Category Archives: Keeping Bonsai Healthy

Non-nutrient Additives

Like any hobby, the bonsai world has its share of fads, snake oil and quackery. None more so than in the multitude of different potions and elixirs offering to bring a bonsai to perfection, for the right (expensive) price. Amusingly, many of these go to great pains to emphasise that they are NOT fertilisers (since actually fertiliser is cheap and relatively easy to buy). Not only that, but many fail to include ingredient lists, reference real data or otherwise explain how their product is supposed to work.

Of course plants need nutrients, these are usually elements and explained in the post Nutrients for Trees. But there are a range of other ingredients which may or may not support your trees’ health, so I thought I’d start a list to help you work out what a product might be seeking to achieve:

  • Blood meal – eugh! A non-ethical source of nitrogen. There really is no need to use animal blood when you can get nitrogen from any rotting organic matter/compost.
  • Charcoal – in the form of biochar – depending on what it’s made from, helps water retention, attracts and provides a home for benefical microbes and provides nutrients & micronutrients (acts as a fertiliser)
  • Cocoa bean shell mulch – supports endomycorrhizae and nitrogen-fixing bacteria for root growth BUT also contains theobromine which is toxic to dogs, cats & fish
  • Conifer oils – tend to be antimicrobial, insect repellent, antifungal etc
  • Ectomycorrhizae and endomycorrhizae – fungi which interact with roots to improve uptake of nutrients – whether a particular species of tree uses endo or ecto mycorrhizae is detailed in this site.
  • Feather meal – another non-ethical source of nitrogen.
  • Flavanoids, flavanols, flavanol glycosides, anthocyanins – support the symbiosis between roots and arbuscular mycorrhizal fungi (a type of endomycorrhizae), as well as with nitrogen-fixing bacteriaref
  • Humic acid – ultra-dense organic matter, converts elements into forms available to plantsref, nourishes microorganisms in the soil and may mimic the phytohormone auxin
  • Japanese Cedar (Cryptomeria japonica) oil – insect repellent, insecticide, antifungal, antimicrobial
  • Japanese Cypress (Chamaecyparis obtusa) oil – antibacterial, antifungal
  • Kaempferol – a flavonoid (see above)
  • Leonardite – source of humic acid (see above) – extracted via open-cut mines
  • Manure – animal manure is a source of organic matter; from omnivores (eating both plants and animals – eg. chickens or pigs) it is higher in total nitrogen and phosphorus than from herbivores (eating only plants – eg. horses, sheep or cows) which have manure higher in total carbonref
  • Pine oil – insecticide/insect repellent
  • Quercetin – a flavonoid (see above)
  • Saponin – an insect repellent
  • Seaweed – a fertiliser which includes micronutrients which don’t appear in standard fertilizer, such as sulphur, as well as plant metabolites which can support the growth of mycorrhizal fungi and nitrogen-fixing bacteria. Seaweed extracts have been known to promote plant growth.ref
  • Succinic acid – helps reduce heavy metal contamination – as a component of Alar was used for improving fruit set in fruit tree (before Alar was banned as a carcinogen)
  • Vinasse – source of organic matter and potassium (can be chemically processed)

Read more about what some products contain here: HB-101 Analysis, SUPERthrive, Green Dream, Biochar and Biogold.

Green Dream

Green Dream is one of those products that is spoken about in whispers as some kind of miracle elixir. It was created by UK bonsai artist Colin Lewis who now lives in the US – and sells the same product there from his website. In the UK it’s available from Kaizen Bonsai. So what’s in Green Dream? Let’s just say you don’t want to use this product if you are a vegetarian, vegan, or interested in animal welfare. In the FAQs on the UK supplier’s site, it lists the following:

  • Blood meal – a source of nitrogen. Personally I feel that animal blood is over the top when nitrogen can be found in compost or any other rotting organic matter. Most likely the blood is an abattoir side-product, and associated with animal cruelty.
  • Feather meal – also a source of nitrogen and a side-product of the poultry processing industry – not known for its animal welfare standards either.
  • Cocoa shells – are the husks from processing cacao beans for chocolate. In theory this might be positive for your trees since cocoa (and its shells – known as Cocoa Bean Shells or “CBS”) contain bioactive compounds such as polyphenols. The polyphenols in chocolate products “comprise mainly catechins, flavonol glycosides, anthocyanins and procyanidins”ref – of these, flavonols and anthocyanins are both flavonoids, known to support the symbiosis of roots with arbuscular mycorrhizal fungi as well as nitrogen-fixing bacteriaref
  • CBS also contain theobromineref, which is toxic to aquatic animals, cats and dogs at reasonably low levels, so please be careful if you are using this product where cats or dogs can access it.
  • Dried organic seaweed – since seaweed is a plant itself it contains all the nutrients plants require – including many of the micronutrients which don’t appear in standard fertilizer, such as sulphur, as well as plant metabolites which can support the growth of mycorrhizal fungi and nitrogen-fixing bacteria. Seaweed extracts have been known to promote plant growth.ref
  • Vinasse – is the by-product of sugar ethanol production. It is a potassium source and a “soil fertility improver because it promotes deep root development, nutrient lixiviation and increases considerably the sugarcane yield”ref however there has been controversy over its use due to environmental damage from the high organic content.ref
  • Slow release compound fertiliser with an analysis of N.6% – P.5% – K.7%.
  • With added trace elements. Iron, Manganese, Zinc, Copper, Boron, Molybdenum (probably from the seaweed and vinasse)

Overall I won’t be using Green Dream Original as I am a vegetarian and don’t wish to use animal products from cruel farming practices for my bonsai. Also, I have a dog and don’t want to put him at risk.

The product is labelled as an organic fertiliser but this doesn’t mean its ingredients are organically produced in the same way as food: What is organic fertiliser?

