Category Archives: Keeping Bonsai Healthy

Spray bottle with detergent

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 plantsref 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 informationref 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:

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).

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.

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 <4.5ref
  • 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 measurementref, 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):

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 potsref 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 airref, 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 (NO3-). 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


I’ve talked about transpiration in quite a few different posts on this site, but a recent thread on 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.

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).

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.

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 evaporationref 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 groundref, 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 -6oC, but after spending 2 days at 4oC, they can go down to -21oC 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 10oC 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 -25.6oC, whereas mature roots are hardy to -18.8oC and young roots are hardy to -6.1oC.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 -15oC when the night-time temperature reached -30oC, whereas the soil only went down to -6oC.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.

Pathogens – nasty tree microbes

A pathogen is a microorganism such as a virus, bacterium, oomycete (water mould) or fungus which causes disease and/or death.ref Examples you might have heard of include Dutch Elm disease (caused by the fungus Ophiostoma ulmi), Horse Chestnut bleeding canker disease (caused by the bacterium Pseudomonas syringae v. aesculi), Ash dieback (caused by the fungus Hymenoscyphus fraxineus), Sudden Oak Death (caused by the oomycete Phytophthora ramorum) and mosaic viruses in vegetables.

The groups of pathogens causing forest tree diseases and their prevalence is shown in the chart below. 85% of these are fungi and the remainder are divided among bacteria, viruses, nemotodes (microscopic worms) and oomycetes. Oomycetes appear similar to fungi but are actually not genetically similar, and are classed in the Chromista kingdom.ref

Link to source article here; original sources Butin, 1995; Capretti and Ragazzi, 2010; Manion, 1991; Tainter and Baker, 1996

Note that as they are not microorganisms, insects are not classified as pathogens, although they can also cause significant damage and death to trees. Insect damage is a subject for another post, but for this one, their main role is in introducing pathogens to trees by delivering microbes on them into wounds they create (this is called being a pathogen ‘vector’).

Every living thing is in a battle for survival so that its genes can be passed on to future generations. This means that both trees and pathogens are constantly evolving to outsmart each other. But fungi and bacteria can also be beneficial and even necessary for trees, so you can’t just try to kill off every fungus or bacteria as this will destroy the ones that trees actually want and need.

The other thing to keep in mind is that plants have a completely different ‘lifestyle’ to animals in the sense that they are plastic and can grow new tissues, synthesise new compounds and respond to pathogens in a very different way to humans. As noted elsewhere, in response to wounds, trees ‘don’t heal they seal’ and they don’t have an immune system in the same way as animals.

But, there are three ways to help your trees avoid damage from pathogens. The first is to prevent them from being exposed to pathogens in the first place. The second is to remove any pathogens which do take hold. The third is to bolster your trees’ defences so they can fight off the pathogen and its effects.

How can you prevent exposure to pathogens? A tree’s first line of defence is its bark, leaf cuticle, and the pectin and lignin in cell walls, which are physical barriers which prevent pathogens from entering its cells.ref Since they can’t run away, this is the main way that trees avoid exposure. But in bonsai we do a heck of a lot of pruning, which unfortunately breaks the physical barrier, leaving the tree vulnerable.

To minimise the chance of nasty microbes attacking your tree as a result of pruning there are some steps you can take. Firstly, practice excellent hygiene and make sure your pruning tools are disinfected regularly, particularly when moving between trees. But be careful that the disinfecting method you use doesn’t damage your tools (carbon steel is particularly vulnerable). Soap and water can work (but dry off the water), or you could use an antimicrobial oil such as tea tree oil (from Melaleuca alternifolia) or oregano oil (from Origanum vulgare) – not only are these antimicrobialref1,ref2 but they also protect steel from corrosion.ref1,ref2

Another approach is to prune during the wintertime. The latter helps because in general most living things are more active in warmer weather – including pathogens. Pruning in winter reduces the likelihood that a pathogen will enter a wound before the tree can seal it off.ref Also many fungi and oomycetes prefer a moist environment, so when pruning try to avoid leaving the wound wet. Angling pruning wounds towards the sun can also be beneficial, since sunlight has disinfecting as well as drying properties (although probably not so much during the wintertime in higher latitudes).ref

Contrary to some advice, wound sealants have not been shown to reduce bacterial or fungal infection on tree wounds.ref This is because a tree wound is not sterile so any sealant can seal pathogens in as well as out. One wonders whether applying an antimicrobial oil to a wound might work, but I cannot find any studies looking into this. Recently I tried applying raw linseed oil to the cut ends of various crabapple branches I was trying to propagate, but these ended up with large communities of mould on them regardless (possibly because I was using high humidity which is perfect for fungal growth).

