some form of heat so that the temperature can’t go below a certain point – in practice this could be moving your pots closer to your house, using a (lightly) heated propagation bed, or putting them in a (lightly) heated greenhouse or outhouse. You don’t want them to think it’s spring so there shouldn’t be too much heat, just enough to keep the temperature above freezing. Be aware that in a warmer environment such as a shed your trees will deacclimate earlier as well, so make sure they don’t go back outside into a frost as they may have lost their cold acclimation.