In researching this article I also looked into another product on Kaizen Bonsai’s website – Green Dream Rapeseed Meal. Rapeseed is also known as canola, it’s a Brassica vegetable whose seeds are grown used for oil, which is used as vegetable oil in cooking, and as a biofuelref Since 28.3 million metric tons of rapeseed oil was produced worldwide in 2020/21ref, there is a heck of a lot of rapeseed meal to dispose of! Rapeseed meal is used as an animal feed, due to its relatively high protein content, and traditionally was used in China as a fertilizer. This is probably because China is the second largest producer and consumer of rapeseed oil and so has a lot of meal.

The meal has plant nutrients in it, because it’s made from plants, you can see a full breakdown here. Aside from the NPK listed on the label, according to the Canola Council it also contains almost every other nutrient a plant needs (see Nutrients for Trees), except for boron and nickel, so a bit like seaweed this can provide some of the trace elements that aren’t always available in the soil or in standard fertilizers.

There is a research paperref breaking down the compounds found in rapeseed meal, of these only one has any known effect on plant growth and that’s kaempferol – we came across it over in the HB-101 Analysis – it encourages the growth of endomycorrhizal fungi which aid nutrient uptake in roots.

One thing to know about rapeseed meal is that when it’s not dressed up as a bonsai fertilizer, it can purchased for quite a bit less. I found 20kg available for less than £14 (including delivery to UK mainland) at this site selling animal feed.

Will water drops on my trees burn the leaves?

This is one you sometimes hear in gardening circles – that you might burn the leaves of your plants if you water them in the heat of the day – because the drops of water act as a magnifying glass, focusing the light onto the plant and theoretically burning it. I have never observed water-droplet-shaped burns on a leaf which one would expect from this kind of behaviour. But what does the science say?

This articleref references another one behind a paywall which says that although droplets can increase light 20x at their focal points, in most of the species tested a layer of leaf trichomes hold droplets above the leaf surface, and beyond the focal point.

In another studyref “sunlit water drops on horizontal leaves without waxy hairs cannot cause sunburn regardless of solar elevation and drop shape.” – this is because “the focal region of water drops falls far below the leaf at higher solar elevations and can fall on to the leaf only at lower solar elevations, when the intensity of light from the setting sun is generally too small to cause sunburn.”

Unfortunately both papers conflict somewhat as the first says trichomes specifically hold water droplets too far away from a leaf to enable sunburn, and the second paper says only horizontal ‘hairy leaves’ can get sunburn (they found this could happen for floating fern but this plant has quite specific trichomes/hairs). In reality most bonsai trees do not have horizontal leaves, instead leaves are at a multitude of angles, and the water droplets if stuck to a leaf would probably be angled away from the midday sun. But do send me a picture if you ever see a bonsai tree burned by a water droplet!

SUPERthrive

SUPERthrive is another product claiming great results for plant health without being a fertiliser. I’m not sure why these companies find it so offensive for their products to be known as fertilisers! Fertiliser just means a product containing plant nutrients. Anyway, what is in SUPERthrive? Here is the ingredient list:

So to start with – it *is* a fertiliser (1:1:1). Aside from nitrogen, phosphorus and potassium it also contains another macronutrient – calcium, and a micronutrient – iron. I’ve taken the explanation of these below from my post What each nutrient does (x17).

Calcium is used for plant structure as it strengthens the cell walls in plants. Its presence (or absence) is also used for signalling of stresses to the plant, allowing it to activate defences against pathogens. There is twice as much calcium in this product than N P or K – so quite a lot.

Iron is present in a large number of different enzymes within plant cells, appearing in chloroplasts (where photosynthesis takes place), mitochondria (where energy is created) and the cell compartment. Iron is therefore a key nutrient for growth and survival in plants – in just the same way it is with humans and other forms of life. Iron is a component of so many enzymes that there is a specific name for them – ‘FeRE’ or iron requiring enzymes (Fe is the chemical symbol for iron).

Iron can be toxic if too much is present, so plants have evolved mechanisms to remove it when it gets too high. There is a much smaller amount of iron than other nutrients in SUPERthrive.

SUPERthrive also contains four varieties of mycorrhizae (fungus which integrates with plant roots). The types included are all arbuscular mycorrhizal fungi which are a form of endomycorrhizae – that is, the fungal cells enter the plant’s roots. This supports healthy root development, improved access to soil nutrients and healthier trees. You can find these mycorrhizae in soil, particularly in established forests. Whether or not a specific type of mycorrhizal fungus will benefit your tree depends on the species of tree. There is a list of which types work with with species on this site.

From a bonsai point of view this list helps us see that endomycorrhizae (ie. the fungi in SUPERthrive) are not associated with plants in the families Pinaceae (fir, cedar, larch, spruce, pine, hemlock) and Fagaceae (beech, chestnut, oak), neither should they work for lime trees (Tilia).

They are associated with plants in the families Cupressaceae (cypress, juniper, redwoods, thuja), as well as acers, ginkgos and most other flowering trees. So you might see a difference in effect depending on the species of tree with which you use this product.

The final ingredient in SUPERthrive is humic acid. Leonardite, the source of the humic acid in this product, “is an oxidized form of lignite, very enriched in HS [humic substances] and characterized by well-known auxin-like effects”ref Lignite is a form of brown coal, which used to be peat but hasn’t become coal yet. Humic acid is a liquid made by dissolving leonardite, so it contains dissolved, concentrated organic matter (dead plants), effectively this is like super-concentrated liquid compost. Could it be similar to compost tea? In fact, yes, liquid from compost has also shown the same auxin-like effectref. Peat analysis shows a chemical composition of carbon, oxygen, hydrogen, nitrogen, sulphur in decreasing order.ref But this doesn’t tell the whole story. Peat and its compressed descendants contain a multiplicity of nutrients as well as other components – basically it’s all the ingredients which go into plants, and are synthesised by plants, compressed and starting to decompose. So this component of SUPERthrive probably has most of the nutrients required for plant growth – although these aren’t necessarily bioavailable. This study found that “Leonardites did not affect significantly any measured variables in comparison to the control”ref So my guess is that if you are fertilising your trees with a comprehensive fertiliser, and giving them some organic matter, the addition of humic acid may not make a difference.