One interesting pathogen avoidance method is to decouple the seasonal timing between the host plant and the pathogen vector (vector means the delivery method of the pathogen, often an insect). One study found that Dutch Elm disease was avoided by trees which flushed early, since they were not as susceptible to infection after this point.

The second way to help your trees avoid pathogens is to remove them once there. This is tricky since by definition a pathogen is microscopic and impossible to see with the naked eye. You can’t go and squash every bacterium on your tree! It is possible to kill pathogens using antibiotic and/or antifungal substances, but effectiveness varies depending on what you need to remove and usually involves unpleasant and toxic chemicals (such as glyphosate) which also kill good microbes.

Biological control is an alternative to chemicals, this means finding another organism which feeds on or somehow damages the pathogen in question. An example of biological control is the introduction of the Myxoma virus into Australia to control the rabbit population. In trees, the fungus Trichoderma is used as a biological control agent, and it is found in bacterial inoculants for plants such as this one. Species of Trichoderma have been found to be effective against Armillaria root rot (also known as the dreaded honey fungus) and pine pitch canker.ref

The final way to help your trees is to bolster the natural processes they use to resist the negative effects of pathogens. Plants produce a huge range of substances which have defensive effects, and can detect pathogens with surprising speed and specificity. When a pathogen is detected by a plant, it first activates a specific ‘pattern-triggered immunity’ response which is believed to be sufficient to defend against a wide range of pathogens.ref This is known as ‘basal resistance’. A second line of defence detects the so-called ‘effectors’ – substances that pathogens create to avoid the pattern-triggered response. This second ‘effector-primed immune response’ causes cell death at the site, limiting the spread of the pathogenref and is known as the ‘hypersensitive response’. Plants also synthesise a wide range of defensive compounds such as resins in conifers, terpenoids or essential oils, saponins and flavonoids (of which 9,000 are known).ref1,ref2 These help them deter pathogens by making their cells poisonous or unpalatable.

So ensuring your plant is not stressed by lack of water, light or nutrients is one way to help it have the resources to defend itself. Another way is to provide it with beneficial microbes such as mycorrhizal fungi and beneficial bacteria. One study found that providing the bacteria Bacillus cereus to tomato plants enhanced their resistance to pathogens by activating the plant growth hormones salicylic and jasmonic acids. Cultivating a healthy rhizosphere (root microbiome) which supports your tree’s health can be achieved by using a well-aerated soil mix and by not constantly repotting. Repotting risks losing the microbiome which a tree has built up over time, for this reason I always try to add back in some of the previous soil when I repot.

So what should you be doing as a bonsai enthusiast to avoid any nasty pathogens ruining your great work? I’d suggest three things. The first is good pruning practices – minimising wounds, avoiding pruning wounds becoming wet or humid, and vigilance in disinfecting tools and pots. The second is to keep your trees vigorous and healthy – particularly before doing large-scale pruning or defoliation. Give your trees lots of water, nutrients and sunlight to help them bolster their defences. And finally help your trees out with the addition of beneficial fungi & bacteria; products containing these can be found online.

aquarium water

Water Sources for Bonsai

While many of us might simply use the hosepipe to water our bonsai, there are actually a range of options for recycling or collecting water for this purpose. I’ve looked into some of these below to understand how suitable water from different sources is for watering your trees.

Dehumidifier water – good unless you have a dessicant humidifier and toxic air

Dehumidifiers remove water from the air in a few different ways. One common type is a compressor dehumidifer. This type pulls air through a filter and over cooled metal coils which cause water in the air to condense onto the coils and drip into a reservoir.ref Since this is effectively distilling the water, it should be relatively pure. Where contamination could come into play is if the coils or the water reservoir are not kept clean, but for bonsai tree watering, this shouldn’t be an issue.ref You should be cleaning the reservoir anyway to avoid Legionnaire’s disease (see below). The water from a dehumidifier won’t have any minerals or nutrients in it (unlike rainwater), so fertiliser would be needed.

