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Transpiration

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Bonsai Tree Growth Stages

Most bonsai trees progress through stages of development, each with a different objective. In general the progression is thicken trunk -> achieve branch & root structure -> achieve branch, foliage & root ramification -> reduce leaf size -> evolve as branches grow/fall off. The faster we can move through the first few development stages, the faster we will have beautiful, well-proportioned bonsai – harnessing the tree’s natural growth is a way to speed this up. We also want to avoid doing things which slow down a tree’s growth during these phases, as this will mean it takes longer to get the tree we want. Read about how trees grow before starting at #1 below. Also consider what do old trees look like?

1. Trunk

Some bonsai enthusiasts collect mature trees for bonsai specifically so they can start with a thick trunk, following a collection process which minimises damage to the tree. The alternative is growing your tree’s trunk. Once a tree has its roots and foliage reduced in size in a bonsai pot, it won’t generate the energy needed to make significant sapwood additions and its girth will only increase by small increments every year. So you really need to be happy with the trunk size first before you stick it in a tiny pot. But – how big should a bonsai tree’s trunk be?

2A. Branch Structure & Overall Shape

Arranging the branches is what gives you the canopy and overall foliage shape that you’re after and the first step in this process is growing (or developing) the branches you want in the positions they are needed. Growing a branch starts with a new bud, which, unless it’s a flower bud, becomes an extending shoot and eventually a new branch. So firstly you need to work out where new buds will grow on your tree and then deal with the extending shoots as needed to get the required internode length.

You may need to remove some buds and shoots if they don’t help achieve the shape you are looking for – this should be done as soon as possible to avoid wasting the tree’s finite energy reserves. You have a trade-off to make here because leaving more foliage on the tree will provide more energy overall which contributes to its health and ability to recover from interference. However, growing areas of the tree which won’t be part of the future design is a waste of energy. You don’t want to remove so much of the tree’s foliage that it struggles to stay alive or develop the areas that you do want to grow out.

When you are creating your branch structure, often you will need to reposition branches – this is done with a wide range of different tools and techniques. A more advanced technique for adding new branch structure is grafting.

Sometimes the trunk itself or larger branches need a rework, to make them more interesting or to make them look more like old trees – for example adding deadwood or hollowing out the trunk. Usually this is achieved through carving.

2B. Creating a Strong Root System

The trunk thickening and branch structure phases both work best when the tree has lots of energy and so letting it grow in the ground or in a decent sized pot during these phases will get you there quickest. This also allows the roots to keep growing, but you want to understand about the role of roots, and root structure & architecture even if you still have your bonsai in a training pot. Particularly in this case, knowing about how to foster the the rhizosphere will help your tree stay vigorous. To maximise the roots’ exposure to nutrients and water you want to encourage Ramification of Roots (lateral root development).

Eventually it’s time to move the tree into a bonsai pot. This requires cutting back the roots, but as long as the roots are balanced with the foliage in terms of biomass, the tree should be OK. Root growth is usually prioritised outside of times of stem/foliage growth, and above 6-9 degrees C. So repotting might be best conducted at times that meet this criteria. Your growing substrate/medium is an important consideration.

3. Ramifying Branches & Foliage

Ramification is when branches subdivide and branch, giving the impression of age and a full canopy – and a well-ramified tree is a bonsai enthusiast’s goal. There are some techniques for increasing the ramification of branches and foliage. But not as many as there are for root ramification.

This stage also involves ongoing branch selection and reshaping (see 2A above). Another consideration is whether to keep or remove flower buds.

4. Reducing Leaf Size

An end stage in the journey to bonsai perfection is leaf size reduction. In nature, leaf sizes reduce relative to the biomass of the tree as it ages but since bonsai are small this effect doesn’t translate since the biomass never gets large enough. The tried and tested method for reducing deciduous tree leaf size is actually to practice one of the various methods of defoliation. A couple of others are covered here in reducing leaf size.

When to conduct these various activities depends on when the tree can best recover from them – which is a function of the Tree Phenology (or Seasonal Cycles).

5. Evolving Branches

Trees are not static organisms – they obviously continue to grow which is what we harness in the above steps. Part of this is that eventually branches may become too large for the design, or they may fall off (Peter Warren notes that Mulberry are known for this). As bonsai artists we want to have this in mind so that branches are being developed which can take their place in the future. This is an ongoing version of step 2A.