Leonardite is mined in open-cut mines, and can be extracted using chemicals, so it’s not particularly environmentally friendlyref, nor sustainable as claimed by this manufacturerref, since it takes 300 million years to create!

Ultimately this product is a fertiliser with extra calcium, mycorrhizae to promote healthy root development (but only for certain tree species), and concentrated liquid compost/organic matter. It probably provides beneficial compounds to bonsai trees – particularly those in families which benefit from endomycorrhizal fungae. But I’d argue these compounds could be obtained elsewhere – from a comprehensive fertiliser, compost tea and a handful of humus (dark, organic material that forms in soil when plant matter decays) from your local forest. It’s probably a useful product though if you don’t have the time or access to other additives for your bonsai.

HB-101 Analysis

Recently a member of my bonsai club was talking about this product, HB-101. It is apparently wildly popular in Japan and supposed to be fantastic for bonsai. It claims to be an “all-purpose natural plant vitalizer”. Since plants synthesise their own requirements for growth from the 17 nutrients, I couldn’t really see how this would work unless this product was a fertilizer, so wanted to dive a bit deeper into this product to work out what it does. Unfortunately the product website is pretty waffly, or possibly just poorly translated, so a bit of investigation was required.

According to the manufacturer HB-101 is made from “essences of such long-lived trees as cedars, Japanese cypress, and pines as well as from plantains.” Without knowing how they define ‘essences’ and which bit of the plantain they use, this doesn’t help much. But more info is in their submission to the United Nations Industrial Development Organisation: “HB-101 is synthesized from organic distillate, which is extracted from the heated raw material of cedars, Japanese cypress, pines, and plantains as raw materials”. So it contains substances from Cryptomeria japonica (Japanese Cedar), Chamaecyparis obtusa (Japanese Cypress) and Pinus thunbergii (Japanese Pine) as well as plantain grass (not plantains like bananas).

In their safety data sheet it shows the product has a pH: 3.0 ~ 4.5, and is toxic to fish, daphnia and other aquatic invertebrates within 48h at 1% concentration or more. The product explanation from the UN submission contains more useful data including a typical analysis chemical breakdown:

  • Kaempferol; 0.1 ~ 0.2 ppm
  • Water-Soluble Nitrogen (as N); 0.001 ~ 0.005 %,
  • Water-Soluble Phosphoric Acid (as P2O5); 0.0001~ 0.0005 %,
  • Water-Soluble Potassium (as K2O); 0.0001 ~ 0.0005 %,
  • Total Sulfur (as S); 0.0001 ~ 0.001 %,
  • Calcium (Ca); 0.5 ~ 3 ppm,
  • Magnesium (Mg); 0.3 ~ 3 ppm,
  • Iron (Fe); 0.01 ~ 0.05 ppm,
  • Zinc (Zn); 0.01 ~ 0.05 ppm,
  • Silicon (Si); 1 ~ 5 ppm

Based on this breakdown, the product appears to be mainly a nitrogen fertilizer (NPK of 10:1:1) which includes all six macronutrients (nitrogen, phosphorus, sulphur, potassium, magnesium, calcium), and two micronutrients (zinc, iron). It also contains two non-nutrient ingredients – silicon and Kaempferol.

I was surprised to see silicon in the list as it’s not considered one of the 17 required plant nutrients. But a bit of digging and apparently it “activates plant defence mechanisms” and “increases the resistance of plants to pathogenic fungi”ref. Interesting! Looks like I will need to write a new post on non-essential-but-benefical nutrients…there are bound to be others aside from silicon.

The other ingredient is Kaempferol. Kaempferol (in case you were wondering) is a flavonoid (substance synthesised by plants) which “has a role as an antibacterial agent, a plant metabolite, a human xenobiotic metabolite, a human urinary metabolite, a human blood serum metabolite and a geroprotector.”ref It has a molecular formula of C15H10O6. Flavonoids are “pigments that color most flowers, fruits, and seeds”ref and they are actually produced by plants themselves, so it’s not obvious to me why adding them to a plant would do anything.

The product explanation claims that Kaempferol ‘activates plant mitochondrial enzymes’. Looking into Kaempferol further, it is referenced in a wide variety of human disease research, including cancerref and brain injuriesref. It does indeed appear to affect the function of mitochondria and provide protection to cells against injury – at least in humans. In this article Kaempferol is said to protect against oxidative stress and various forms of toxicity by affecting mitopaghy (the removal of damaged mitochondria), suppressing fission (cell reproduction) and apostosis (cell death). In humans it seems to have the almost magical property of helping healthy cells to survive while inducing the death of cancer cellsref.

Admittedly plants (like all complex life) do contain mitochondria, which generate the energy needed to power cells. So if a substance affects mitochondria in people, it may well have similar effects in plants. The question is whether externally delivered Kaempferol can actually enter the plant and get into its cells, which it would need to do in order to make any difference. As flavonoids are synthesised by plants, and since plants don’t absorb this kind of substance – instead absorbing the raw materials to make it themselves – I don’t think Kaempferol can be having any impact on the plant mitochondria.

BUT – what it might be doing is having an effect on other living things that affect tree health. And hey presto, a bit of searching uncovered that Kaempferol has been shown to improve root development by supporting Arbuscular Mycorrhizal Fungi (AMF) ref – this is a type of fungi in soil which can help expand the volume of soil from which nutrients can be extracted (Thomas).

In one study a range of flavonols were tested for their contribution to the growth of mycorrhizal fungi and Kaempferol was shown to make a modest improvementref but another flavonoid called Quercetin was even better.