A dessicant-type dehumidifier pulls water through a dessicant material – such as zeoliteref (if you’re wondering where you’ve heard that name before, it’s used as a bonsai soil additiveref). Water is absorbed in the dessicant then this material is heated and the water drips out into the reservoir. It’s not exactly the same as distilling because the water is in contact with the dessicant as it is condensed and could hold dissolved compounds. Researching the properties of zeolite will take you down an entirely new internet rabbit hole (including 1.24M research paper results on Google scholar). But this substance is known for extracting heavy metals and other contaminants from liquidsref1 , ref2. So in theory the zeolite could hold other molecules which could be released into the water as it condenses. This might be an issue if you are using dehumidifier water from a location with particularly toxic or polluted air. If not, the risk to your bonsai should be fairly low. As above cleaning the water reservoir and dessicant regularly is important.

Tumble dryer water – good if condensing, less good if venting

Tumble dryers also work in slightly different ways but the main mechanism for extracting water is that warm air evaporates water from the clothes, then this air either passes over a condenser or is vented outside, in both cases water condenses from the air as it cools.

With a condensing dryer, the process is another form of distillation so should be relatively pure water and fine for use on bonsai.

If you are using a vented machine however there may be microplastics, particulates and lint from the drying clothes coming through the venting pipe. In 2021 tumble dryers were found to be a leading source of microfibre air pollution.ref You might not think this would affect your tree very much, but it has been found that nanoplastics and microplastics can enter plants through their roots, carrying a range of toxic substances including pesticides, polybrominated diphenyl ethers (PBDEs), endocrine-disrupting chemicals (EDCs), polycyclic aromatic hydrocarbons (PAHs), phthalates, and bisphenol-A. PLA microplastics have also been shown to negatively impact arbuscular mycorrhizal fungi diversity and community structure.ref If you want to avoid microplastics in your pots and tree roots you probably don’t want to use vented tumble dryer water.

Air conditioner water – good

Air conditioners work similarly to dehumidifiers and dryers – they pull air through a mechanism which cools it, and in doing so water is condensed from the air. So it’s fine to use for watering (although as above will contain no nutrients).

Boiler condensates – not good

I’m not sure how many people would be trying to use boiler condensates for watering but just in case you think of it, boilers which use fossil fuels produce condensates containing carbonic acid, sulphuric acid and nitric acid, all of which reduce the pH of the water coming from the system.ref Although slightly acidic water (in the range 5.5-6.5) has been show to optimise plant growthref, boiler condensates can be as low as 3 which is toxic to plants.ref

Rainwater – good – maybe the best

Rainwater is a different proposition to the previous water sources. Whilst it is distilled from air just like the others, it has to fall through the atmosphere to reach the ground. As it does this, rain absorbs compounds present in the air in particulates and gases. It also runs off roof surfaces, down drains and pipes and into storage tanks. So rainwater collects contaminants along the way. As atmospheric carbon dioxide is one of the molecules rain collects, it has an average pH value of about 5.6 (just at the lower end of preferred plant water pH which is 5.5-6.5)ref

The chemical composition of rain varies geographically even before it hits the ground. For example in Samoa the rainwater composition is highly influenced by marine sources, which makes sense since it’s an island.ref Locales near the ocean have rainwater with a similar (diluted) composition to sea waterref but those inland vary depending on natural and manmade influences such as industry, topography and weather. Raindrops have also been found to contain airborne bacteria and fungi.ref

Rainwater in the UK is monitored for ions of sodium, calcium, magnesium, potassium, phosphate, nitrate, ammonium, sulphate, sulphur dioxide and chloride, and for acidity/pH and conductivity.ref The latest measurement at my nearest monitoring station showed the following contents from a 3mm sample – nutrients are there but nowhere near the same levels as a seaweed fertiliser.