Pruning

Once your tree has grown in the general direction and shape you want, you can refine it through pruning. Cutting into a tree can affect its health & vigour, so it’s helpful to understand exactly what happens to a tree when you do this. A really excellent paper explaining the effect of pruning is available from Purdue University^ref but to summarise, pruning has the following effects on a tree:

it removes photosynthetic material (leaves) thereby reducing the tree’s ability to generate energy
it reduces transpiration (the evaporation of water from the canopy) and the rate of water transport up the tree
it disrupts the pathways of plant growth regulators, causing regrowth but also consuming stored energy
if the main xylem vessels in the trunk are cut, it causes embolisms which reduce the water carrying capacity of the tree
it exposes the internal vascular system to the environment where bacteria and fungi can enter (by causing a wound)
on some conifers, pruning the shoot or branch removes options for future bud growth because dormant buds and meristem tissue is often concentrated in the more recent growth

Minor Pruning

Minor or leaf pruning is used in bonsai to keep the shape of a tree according to a design, but also to create ramification and reduce leaf size (or, keep leaves small). As per point 3 above, pruning leaves drives the tree to refoliate and it does this by activating dormant or suppressed buds. In deciduous trees there is usually a bud in every leaf axil and this will go on to produce at least 2 shoots, so you also get increased ramification. With only stored reserves to use for refoliation, shared across twice as many buds, leaf size will be reduced. Read more in: ramification of branches and foliage.

Major Pruning

Major pruning which involves cutting off branches or significant parts of the foliage may have more impact on the tree. The first thing is that removing large amounts of foliage will reduce the tree’s ability to generate energy. It will also reduce the tree’s energy requirements but not by as much as is lost (since leaves are working for the whole tree and not just to sustain themselves). See this article: Defoliation.

Major pruning is often required to get the design you want for a bonsai. So is it better to grow out then cut back, or cut back then grow? Growing first generates lots of energy but also lots of wasted growth, which is eventually removed. Cutting first saves energy by directing it all to the places you want to develop on the tree, but it reduces the total amount of energy available for growth.

To test this look at the following calculation. If you start with two identical 50-leaved plants, and the goal of reaching a particular level of refined foliage in 5 years time, you have two options. Scenario 1 lets the plant grow unpruned all the way to the end of the period then has a major prune back down to the target level of foliage. Scenario 2 prunes every year, gradually building up to the target level. Although they start and end in the same place, the first plant has generated a whopping 195,250 ‘leaf units’ of energy for growth – 12x what the second plant has generated.

As much as 80% of the energy created by leaves is exported to the other organs of the plant^ref. These energy units could have been used in places that don’t eventually get removed in the ‘Cut’ scenario, such as thickening the trunk, storing reserves for stronger budding or refoliation.

The most obvious risk with major pruning is the fact that you are effectively wounding your tree. Read more about how it responds in repairing (?) damage.

What kind of pruning tools should you use? Learn about the difference between carbon and stainless steel bonsai tools here.

Roots

The roots of your tree are *just* as important as the above-ground parts, with a lot of responsibilities which aren’t immediately obvious. I’ve summarised the main ones here but there is a lot more detail in separate posts with links provided below. So why are roots so important?

they absorb water from the soil to meet all the tree’s needs (both for photosynthesis and transpiration)
they absorb all the nutrients that the tree needs from the soil (using a different process to water, hence a separate point)
they transport nutrients & water up to the above-ground parts of the plant, and photosynthates (the products of photosynthesis) down to the root tips
they produce exudates (secretions) which sense and control the rhizosphere (the environment in which the roots are growing)
they produce plant growth regulators for signalling and enabling growth within the plant
they store food for later use
they provide structural strength and stability for the tree by attaching it into the soil

Points 1 and 2 are fundamental to the health and growth of the tree – the roots are the mechanism for the tree to obtain all of the water and nutrients it requires (despite the mythic popularity of foliar feeding, this is only a way of augmenting nutrient absorption and not a primary mechanism). The mechanics of how they do this is described in more detail in how roots absorb water and nutrients – in summary it’s the fine roots and their root hairs which do the majority of the absorption since they have the closest and most expansive contact with the soil. There needs to be enough root surface area to supply the stems, shoots and leaves with the water and nutrients they require.