Kaempferol does come from conifers which are in this product’s ingredient list. But it’s also high in green leafy vegetables like spinach, kale and dillref. So my guess is that a lot of it comes from the plantain grass in this product. Quercetin, which was more effective for nurturing mycorrhizal fungi, is apparently found in red, green, and purple-pigmented plants – for example, red onions. Unfortunately though, it seems that Quercetin (and Kaempferol) are not very easy to extract – industrial methods (likely the one used for HB-101) require ethanol but if you’re keen here is a method using ‘subcritical water’ (liquid water under pressure at temperatures above usual boiling point, 100 °C (212 °F)

So what else might the cypress, pine and cryptomeria be contributing? The company’s product page gives a few other clues – specifically it mentions saponin, pine oil and succinic acid, although these are not listed on the chemical analysis data sheet.

Saponins are substances produced by plants which are “responsible for plant defense against antagonists; such as mollusks, pathogens and insects”ref They are contained in a wide range of plants but do not appear to be present in conifersref. By contrast saponins are found in plantain grass (and actual plantains!)

Looking at what the conifers are providing to this product – pine oil has larvicidal and mosquito repellent propertiesref, Cryptomeria japonica oil is insect repellant and insecticidalref, antifungal against tree pathogenic fungiref as well as antimicrobialref and Japanese Cypress oil is antibacterial and antifungalref (and apparently also good for hair loss!).

And finally, succinic acid. This has been shown to improve tree tolerance (in Larix olgensis) to heavy metal contamination – specifically with Leadref and Cadmiumref. This doesn’t seem to me a particularly useful attribute for bonsai, since we’re using inert bonsai medium. Dig a bit further and you find succinic acid was combined with 2,2-dimethylhydrazine to make a plant growth regulator called Daminozide (also known as Alar). It regulated the growth and set of fruit but has been banned due to concerns about cancer risk. This isn’t on the ingredient list so I don’t think it’s present in the product. I can’t find much useful information on what succinic acid might be contributing.

So what’s my overall analysis? Initially I was sceptical because I don’t like products that rely on fluffy advertising and don’t explain how they work. It annoyed me that the product doesn’t contain an ingredient list, and it doesn’t admit to largely being a fertiliser, which is most definitely is.

However one of the key features of this product is that parts of it are derived from a distillation process, which enables the extraction of beneficial compounds from the conifer leaves and wood as well as from the leafy plantain grass. These should help boost a tree’s defences to insects, pathogenic microbes and fungi, and improve the mycorrhizal activity in the pot to supports better root development (and thus healthier trees). It also has a lot of nitrogen, which as outlined in nutrients is hard for plants to obtain.

So somewhat grudgingly I have to say it might actually work.

Photosynthesis

Another epic topic which has occupied scientists for the best part of 400 years, the equation for photosynthesis itself was not understood until the 1930s.ref

Photosynthesis is the process of turning energy from sunlight into chemical energy, a process which famously is performed by plants, specifically by plant cells containing ‘chloroplasts’. Chloroplasts contain a substance called chlorophyll. Chlorophyll is often described as a ‘pigment’ – but this makes it sound like its only role is to colour things green! Which of course it isn’t – being green is just a result of its real function which is to absorb sunlight of a certain spectrum. Chlorophyll absorbs visible light in two regions, a blue band at around 430 nanometers and a red band around 680 – according to Vogel. Other sources state the bands are 680 nm and 700 nmref. Everything else is reflected, which looks green. Chloroplasts are believed to have originally been cyanobacteria which were incorporated into plant cells to provide photosynthetic capability (Lane, 2005). To see where chloroplasts and chlorophyll are present within a leaf, check out this post on leaf structure.

The sequence of reactions which happen during photosynthesis are known as the Calvin-Benson-Bassham cycle, after the scientists who discovered it. The simplified equation describing photosynthesis is as follows:
6CO2 + 6H2O –light energy–> C6H12O6 + 6O2.

This suggests that photosynthesis is the exact opposite of respiration, creating glucose instead of consuming it. But actually this equation is incorrect as photosynthesis does not create glucose. The Calvin et al cycle produces a molecule called G3P which is a 3 carbon sugar which can be used to create other molecules. Anyway, that’s probably not super-relevant for bonsai! The main insight from the equation above is that light energy is needed for plant survival and growth – this is why trees do not like being inside unless they have a suitable artificial light source.

Another point to note is that plant cells, like all living cells, respire. Plant cell respiration is like animal respiration, that is, cells consume oxygen and glucose to produce energy, while emitting carbon dioxide and water. Cells use respiration to generate the energy they need for metabolism (basically, keeping the cell alive and functioning). All cells in a tree respire, including the leaves, roots and stems, 24 hours a day. This means trees offset some of their photosynthesis by respiring – in particular at night when no photosynthesis occurs.

On balance, leaves produce a lot more sugars and oxygen through photosynthesis than they use during respiration – and this provides the energy they need to maintain themselves and grow. In fact the point at which they *don’t* produce more than they use through respiration is as low as 1% of full sunlight (Vogel). Note that this low level can be reached if a leaf is entirely shaded by 2 other leaves, since the typical light transmission through a leaf is only 5%.

Another interesting fact about photosynthesis is that leaves can only use about 20% of full sunlight before the photosynthetic system saturates (Vogel). This number actually varies depending on the species and leaf type. So leaves below the top layer (likely to be ‘shade leaves’ described in this post: Leaves) can still get enough light from partial exposure, if they are slightly shaded or lit for only part of the day, to saturate and max out their photosynthetic capability. The majority (99%) of the energy absorbed is used to maintain the leaf itself, so only 1% is released for growth.

But – back to the photosynthesis equation – which is an extremely simplified version of what is actually happening. Photosynthesis requires 150 discrete steps involving a similar number of genesref. A good reference is this Nature article.

Photosynthesis takes place in two stages. First the ‘light dependent’ reactions happen. Light energizes electrons within the chlorophyll, and these electrons are harnessed to produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) which are molecules used by cells for energy and as an electron source. The chlorophyll replaces its lost electrons by taking some from water – effectively they ‘burn’ or oxidise water, leaving the oxygen behind as a waste product (Al-Khalili).