The other factor to consider is where you store your rainwater. Outdoor water reservoirs are usually colonised by bacteria, fungi and other organisms, and have rotting plant matter, bird faeces, dead insects and other detritus. This may also be a good source of fertiliser, depending on how concentrated your detritus gets, or a source of toxic microbes and algae, particularly if there isn’t much flow and replenishment of the water.

But overall rainwater is a good choice for watering bonsai. It has a favourable pH for plants, and is a mild fertiliser containing a range of macro and micro nutrients. As it doesn’t have carbonates like groundwater, using rainwater can help you avoid limescale marks on leaves & pots if you live in a hard water area. Just try to avoid leaving it standing or stagnant for long periods particularly if the temperature is above 20oC (see Legionnaire’s disease below).

Aquarium water – maybe depending on your tank

The subject of aquarium water almost warrants its own post. Anyone wanting to better understand the chemical and biological parameters inside a planted aquarium should read the brilliant book “Ecology of the Planted Aquarium: A Practical Manual and Scientific Treatise” by Diana Walstad.

The water in your aquarium is likely to be completely fine for bonsai if it’s fine for fish. They have high standards – and can’t handle large pH ranges, excessive levels of nitrites, ammonia or excessive nutrients like heavy metals. In fact this water can be an excellent source of nitrogen since fish in aquariums excrete ammonia, which is the main source of nitrogen for most fertilizers. Usually when doing a water change on an aquarium the levels of rotting organic matter have accumulated and ammonia levels are at their highest – one of the main reasons for doing water changes is to reduce them. Planted aquariums which use soil as a substrate and have fish (and fish food as an input) also contain other macro and micronutrients, to the extent that fertiliser isn’t necessary. So aquarium water could be a good addition to your bonsai, as a fertiliser.

On the other hand, if you have certain plants in the aquarium, some are known to be ‘allelopathic’ – that is they produce compounds to inhibit other organisms. For example the water lily Nuphar lutea kills duckweed and lettuce seedlings (Walstad, 2012). Aquatic algae and bacteria can also behave allelopathically, to the extent that Diana Walstad keeps her prized plants in their own substrate and even in separate tanks to stop them being killed by competitive organisms. So there is a risk with water from planted aquariums or aquariums with algae that there may be allelopathic compounds which damage your tree. It’s hard to make a recommendation since it’s impossible to know without testing whether toxic compounds are in your aquarium water. If you have no algae or plants, then the water is probably fine.

Greywater – not unless you treat it

Grey water is waste water from your bath, shower, washing machine, dishwasher and sink.ref It can contain detergents, oil, dirt, organic matter like food, skin particles, microplastics, bacteria, fungi or anything that you wash off yourself, your dishes or your clothes. As such greywater isn’t suitable for watering your bonsai. Some larger properties like hotels recycle their greywater, but it’s treated first using UV light, chemicals or serious filtration systems.ref So unless you have access to a high quality greywater filtration system, do not use this to water your trees.

Blackwater – no

The things you learn about when researching bonsai websites. We won’t go there – just – no.

Seawater – not unless you have a desalination plant

Unless you are growing kelp forests, do not water your bonsai with sea water. Excess salt is extremely detrimental to plants, and can kill them.ref

Lakes, streams, rivers, boreholes, wells or ponds – it depends

Natural water sources such as these are not treated and can contain all sorts of things aside from water, but this really depends on what runs into them, what happens and lives on and in them. Some lakes & streams are very clean, others have industrial runoff, pesticides, sewage overflowref, algal blooms from excess fertiliser runoff and worse. What you want to avoid with a water source is dissolved contaminants which might harm your tree – usually these will derive from human activity going on upstream. So before using such a water source, it would be wise to investigate what might be entering it and perhaps to invest in some testing.