Point 3 reflects the fact that roots are part of a tree’s vascular system, that is to say, they transport the fluids necessary for growth around the tree. Above the ground the vascular system is present in stems, shoots and leaves, and below the ground it is present in the roots. Water and nutrients are transported up from the roots through the xylem and photosynthates (the products of photosynthesis) are transported down from the leaves and other storage organs in the tree via the phloem, to provide the energy and nutrients for the roots to grow and function. Do roots grow all year round? Find out here: when do roots grow?.

Points 4 & 5 show that roots are very much an active participant in tree growth and not simply a set of supply pipes. They produce both cytokinin and auxin (read more in the post about plant growth regulators), they also produce a huge variety of substances known as exudates which both sense and control the rhizosphere (the environment in which the roots exist). Researchers believe that roots use exudates to “regulate the soil microbial community in their immediate vicinity, withstand herbivory, encourage beneficial symbioses, change the chemical and physical properties of the soil, and inhibit the growth of competing plant species”^ref. Read more about exudates and how they are produced in root exudates.

Point 6 reflects the fact that roots are used to store food, in the same way that the trunk and branches do this aboveground (throughout the ‘woody skeleton’ (Ennos)). I was going to tell you that a lignotuber is an example of this and show you a lovely picture of my eucalyptus, but then I read “contrary to common assumptions…the lignotuber in young eucalypt trees did not appear to be a specialized starch storage organ. Rather, the lignotuber resembled an extension of the stem because its starch concentrations and temporal fluctuations mirrored that of the stem.”^ref How roots store food and how much of a contribution to the plant’s overall storage capacity they make is debated. More on that in Root Food Storage (or, can I root prune before bud break?)

Finally as per point 7, the roots are responsible for physically holding the tree steady and stable against wind and gravity. They do this in many ingenious ways by adopting different root architectures – combining vertical taproots, lateral roots & sinker roots, creating ‘buttress’ roots, sending roots far from the trunk when needed and managing new root development in ways which stabilise the tree. More about this in root structure and architecture.

What all of this means from a bonsai perspective is that you need to pay just as much attention to the health and care of the roots of your tree as you do to the above-ground parts. Never mind developing a strong nebari for aesthetic purposes, you need to ensure that even though the roots of your bonsai trees are squashed into teeny-tiny pots, they are still able to perform the vital functions outlined above. Neglecting the roots will negatively affect the overall health of your tree.

Practically speaking, this is why you should aim to develop a well-ramified fine root ball, to provide the tree with lots of root surface area for nutrient & water uptake – taking into consideration the amount of biomass above-ground as this will determine how much root mass is needed.

The growing medium plays a huge role as well – this is your tree’s rhizosphere. It should provide the water, nutrients and micro-organisms the tree needs as well as (some) oxygen for root cell respiration, and ideally should not be disrupted so much so that exudates and microbes (fungi or bacteria) are lost. The risk associated with bare-rooting a tree (or excessive repotting) is that it destroys the rhizosphere all at once, leaving the tree vulnerable to pathogens and forcing it to regenerate exudates it has already created (which can use up to 40% of its stored carbon).

Your tree’s roots need to have a regular supply of nutrients, so they require fertiliser of some kind. Even if good compost is added during potting, the small size of bonsai pots will mean the nutrients won’t stay in there for very long. Trees will need added fertiliser – either home-made (for example, regular doses of diluted compost leachate), or purchased. And obviously – watering is critical. Given the role of symbiotic partners (such as fungi & bacteria), you can also add these to the soil – if your tree senses their presence and wants them to stick around – it will probably produce exudates to achieve this.

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 cell^ref), 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 lab^ref1,^ref2 Some of the early theories about them were comprehensively demolished in a seminal article by Anthony Trewavas^ref (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:

Auxin^ref – 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.
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
Brassinosteroids^ref – 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.
Gibberellins (GA)^ref – involved in multiple processes including seed germination, stem elongation, leaf expansion and flower and fruit development.
Strigolactones (SL)^ref – induce hyphal branching of arbuscular mycorrhizal fungi and are shoot branching inhibitors.
Abscisic Acid (ABA)^ref – involved in the induction and maintenance of seed dormancy, stomatal closure, and response to biotic and abiotic stresses.
Jasmonates (JA)^ref – shown to be inhibitors of growth but also involved in development of flowers and defense responses against herbivores and fungal pathogens
Salicylic Acid (SA)^ref – associated with disease resistance
Ethylene^ref – 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 roots^ref, 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 Mume^ref. 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.