After this the ‘light independent’ reactions happen. These are facilitated (sped up) by an enzyme known as RuBisCO. Using the energy sources created in the first step (ATP and NADPH), RuBisCO ‘bolts’ together the hydrogen from the water, and the carbon and oxygens atom from the carbon dioxide, along with phosphorus, to make the 3 carbon sugar glyceraldehyde-3-phosphate (C3H7O7P) (Lane, 2005). This is known as carbon fixation.

In reality there is also a third stage where the ingredients for the cycle are regenerated so that it can continue running.

The full equation (the Calvin-Benson-Bassham cycle) – is:
3 CO2 + 6 NADPH + 6 H+ + 9 ATP + 5 H2O → C3H7O7P + 6 NADP+ + 9 ADP + 8 Pi   
(Pi = inorganic phosphate, C3H7O7P = glyceraldehyde-3-phosphate or G3P)

Sorry for geeking out for a minute there!

An important point is that photosynthesis doesn’t just go up with increasing sunlight, it has a set of limiting factors which were described by Blackman in 1905 in his articleref “Optima and Limiting Factors”. He said “When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest”. The limiting factors he described for photosynthesis included light intensity, carbon dioxide and water availability, the amount of chlorophyll and the temperature in the chloroplast.

The amount of sun needed to max out photosynthesis from a light intensity point of view is not as high as you would imagine. Vogel says a leaf “absorbs about 1000W per square metre from an overhead sun shining through a clear sky” and photosynthesis only consumes 5% of this – 50W per square metre. Obviously this is going to vary depending on the species and whether a leaf is a sun leaf or a shade leaf.

But what does it mean for bonsai? Well – you might have spotted some other elements in the photosynthesis equation. NADPH (formula C21H29N7O17P3) contains nitrogen and phosphorus, as does ATP (formula C10H16N5O13P3). RuBisCO relies on a magnesium ion to perform its role as a catalyst. There are a bunch of other enzymes and co-factors which are required to support photosynthetic reactions as well – which explains why certain nutrients (including Nitrogen, Phosphorus, Magnesium, Potassium, Chlorine, Copper, Manganese and Zinc) are critical for trees in varying amounts – more here: Nutrients for Trees.

And if you’re concerned about whether your trees have enough light for photosynthesis, you can see exactly how much energy is arriving on your outside bonsai at this site. Find your location and download the PDF report and you’ll see the irradiation levels – which can be useful in understanding how your bonsai trees will fare. As an example, the max in my location is 328Wh/m2 in April which explains why the olive trees in London are so unhappy looking – if you look at irradiance in Greece, where they thrive, it barely ever goes under 328Wh/m2 even in winter! Trees in Greece receive nearly double the amount of irradiance than those in South West London.

One final point of interest with admittedly little relevance to bonsai – some plants (nineteen different plant families, independently of each other) evolved an improved photosynthetic process which is known as C4 photosynthesis. This concentrates CO2 nearer to the RuBisCO enzyme, reducing its error rate. Unfortunately most trees use C3 photosynthesis with its associated inefficiency – other than Euphorbia, which are apparently “exceptional in how they have circumvented every potential barrier to the rare C4 tree lifeform”ref.

Plant Growth Regulators (or Phytohormones)

You’ve probably heard of rooting hormone powder, or auxin, or gibberellins – these are all ‘Plant Growth Regulators’. Plant Growth Regulators used to be known as ‘phytohormones’ which means plant hormones. This has been quite a contentious topic among plant biologists.

A hormone in an animal is a chemical messenger, a substance which acts as a signalling or control molecule to cause an action to take place. “Hormones carry out their functions by evoking responses from specific organs or tissues that are adapted to react to minute quantities of them”ref. In animals, hormones are produced at a specific site (a gland, like the pancreas), work at low concentrations, and have a predictable dose-response. That is, an increase in hormone will result in the more of the related action (eg. more insulin leads to more sugar being taken up by the liver).

There *are* substances synthesised by plants which are involved in regulating growth – plant growth regulators – but they don’t work in the same way as animal hormones. It’s said that the assumption that they did sidetracked plant researchers for decades.ref Plant ‘hormones’ are synthesised in multiple sites in a plant (and potentially in every cellref), have multiple actions on different cells (they don’t act on just one organ or tissue), don’t exhibit predictable dose-response behaviour like animal hormones and are involved in significant interactions or feedback loops with each other.ref

What this means is that it’s quite hard to unpick what they do and how they work. The roles and mechanisms of plant growth regulators are still very much current research topics, as can be seen at two of the research groups at Cambridge University’s Sainsbury labref1,ref2 Some of the early theories about them were comprehensively demolished in a seminal article by Anthony Trewavasref (in particular the theory of auxin-derived apical dominance which was later proven wrong as explained below).

We now know that plant growth regulators act in concert with genes and the proteins they express, and not as an independently acting substance (one of the genes involved in cytokinin synthesis is known as LONELY GUY…).

So what are the plant growth regulators, where and how are they made? There are nine main plant growth regulators you may come across in your reading:

  1. Auxinref – classically called ‘the growth hormone’ and a signal for division, expansion, and differentiation throughout the plant life cycle – involved in root formation, branching, the tropic responses, fruit development, shoot meristem function, the development of cotyledons and senescence. The most common form is Indole-3-acetic acid (IAA). Auxin acts in a ‘ying-yang’ relationship with cytokinin (see below)ref as well as with gibberellins. More about auxin below.
  2. Cytokinins (CK)ref1 ref2 – involved in cell division, shoot initiation and growth (including maintaining the stem cell niche), nutritional signaling, root proliferation, phyllotaxis, vascular bundles, leaf senescence, branching and nodulation, seed germination, nutrient uptake, and biotic and abiotic stress responses. 6-BAP or 6-Benzylaminopurine is a synthetic cytokinin which is used in micropropagation and agriculture. Coconut water (not milk) has been found to be a natural source of cytokinins.ref
  3. Brassinosteroidsref – involved in a wide spectrum of physiological effects, including promotion of cell elongation and division, enhancement of tracheary element differentiation, retardation of abscission, enhancement of gravitropic-induced bending, promotion of ethylene biosynthesis, and enhancement of stress resistance.
  4. Gibberellins (GA)ref – involved in multiple processes including seed germination, stem elongation, leaf expansion and flower and fruit development.
  5. Strigolactones (SL)ref – induce hyphal branching of arbuscular mycorrhizal fungi and are shoot branching inhibitors.
  6. Abscisic Acid (ABA)ref – involved in the induction and maintenance of seed dormancy, stomatal closure, and response to biotic and abiotic stresses.
  7. Jasmonates (JA)ref – shown to be inhibitors of growth but also involved in development of flowers and defense responses against herbivores and fungal pathogens
  8. Salicylic Acid (SA)ref – associated with disease resistance
  9. Ethyleneref – multiple effects on plant development including leaf and flower senescence, fruit ripening, leaf abscission, and root hair growth.