It’s thought that water obtained from aquifers underground (eg. via wells or bore holes) is of higher quality if it comes from a deeper confined aquifer than from a shallow one.ref Shallow aquifers are more open to contamination from pollutants.

pH is a consideration when using a natural water source. According to the Kentucky Geological Survey: “Streams and lakes in wet climates such as Kentucky typically have pH values between 6.5 and 8.0. Soil water in contact with decaying organic material can have pH values as low as 4.0, and the pH of water that has reacted with iron sulfide minerals in coal or shale can be even lower. In the absence of coal or iron sulfide minerals, the pH of groundwater typically ranges from about 6.0 to 8.5, depending on the type of soil and rock contacted. Reactions between groundwater and sandstones result in pH values between about 6.5 and 7.5, whereas groundwater flowing through limestone strata can have values as high as 8.5.”ref Since you want to keep water pH between 5.5 and 6.5 for plant watering, it would be a good idea to test the pH of any natural water source you are using.

A note on Legionnaire’s disease

Legionnaire’s disease is a potentially fatal form of pneumonia caused by inhaling mist or droplets of water containing Legionella bacteriaref. This bacteria can grow in any non-sterile waterref, feeds on algae, rust, scale or sludge, and thrives in temperatures between 20oC and 45oC.ref So it could be present in some of the above sources, particularly when water is left standing. It’s worth being more careful when temperatures are above 20oC as you really don’t want to get this disease, which may already be a risk to bonsai practitioners since another source of Legionnaire’s is compost & potting soil.ref In warmer weather it may be safer to use tap water for bonsai.

So to wrap all of this up from a bonsai perspective – feel free to use rainwater or water which comes from the distillation process of condensing humidifiers, tumble dryers or air conditioners. Be careful though if this water has been standing for a long time in temperatures above 20oC. Feel relatively free to use aquarium water (but keep an eye on your trees to monitor for allelopathic effects) or dessicant-type dehumidifiers. Test the water if you’re using an untreated natural water source like a river, lake or well. Avoid using water from vented tumble dryers. And definitely do not use sea water, boiler condensate, untreated grey water or black water. Happy watering!


The Endosphere

Although it might sound like we’re veering into science fiction territory, the endosphere is actually part of a plant’s microbiome, like the rhizosphere and the phyllosphere. It is the community of microbes which live inside the plant itself – that is, between and in its cells. It’s only in the last few decades that research on the endosphere has accelerated – this has found that in fact a wide variety of microbes including bacteria and fungi live inside plants for at least a part of their lifecycle.ref They are known as endophytes – and some of these are symbiotic whilst others can be pathogens.

Endophytes are found throughout the plant, in leaves, roots and stems, in spaces between cells as well as within cells themselves; the greatest number are found in roots, then leaves, then fruit/flowers. The types of microbes in residence depends on the microenvironment in each part of the plant, the specific physical and chemical characteristics in each environment attract different microbes.ref

To enter the plant in the first place, microbes come from outside, through the root tips and hairs, through stomata and trichome pores in leaves, fruit & flowers, through holes in the stem made by insects, or by producing enzymes which break down plant cell walls to create an opening. Often these microbes are present in the rhizosphere or phyllosphere, and they migrate into the plant for all or part of their lifecycle.ref Usually they live between cells, but some examples of bacteria and fungi entering plants cells have also been found. Endophytes can be transmitted vertically (from mother plant to seed), and horizontally (from the outside environment).ref

Of all the spheres, the endosphere is the hardest to study, so there isn’t a huge amount of research which demonstrates what endophytes actually do when they are inside plants and how the host plant might benefit. Some findings are that endophytes are able to detect Reactive Oxygen Species (“ROS”) and may be able to help plants fight high ROS levels (eg. acting as an anti-oxidant).ref Others have found endophytic fungi which produce the plant growth regulators gibberellic acid and indole acetic acid (auxin), and that this contributes to greater root & shoot mass.ref1 ref2 One study found an endophyte which conferred resistance to Dutch Elm Disease in vitroref. Finally a large number of endophytes associated with trees have been found to produce Taxolref, the best-selling cancer drug ever manufacturedref and this promises to be a way for greater volumes of the drug to be created.ref So like bacteria & fungi across the microbiome, these microbes appear to be pop-up pharmacies within the tree.

The endosphere probably doesn’t need to be your prime concern from a bonsai perspective. Like the other components of the tree’s microbiome, you want to foster a healthy one, which benefits the tree, and not an unhealthy one. Doing this mainly involves not killing them off!