Leaves

Rather naively, when setting up this website, I made a note to create a post on the topic of ‘leaves’. Several days of research later it’s clear, there can’t just be one post about leaves! Why? Well, leaves are the reason that complex life on earth exists. Their ability to photosynthesise – to turn energy from the sun into energy that living things can use – is fundamental to our planet. So – there is a lot going on inside a leaf. My starting point for this post is the wonderful book ‘The Life of a Leaf’ by Steven Vogel, and I am going to cover the purpose of leaves, respiration, transpiration, and how leaves deal with environmental challenges like heat, wind and cold. There is a lot more to know about leaves, to read this check out all the other posts tagged with ‘leaves’, or start with leaf structure.

So, what is the purpose of leaves? Simply, they are the energy generating mechanism for plants. Through the process of photosynthesis they use energy from the sun to ‘burn’ water, removing its oxygen atom and bolting the remaining hydrogen onto carbon dioxide to create sugars. These sugars are used to power the life and growth of the plant. And as a side-product, oxygen is created, for animals and humans to breathe. Photosynthesis only happens when a leaf is illuminated by the right kind of light – in nature, this is sunlight during the day. If you want to geek out with more detailed information about how photosynthesis actually works, try this post: photosynthesis

A key concept when it comes to leaves is transpiration. It turns out that up to 97% of the water used by plants is actually evaporated back into the air – a process known as transpiration. The reason for this is because plants need to get CO₂ into their leaves in order to supply it to the chloroplasts (organelles which perform photosynthesis) and to make this possible they need to open holes on the leaf surface to allow CO₂ molecules to enter – these holes are known as stomata. When the stomata are opened, water naturally evaporates from within the leaf – it is the negative pressure in the xylem caused by this evaporation which pulls the water (and nutrients) up from the roots. What this means for bonsai is that your trees need a lot more water than you might think based on their size – remember up to 97% of it will be evaporated out. But if you water on a sunny day, will you burn the leaves? Find out here.

Did you realise that individual trees have leaves of different shapes and sizes, depending on their position on the tree? Shade leaves are larger, thinner, darker green (containing more chlorophyll) and less lobed, with fewer stomata than sun leaves; conversely sun leaves are smaller, thicker, lighter yellowish green, more lobed, and have more stomata – each leaf is more efficient in their niche of light exposure (Thomas). In her book The Arbonaut, Meg Lowman describes a coachwood tree with each leaf having its own unique formula for success – the lower leaves are long-lived and better structured to harvest low amounts of filtered light, the upper leaves are short-lived “extremely high-powered chlorophyll factories…[which]…produce sugars that keep the entire tree alive, healthy and growing”. And between these, variations to suit the different light conditions. All of this is called ‘photomorphogenesis’, or a “developmental process in plants in which the incident light determines the growth of the plant”^ref. One research team even discovered a gene which could force the growth of sun leaves^ref and resulted in 30% greater photosynthesis.

What this means for bonsai is unclear as it’s not obvious from the research at what scale these differences can manifest. In theory there will be a layer of leaves around the canopy of the bonsai tree which could be sun leaves, and leaves below and within it which could be shade leaves. If it is literally only the sun exposure on a bud which determines what type of leaf is grown, then theoretically these two different leaf types could appear on a bonsai. If so, then removing the top layer of leaves – as is often recommended in bonsai, to ‘let light into the tree’ may not be the best course of action. If the leaves below have grown as shade leaves they may struggle to tolerate their new position in full sun. Anyone noticed sun & shade leaves on their bonsai? Let me know at info@bonsai-science.com.

‘Leaf morphology’ – or the shapes and sizes of leaves – is another rabbit hole you can joyfully enter via Google if you so desire, but to sum things up, leaves have differing physical attributes, each lending the leaves different properties – “leaf traits may reflect the adaptation mechanisms of plants to the environment.”^ref

A key measure related to leaves is the ‘specific leaf area’ – the ratio of total leaf area (ie. the total amount of leaf surface) to total leaf dry mass (the total weight of the leaves without water). SLA is used as a measure of the overall health of a plant, as it reflects the efficiency of carbon gain relative to water loss^ref.

What is observed is that different trees have different strategies for optimising specific leaf area – in addition to photosynthesis mentioned above. Three interrelated attributes include leaf size & shape, leaf venation (how the xylem & phloem ‘veins’ are structured) and stomatal conductance (the number and distribution of stomata on the leaf).^ref The model in this article suggests several predictions related to these attributes.