Slightly maddeningly none of these substances do just one thing – they’re all involved throughout the plant!

So how and where they are made in a plant? This isn’t simple either. In fact local biosynthesis is thought to be critical for plants, whereby plant growth regulators are made at the site where they are needed. For example both auxin and cytokinin are synthesised in leaves *and* in rootsref, and can be made by chloroplasts and mitochondria, organelles which occur throughout the plant.ref Chloroplasts can make precursors to auxin, abscisic acid, jasmonates and salicylic acid.ref

Even though plant growth regulators don’t act in a predictable dose-response way like animal hormones, they still have a role in shaping plant growth in tissues which are sensitised to respond to them. Theoretically by understanding these responses we can manipulate a tree’s growth. And this is what they do in plant tissue culture (more on that below).

You may have heard the theory of auxin-controlled ‘apical dominance’, which holds that auxin produced by leaves at the apex inhibits lateral buds. This theory was strongly criticised by Trewavas in 1981: “The only hypothesis of apical dominance which has retained some measure of support is the nutritional one. A number of plants placed under conditions of reduced nutritional status adopt a growth pattern of strict apical dominance.” His point of view was further supported by a 2014 study which found that “sugar demand, not auxin, is the initial regulator of apical dominance”.ref The researchers found that after removal of the shoot tip, sugars were rapidly redistributed over large distances and accumulated in axillary buds within a timeframe that correlated with those buds releasing. But auxin didn’t travel fast enough to be responsible for bud release. So basically they found that apical dominance arises because the main shoot is greedy for sugars, and due to its position at the end of the vascular system it can prevent lateral buds from taking the sugars needed to release and grow.

Auxin does play some role though, and the theory is that its role related to the fact that it’s the only plant growth regulator which displays polar transport. That is – it moves from the apex to the base of the plant, via the phloem, and can travel the entire length of the plant, ending up in the roots. This gives auxin a special role related to the spatial aspects of growth, and auxin ‘maxima’ (locations where auxin accumulates) are sites where new buds, flowers or lateral roots emerge. In fact, auxin and cytokinin work in concert throughout the plant, from shoots to roots, with apparently opposite effects in each location “like yin and yang”.ref

An excellent reference point for this subject is the world of plant tissue culturing. This is where small pieces of plant tissue are sterilised and cultured in a medium containing plant growth regulators, which cause the tissue to grow into a ‘plantlet’ (sometimes in a test tube, if the source material is small). Further steps multiply the plantlet into several plantlets, which are then encouraged to create roots, multiplied again and/or planted out as seedlings to harden off. This process is used for industrial plant cloning where large numbers are required, and in the aquarium trade to avoid contamination with snails and other microbes (see Tropica’s website).

In plant tissue culturing, plant growth regulators are used to induce the relevant growth stage, which ones work for each species in which stage is documented in the ‘protocol’ for that species. In all cases specific ratios of cytokinin:auxin (and sometimes gibberellins) lead to different developmental stages – shoot growth, lateral shoot growth and root growth.ref1, ref2 To give you a bonsai-oriented example, one study determined a protocol for the micropropagation of Prunus Mumeref. They were able to multiply fresh prunus mume shoots in a petri dish using a 4:1 ratio of cytokinins to auxins, and then root them using auxin.

So – apologies for the rather long read, it is quite a complicated subject! What can we take from all this for our bonsai practice? Firstly we can stop the brain-bending trying to understand how auxin controls apical dominance because it doesn’t – access to sugars does this instead.

Also we can use the yin-yang rule – high cytokinin:auxin encourages buds & shoots, and high auxin:cytokinin encourages roots. So I’m going to start adding some auxin rooting gel into my air layers and cuttings. Cuttings have never worked for me in the past so maybe this will be the secret sauce I need. I’m also going to try some cytokinin gel to encourage lateral budding on my trees.

If you are looking for products to give this a try, make sure the product actually contains the plant growth regulator you want. For example, Clonex contain auxin, and some of the orchid budding pastes such as Keiki paste contain Kinetin (a cytokinin). Many other ‘rooting hormones’ or plant hormones products on the market have no ingredient list at all, so avoid those. You can also find these products online in shops dedicated to hydroponics, where cloning and plant tissue culturing is a technique used by practitioners, or in lab supply shops such as microscience or Phillip Harris in the UK. You can even make your own hormone gels following these instructions.

Another trick you can use is that gibberellic acid can be used to break dormancy in seeds, if you really don’t have the patience to wait for natural dormancy to break. Or give coconut water a try, this has been found to have a similar effect in a range of species. For hard coated seeds in particular, usually it’s best to search Google Scholar for a researcher who has experimented with different approaches, since what works is very species-dependent.

Repairing damage (or not)

When a tree is damaged or injured in some way, various responses happen, but none of these would be characterised as ‘repair’ in the same way one sees the human body repair itself. Trees create new growth to compensate for damage, and seal off damaged areas to prevent infection or further damage occurring. I like the way Wayne K. Clutterbuck put it in his article about tree wounds – “trees don’t heal, they seal”.ref

If leaves detect high wind, excess UV or frost, they furl up which protects them from damage. Similarly, they can respond to insects or other invaders by producing defensive compounds or thickening their leaves; defence is an important part of plant survival. But if eaten, ripped, scorched or frostbitten, leaves have no repair mechanism, as they do not have a meristem with active stem cells which could initiate new growth. Instead a tree will rely on other leaves, or grow new ones to replace the damaged ones. Deciduous trees simply drop their leaves every year, along with any damage they have incurred, and grow a new set in the spring.