The Phyllosphere

The phyllosphere is the community of microbes which live in and on a plant’s leaves. I had no idea that this even existed before writing the section for this website about the microbiome. Of course, if you think about it for a microsecond, it must! Our world has more microbes than anything else by several orders of magnitude, so, there must be microbes in a tree’s leaves. But the phyllosphere has been less publicised due to the intense interest in the rhizosphere (root microbiome) and in its beneficial microbes which can help plants grow by manipulating the soil and root environment.

The phyllosphere is different to the rhizosphere in that its main microbial members are bacteria and not fungi, although fungi are present, along with some archaea. It has been estimated that there are 1 million -10 million bacterial cells per cm2 of leaf surface.ref And worldwide, the phyllosphere is an important microbiome, with a possible 1026 cells! But it’s a relatively hostile environment, with fluctuating temperature & humidity and limited nutrients on the leaf surface. The shape and structure of the leaf at a microscopic level provides a range of microhabitats for bacteria, including the bases of trichomes, stomata, hydathodes (leaf pores), grooves along the veins, epidermal cell junctions, and cuticle depressions.ref A study into tree phyllospheres found 129 bacterial species were significantly associated with the gymnosperms including Armatimonadetes, Actinobacteria, Bacteroidetes, Acidobacteria, TM7, TM6, Deltaproteobacteria, OD1, Fusobacteria, and FBP and 79 with the angiosperms including Chlamydiae, Proteobacteria, Gammaproteobacteria, Alphaproteobacteria, and Firmicutes.ref

Bacteria on a leaf surface, from:

What determines the microbial mass and mix on leaves is a combination of different factors, including the nitrogen content of leaves, the specific leaf area (related to carbon availability), wood density and seed massref and the largest part of the variation seen between phyllospheres comes down to the host species. Conifers have a different phyllobiome than other species, for example they have less ice nuclei active bacteria (bacteria which can cause ice crystals to form) and they have Frankiaceae which is involved in nitrogen fixing in the soil.ref Location also plays a role, with urban trees displaying a different phyllosphere makeup – correlated to ultrafine particulate matter and black carbon on the leaves.ref

Bacteria usually require an available carbon source. You might be surprised to know that similar to roots, leaves also produce exudates (substances they exude into the environment). These include a wide range of carbon compounds, such as carbohydrates, amino acids, organic acids, and sugar alcohols, primarily products of photosynthesis, as well as proteins, oils, secondary metabolites and mucilage.ref These carbon sources are not the only ones – the Methylobacterium species can use methanol exuded from the leaf from the breakdown of pectin as its only carbon source.ref One of the bacterial families found on birch – Rhodospirillaceae – is able to photosynthesise, removing the dependence on leaf carbon sources. Another study discovered that certain phyllosphere bacteria can use diesel for their carbon source!ref

Similarly, bacteria in the rhizosphere produce a range of substances just like they do in the rhizosphere – biosurfactants which reduce surface tension, degrade hydrocarbons and improve moisture levels and dissolved nutrients on the leaf surface, plant growth regulators which open up the leaf cells and cause them to leak nutrients, enzymes which help break down nutrients and protect the bacteria from solar radiation, and phytotoxins (if the bacteria is a pathogen).ref

The benefits of phyllosphere microbes to their host are similar to those in the rhizosphere – for example Acetic Acid Bacteria have been found to perform nitrogen fixation within the needles of Pinus flexilisref, others confer resistance to Bursaphelenchus xylophilus-induced pine wilt diseaseref, some phyllosphere fungi produce zeatin, a cytokinin (plant growth regulator)ref and others auxins, some also produce anti-freeze proteins which lower the freezing temperature on the leaf.ref Bacteria are implicated in the bioremediation of harmful chemicals or pollutantsref, improved tolerance to stress, production of proteins which trigger the plant to mount defences against pathogens as well as those which attract populations of beneficial fungi.ref

So, just like the rhizosphere, the phyllosphere is a very active place with many microorganisms playing different roles and constantly interacting in a dynamic ecosystem. What this means for bonsai is that there likely are organisms in the foliage which benefit your plant. Similar to the advice in general around the microbiome, applying fungicides, anti-bacterials and chemical pesticides can kill phyllosphere organisms so avoiding this is a good idea.