One of these is that a leaf with a larger width to length ratio can support higher carbon production factors, stomatal conductance, and leaf area than a leaf with smaller width to length ratio. So long, narrow leaves are less efficient and have higher ‘stomatal resistance’ – ie. they are less efficient at transporting gas and water vapour in and out of their leaves. This also applies for thick leaves, which have higher stomatal resistance than thin leaves.

That’s not all bad though, because higher stomatal resistance is correlated with smaller xylem, which are more resistant to embolism. These tradeoffs help different trees thrive in different conditions and drive evolutionary adaptation.

The implications of this model that on average narrow leaved species will do better in more arid conditions with less available water, since they transpire more slowly and are less at risk of embolism, and as water becomes more available the broadleaved trees will do better. So conifers may do better on a mountain top, and angiosperms such as beech will do better in the valleys. Translated to bonsai medium, this suggests that a narrow leaved tree (including most conifers) will prefer a more well-drained medium (hence the advice that conifers don’t tend to like getting ‘their feet wet’). Your tropical trees on the other hand – like a Ficus – may be perfectly happy with extremely wet conditions (as long as they don’t become anoxic – ie. so wet they prevent any oxygen from entering).

Leaves have more to contend with than ‘just’ photosynthesising and transpiring – they also have to deal with extremely large temperature variations – “on windless days, ordinary leaves on ordinary trees quite commonly run around 10°C, nearly 20°F, above the air that surrounds them. Under exceptional (exceptionally bad) circumstances they can reach twice that.” So explains Vogel, who then describes in detail the different strategies used by leaves to cool down – starting with evaporation but also including reradiation (radiating heat back into the air) and convection (transferring heat to the air from leaf surfaces and edges). He suggests that smaller, more lobed leaves are better at cooling than large wide leaves – and that large wide leaves avoid horizontal positioning in order to reduce overheating. What this means for bonsai is probably just that the natural leaf shape and positioning of a particular species helps it achieve a temperature balance (as well as photosynthesis) – and also perhaps that highly ramified trees with lots of smaller leaves may have improved temperature regulation (due to improved convection).

Another environmental challenge faced by leaves is freezing temperatures through the winter. One obvious strategy for avoiding these is to be deciduous, but there are plenty of evergreen trees out there and many of them thrive in extremely cold conditions (eg. the boreal forest where snow cover lasts for months and “gymnosperms such as Abies, Larix, Pinus, and Picea dominate”^ref). Of the various strategies deployed to avoid damage from freezing (four at least are described in detail by Vogel, as well as in this paper^ref), the strategy most relevant to bonsai is that the tree generates substances within its cells and organs – such as dissolved sugars, resins and anti-freeze proteins – which reduce the freezing temperature in cells, reduce ice crystal formation and prevent ice crystals from growing within cells. The reason this is relevant is because one of these mechanisms – the creation of anti-freeze proteins – requires a gradual acclimation to the cold. This allows the tree to turn on the genes which produce the anti-freeze proteins, and gives it enough time to accumulate them. So it is a bad idea to move a conifer from a warm/protected area directly into the freezing cold. But if that tree has been in the same place as the seasons change, it will likely have time to build its defences against freezing temperatures.

The final environmental challenge described in fascinating detail by Vogel is the wind. Leaves are adapted to deal with wind so that they can continue to photosynthesise but also not tear, or build up enough resistance to pull the tree over. They do this by bending with the wind into aerodynamic shapes which minimise drag – Vogel’s research includes a great photo illustrating how four different groups of leaves move into cone shapes as the wind increases. One key point for the bonsai enthusiast though – leaves are more resistant to the wind once they have hardened off – if they are new spring leaves they are more prone to being wind damaged.

It’s not just the leaves which contend with all these environmental challenges though, it’s all the microbes on the leaves, in its phyllosphere.

I could write pages and pages more on this fascinating aspect of tree biology but for now I am going to leave it there.

Old Trees

A premise in bonsai is that the best bonsai look like old trees. In my opinion a lot of ‘best’ bonsai look more like fantasy trees from a Studio Ghibli film, and not at all like real trees. I live near Richmond Park in London which has 1300 veteran trees, of which 320 are considered ‘ancient’ (in the third and final stages of their lives)^ref and I know that they have reverse taper, weird branching, knots, breaks and all sorts of attributes which bonsai rules would aim to avoid. So what do we know about old trees and how they actually look?