If a stem or shoot is removed, the tree grows another one from a bud, it cannot replace the one which was removed in exactly the same place. The same principle applies to roots. As outlined in ramification of roots the act of cutting roots causes more lateral roots to grow to compensate.

The wounding of a tree’s trunk or major branches has more important consequences for the tree than just a leaf or stem.ref The tree detects that it has been injured because pressure changes within its cells, and the normal flow of hormones through its phloem and cells is interruptedref. This articleref (admittedly from 1985 but has some nice illustrations) explains what happens – first the cells nearest to the wound adjust their biochemistry to become antimicrobial, then a barrier zone is formed around the wound which prevents microorganisms from breaching the zone. The tissue around the wound is discoloured by these compounds – a good illustration is below. The tree has been damaged by drying cracks in the bark and boring insects. It has reacted by creating a sealed-off dead zone indicated by the darker wood, to repel and prevent further ingress by insects. You can also see that the cambium has generated new xylem and phloem annually which has curled over the edge of the wounded area.

https://www.nrs.fs.usda.gov/pubs/gtr/gtr_nrs97.pdf

Cut paste is a product which is sometimes advocated by bonsai enthusiasts, but there isn’t much to be found in the way of evidence for its effectiveness. Most research papers on the topic come from the 1930s or before, but there are a few – seemingly all from Korean researchers – which identify positive effects from a fungicide called thiophanate-methyl which was found to improve wound closure on Acer palmatumref. The mechanism wasn’t detailed in the study but presumably it worked by protecting the wound from fungal pathogens. I couldn’t recommend this though, partly because you risk dripping it into the soil and onto your your friendly mycorrhizal fungi but also because this substance is toxic to inhale, carcinogenic and causes birth defects.ref

Research shows that wounds are easier for a tree to respond to in warmer weather – in one study at 15 degrees C wound response was strong but at 5 degrees C during dormancy, wound response was minimal.ref

What all of this means for us bonsai practitioners is that when we do major carving or trunk/branch chopping on live wood, we should give the tree the best chance of sealing the damage off and preventing pathogens from entering. To do this we can do it in warmer weather, when the tree is in active growth.

The Microbiome and Symbiotic Microbes

It has been known for over a century that tree roots are colonised with microbes, particularly fungi, but it’s only in the last twenty-five years or so that this idea has captured the public imagination, with Suzanne Simard’s discovery that trees can actually communicate and share resources via their fungal networks.ref

Of course, our knowledge about microbes – a collective name which refers to any living thing so small that a microscope is needed to see it – has massively increased in recent years. Studies into the human microbiome have shown that our own cells are outnumbered ten to one by the cells of microorganisms which live in and on us (Collen). These are mostly bacteria but also include viruses, fungi and archaea, and some of them perform important roles in human health – for example comprising a key part of our immune system.

The same concept applies to trees. Microbes are everywhere on and even in trees, above-ground and below-ground, and some of these are beneficial to the tree, whilst others are detrimental. Microbes colonize the germinating seed right at the beginning of the tree’s life, then move on to colonize the radicle (root) as it emerges and then the cotyledons (first true leaves). Over the tree’s life the species and number of microbes will shift and change. It has been shown in a recent pre-publish study that 95% of the fungi and bacteria present in acorns were transmitted to seedlings, and it is expected that further research will show this is inherited from the parent tree.ref

So not only do seeds inherit their genes from their parents, they also inherit their microbiome.

The microbiome (community of microbes) of trees comprises the phyllosphere (microbes in the foliage), rhizosphere (microbes in the roots), and the endosphere (microbes within the plant itself). Within these live a wide variety of bacteria and fungi, co-habiting, interacting, supporting and competing, with a range of different impacts to their host. A newly emerging term in this field is the ‘holobiont’ – this is a host with its microbiota and recognises that they interact with each other as well as the host. A tree and its microbiome are a holobiont.

https://neutrog.com.au/2020/04/23/the-plant-microbiome/

To understand more about the microbes in each sphere and what they do, read the three posts I linked to in the previous paragraph, each has guidance relevant to their different domains.

From a bonsai point of view, we want to help our trees cultivate a healthy community of beneficial microbes in their microbiome, since this helps them access nutrients, fight pathogens and stress and thrive. There are three things we can do to help with this. The first is to avoid killing the microbes! For example, adding pesticides, chemicals, anti-biotics, weed-killers, anti-fungals etc could damage your mycorrhizal and bacterial communities. There are hundreds of studies showing that glyphosate kills off AMs and ECMs, and it has been shown to negatively influence microbial survival directly as it inhibits an enzyme of the ‘shikimate’ pathway, which produces essential amino acids in both plants and the majority of microbes.

The second thing is that you can add mycorrhiza and beneficial bacteria to your bonsai soil, particularly if you are repotting and losing the existing communities, also if you are creating new bonsai through collection, seed growing, air layering etc. You can buy dried mycorrhiza and bacteria mixes which can be sprinkled into the pot and watered in – I have my mycorrhiza in a salt shaker and my bacterial inoculant in a pepper shaker. The research is a bit mixed about how effective this is since microbes don’t necessarily establish the required density to contribute to plant defences & health, but you can optimise their chances by ensuring your substrate has plenty of nooks & crannies for bacteria to live (eg. this is one of the main claims for the benefits of biochar). Check the product you are buying to ensure it matches the type of mycorrhiza your tree associates with (some products have both ECM and AM). Alternatively, if you can find some soil or humus from an unfertilized, chemical-free forest with similar species, grabbing a handful and stirring it into your bonsai soil will also add benefical microbes .