The Woodland Trust Ancient Tree Inventory^ref identifies the following general characteristics for ancient trees:

Crown that is reduced in size and height
Large girth in comparison to other trees of the same species
Hollow trunk which may have one or more openings to the outside
Stag-headed appearance (dead branches in the crown)
Fruit bodies of heart-rot fungi growing on the trunk
Cavities on trunk and branches, running sap or pools of water forming in hollows
Rougher or more creviced bark
An ‘old’ look with lots of character
Aerial roots growing down into the decaying trunk

For species-specific attributes check out the Ancient Tree Inventory website^ref which outlines specific attributes for eleven of the most common UK species (Oak, Ash, Beech, Yew, Sweet Chestnut, Alder, Hornbeam, Scot’s Pine, Hawthorn, Field Maple & Lime).

Another study^refoutlines some of the expected characteristics of ancient and veteran trees as “a hollowing trunk, holes and cavities, deadwood in the canopy, bark loss and the presence of fungi, invertebrates and other saproxylic organisms.” And citizen submitted recordings of tree measurements across the UK in the same study showed that “ancient trees have larger girths in general than veterans, which in turn are larger than notable trees.”

Trouet in her book Tree Story says that in old trees the top of the tree has ‘caught up’ with the bottom, so the trunk becomes more columnar, whereas a middle-aged tree is more tapered. She also says that branches thicken. This perhaps goes against the bonsai edict of taper above all else.

Another study reports that mature trees have only short shoots – these have smaller leaves and more foliage per shoot that more immature long shoots. Read more in shoots.

So to create a bonsai which looks old, you want it to have a very wide, columnar but hollowing trunk, rough or deeply grooved bark, holes, cavities, dead & broken branches, a compact canopy with deadwood, small leaves on short shoots, and ideally some fungi and a busy community of invertebrates.

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 study^ref 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:

You might cut away too much stored food which the plant might need to grow
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
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)!

Nutrients for Trees

Before we dive into this subject, it’s important to know there are two aspects of ‘feeding’ trees and the one we are going to cover in this article relates to the nutrients that plants need in order to generate new cells and growth.

The other aspect relates to the nutrients which can contribute to a better growing environment for the plant – for example, increasing mycorrhizal fungi, reducing pathogens, and improving the community of bacteria interacting with the roots of the plant. You can read about those in Non-nutrient Additives.

In general true nutrients – as they are referred to in the literature – are elements – ie. they are not able to be decomposed into smaller components, and you will find them all listed on the periodic table of elements. There are 17 elements which are recognised as plant nutrients, and from these trees synthesise their own biomass as well as everything that’s needed to make it (like secondary metabolites, enzymes, carbohydrates, plants produce literally thousands of compounds).

Three of the required elements are derived from air and water (oxygen, carbon and hydrogen). Of the remaining 14, there are six macronutrients (present in higher volumes) and eight micronutrients (required in smaller amounts).

This is important for bonsai because trees usually obtain all of their nutrients from air, water or soil. Since bonsai are not planted in soil, they are vulnerable to nutrient deficiencies. To learn more about how trees absorb nutrients, check out how roots absorb water & nutrients. For a description of each nutrient, why it’s needed and how it’s obtained by plants when not in a pot, please read the post on what each nutrient does. If you don’t want the detail, below is the (relatively) short version…

There are two ways in which nutrients are used by a plant and these relate to what the tree is, and what it does.

What are trees made of?

The main structural components of trees are lignin and carbohydrates. Lignin is a polymer which combines three types of alcohol (p-coumaryl, coniferyl & sinapyl) in different ways, which in turn combines with cellulose (a carbohydrate) to form wood. Lignin makes up 25–35% in gymnosperms^ref and 20–25% in angiosperms, cellulose makes up most of the rest, with 4-10% of other components. Whole text books have been written just on the topic of lignin and it isn’t fully understood as a substance – it could be considered the ‘secret sauce’ to tree success. Feel free to spend $183^ref to learn more…or buy another nice tree instead!

Both lignin and cellulose are made up of carbon, hydrogen and oxygen, all of which is obtained from air & water through photosynthesis.

What do trees need to function?

A tree can’t become a tree with just carbon, oxygen and hydrogen, even though those elements make up most of its structure. It needs chemical reactions to take place throughout its life to create the cellulose and lignin, and to manage all of the processes needed to maintain life. This is why it needs other elements in addition to C, O & H.