The third thing that can be done is to create an environment for your trees which microbes prefer. Good soil, a good level of moisture, drainage, a carbon source (in most cases – roots) and not too much disruption of the roots, good lighting and avoiding large temperature variations, and air flow around the foliage.

Microbes aren’t all sweetness & light though, some are pathogenic not just to plants but to humans as well. Improperly composted manure can introduce bacteria including Salmonella, E. coli and Enterococcus. More relevant to bonsai enthusiasts is the fact that the Legionella bacteria which causes Legionnaire’s disease (a potentially fatal pneumonia) is present in many composts including those made from wood, bark, green waste and peat.ref As a result, whilst we certainly should appreciate our friendly microbes for their role in our bonsai practice, we should also make sure to wash hands and tools thoroughly, and avoid breathing in any organic matter such as compost. When mixing bonsai substrate, doing this under a cover, outside or in a bag is preferable to doing it in a way which sends dust particles into the air.

Repotting Tips

Ah repotting, such a fertile subject for ‘bonsai lore’! Any new bonsai enthusiast is soon taught (particularly in temperate locations), that all repotting should be completed in the spring, just as the buds are starting to leaf out. Here is some of the advice provided on popular bonsai websites:

  • “In general, it is best to repot right before your bonsai begins growing vigorously. In most cases this is spring.”
  • “The best time to repot a Bonsai is early in the spring, while trees are still dormant, and the buds begin to swell. At this stage trees are not sustaining full-grown foliage, so the damaging effect of repotting will be minimized.”
  • “Bonsai cannot be repotted at any time of the year; for the majority of species, there is a small period of time during the Spring where the roots can be disturbed and pruned with reduced risk of danger to the tree’s health.”

Unfortunately there isn’t any evidence that I can uncover to support these claims, and scientifically there may be good reasons to repot at other times of the year. But let’s start from first principles. Why repot in the first place?

Bonsai enthusiasts repot to avoid their trees becoming pot-bound – ie. the roots filling the pot. Why? There aren’t many research papers on this subject but luckily the eminent Australian research organisation CSIRO performed one studyref as a meta-analysis of 65 other studies to which they had professional access. They found what might appear to be the bleeding obvious – that increased pot size resulted in increased biomass – that is, the plants grew more when they were in bigger pots. More growth led to more leaf mass, greater levels of photosynthesis and more leaf nitrogen. In one experiment, doubling the pot size increased photosynthesis rates by 30%.

They also found that neither nutrient nor water availability nor higher temperatures could (fully) explain these pot size effects on photosynthesis and growth, and hypothesised that root confinement per se may cause growth retardation, with reduced photosynthesis as a consequence. Well – this is actually one of the benefits of keeping bonsai trees in small pots – it does reduce growth in both stem and root.

But in bonsai we need to find a balance. We want our trees to be healthy, we need them to develop and grow so that we can continue to refine them over time. If their roots take up 90% of the pot space, there is less space for nutrients, air and water. In one study on tobacco plants, pot-bound plants experienced premature senescence (their leaves fell off early), photosynthesis markedly declined as did the activity of Rubisco (a key enzyme involved in carbon fixation).ref

If we repotted all our trees into larger pots every time they got pot-bound, we’d be living in a potted forest and there would be no bonsai to be seen. Bonsai enthusiasts root prune to achieve the same outcome; root pruning creates space in the pot for soil, nutrients and water, and gives the remaining roots the opportunity to grow. This allows us to keep trees in small pots without halting their growth.

So it seems clear that root pruning is beneficial for bonsai in terms of longevity and growth (root pruning also encourages ramification). So if you are going to root prune, what negative effects might result? There are a few key ones:

  1. You might cut away too much stored food which the plant might need to grow
  2. You might not leave enough root mass to supply the leaves with water for transpiration – or another version of this one is that the plant might not have enough time to regrow roots in order to meet its needs
  3. You might expose cut roots to damaging microbes

The first point is covered in my post Root Food Storage (or, can I root prune before bud break?). Whilst roots do hold carbohydrates they are by no means the only place where these are stored, with branches and stems also storing significant amounts. Furthermore, the point at which they are most depleted (which is when one would theoretically prune them, to avoid losing carbohydrates) is the end of summer (see the post for charts for different species). Pruning roots in spring just before leafing out actually deprives the plant of those carbohydrates for the leafing out or flowering process.

The second point is concerned with ensuring there is enough water uptake to meet the transpiration needs of the foliage. This can be managed by pruning foliage to reduce transpiration, although it’s tricky in pines. Any other technique which reduces transpiration can help – reducing the temperature or wind, increasing humidity (for example by putting a plastic bag over the tree, a practice which is used when trees are collected).

Of course, a tree can grow new roots – and when they do so is covered in another post When do roots grow? I was interested to find that roots grow *after* leaves have had their growth spurt. So if you were trying to optimise root growth straight after pruning, the end of summer, beginning of autumn would be the best time.

So based on points 1 and 2 actually the end of summer or early autumn would appear to be the best time to root prune, depending on the species. The main risk with this approach is that of frost damage to newly grown roots if you leave it too late. But since this is when most root growth happens anyway, I’m not sure it’s really a risk.

A maxim I have is ‘the right time to do something is when you have time to do it’. Personally I have repotted trees in every season because I have a day job and a family and I certainly don’t have days on end to be repotting every tree I own at the same time in Spring! Unless you are being extremely brutal with your root pruning (in which case, do something to reduce transpiration), probably you can do it whenever it works for you.

Which brings us to the ‘how’. You might think that the choice of pot is purely aesthetic, but there is some science to it as well: see choosing a pot. Simply, you want to secure the tree into the pot without damaging its roots (sometimes harder to achieve than it sounds), fill the pot with growing medium making sure to get it into any open spaces, and give your tree a good water. Maybe add some mycorrhizal fungi (depending on the tree species), bacteria and slow-release fertiliser, then let it recover from repotting for a while and avoid constantly fiddling with it (hard I know)!