Key reactions within a tree include photosynthesis, nitrogen capture, cell division and defence against pathogens – but there are millions of chemical reactions going on inside a tree at any given time. Many of these depend on enzymes, a type of protein which acts as a catalyst – that is, it enables something to happen without being consumed by the reaction itself.

As a protein, enzymes are made up of amino acids, which all contain carbon, hydrogen, oxygen, nitrogen and (sometimes) sulphur. This is where the first two nutrients come in – nitrogen is needed in every enzyme, and sulphur is needed in some.

Enzymes are really interesting because they don’t just make reactions happen, they also speed them up in really clever ways (to learn more see Jim Al-Khalili’s book Life on the Edge: The Coming of Age of Quantum Biology^ref). To do this, they use other elements, including metals like potassium, iron, copper, manganese, magnesium, nickel & zinc. Boron, chlorine and molybdenum are non-metal enzyme co-factors (elements which enable enzymes to function).

The remaining two nutrients are calcium and phosphorus. Calcium is used by plants in a similar way to humans, as calcium pectate it acts as a skeleton, strengthening cell walls. It also acts as a chemical messenger as part of processes related to root and bud growth and responding to stress. Phosphorus has multiple roles, as a component of the molecule ATP (adenosine triphosphate) which is used by all living things to store and transfer energy, as the structural framework for DNA & RNA and as part of the carbon fixation process.

How to obtain nutrients for trees?

So we know that plants need significant levels of nitrogen, phosphorus, sulphur, potassium, calcium and magnesium and they also need smaller amounts of boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc. According to Hallé in his absolutely brilliant book ‘In Praise of Plants’, Nitrogen is a key limiting nutrient for plants. He says “Although they easily assimilate carbon, nitrogen remains a constant problem for plants. It is the reason that they are poor in proteins and rich in carbohydrates. The situation is the opposite in animals…”

Standard non-organic fertiliser does not contain all of these nutrients, so you need to make sure they are added somehow – either via an organic fertiliser or a combination of additives like liquid seaweed, compost or fermented/decomposed manure. Take care you understand what the definition of an organic fertiliser actually is. Unfortunately most fertilisers do not reveal their composition on their packaging, so look for one which does. My personal investment in fertiliser is my purchase of a Hotbin hot composting bin – this creates organic compost in 3 months and produces leachate which can be used as a liquid fertiliser. Also consider your carbon footprint. As mentioned in the what each nutrient does post, Ammonia production for chemical fertilizers is a major contributor to global warming as it uses fossil fuels as the main ingredient, as well as massive amounts of energy to produce, and contributes to ecosystem damage through nutrient runoff. Using compost with some manure is a more environmentally friendly way to provide nitrogen to your trees.

Consider your watering as well. Trees need *some* chlorine and grow better if they have a bit more than merely what they need – tap water can provide this nutrient if you use it for watering. You should check your local water company report as you may be able to obtain other nutrients from your water as well, including magnesium.

Whilst there are many research studies identifying the effects of nutrient deficiencies, there aren’t many with evidence for nutrient toxicity so it’s hard to work out if this is a myth or reality. Living things tend to have a system of homeostasis to manage levels of chemicals to avoid them getting too high (or low depending on the substance) so it may be that toxicity is rare due to homeostatic mechanisms removing excess nutrients.

Some bonsai practitioners are fans of foliar feeding. This can be useful in certain circumstances, but the better approach is to add nutrients to the soil.

How much is enough?

This is where the evidence starts to get very thin on the ground. Nutrient requirements vary between plant types and their ability to obtain nutrients is dynamic – it depends on the presence of other nutrients, temperature, pH, energy availability, the size of the plant – there are a lot of variables. Probably the best approach is to follow the guidance on the fertiliser you are using.

Reabsorption of Nutrients

A word of warning to those who like to remove those ‘messy’ dying leaves on deciduous trees at the end of the season. Towards the end of the growing season, once the tree has made enough wood and grown enough leaves, and, from the tree’s perspective, done everything it can to reproduce, it stops focusing on growth and goes into an orchestrated shutdown phase.

During this phase substances from the leaves are reabsorbed into the tree, to be stored in the rays and roots for use again next year – enzymes are again deployed to effect this absorption. This is why leaves change colour as different substances are reabsorbed. What is left in the leaf when it finally drops is actually quite low in nutrients. So let the tree drop its leaves as it wants, to optimise its health for next year.