I’ll admit, there isn’t a lot of science to this post. Bonsai itself isn’t much of a topic for research and it gets used as a name for a type of decision tree which confuses searches.
To answer this question I will borrow Colin Tudge’s definition of a tree: “‘Tree’ is not a distinct category, like ‘dog’ or ‘horse’. It is just a way of being a plant.”
Similarly, “Bonsai is not a distinct category, like ‘lichen’ or ‘flowers’. It is just a way of being a tree.” The bonsai way of being a tree involves living in a very small pot – in a space that’s much much smaller than nature ever intended – and, that’s pretty much it. Everything else is aesthetics!
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 Inventoryref 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 websiteref 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 studyrefoutlines 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.
There are a few different terms bandied about to describe buds which pop up in unexpected positions on a tree – ‘adventitious’, ‘dormant’, ‘suppressed’ ‘preventitious’ ‘proventitious’. Epicormic growth is actually just growth which forms on old-growth units of the tree – not in the current season’s growth.ref It’s great for bonsai because it helps keep the foliage condensed and well-ramified and allows you to develop branches which are closer to the trunk, to keep the profile compact.
This study proposes standardising on the terms ‘adventitious’ and ‘preventitious’. The key difference between the two types is how they develop – preventitious buds originate exogenously (due to an external trigger) and descend from a shoot apical meristem, while adventitious buds develop endogenously (due to internal triggers) from previously non-meristematic tissue.ref
A key concept here is that of the meristem which we encountered back in How trees grow. A meristem is an area in a plant containing stem cells – cells which can become any other type of cell. Trees maintain four active meristems which are continuously producing new stem cells as well as differentiated cells used to build the plant (the shoot apical meristem, the vascular cambium, the cork cambium and the root apical meristem). These allow the plant to respond to damage by growing new organs. The different types of epicormic buds arise from epicormic meristems, which are traces of meristematic tissue which are not located in the active portions of the above-ground meristems.
Preventitious buds arise from meristematic traces which are triggered to become full buds and sprout at some point in the future, for example if the tree is wounded or damaged and needs to generate more foliage for photosynthesis. In fact these preventitious meristems may come from axillary buds which aren’t activated during their growth season. Particularly in angiosperms, many more buds are generated than are activated in a growth season, and these buds may stay dormant until needed. Not only that, but mature buds approaching bud burst have tiny buds inside them as well, which become next season’s buds if a bud extends. If it doesn’t extend, there are 2 years of dormant bud tissue available for future activation.ref
Adventitious buds happen when a callus or other wound response creates meristematic tissue which connects to vascular growth (eg. the cambium) and establishes a trace similar to preventitious buds. In the future this trace can become a shoot.
This study identified four different strategies for epicormic bud development – external clustering, isolated buds, detached meristem and epicormic strands.
External clustering is where “trees produce relatively small, persistent axillary buds, which develop into epicormic complexes consisting of numerous buds and shoots”. Even though they are not visible, every year they extend with annual growth creating more meristematic tissue and/or leaf primordia (embyonic leaves), and sometimes shoots as well. These bulges on tree trunks are a familiar sight on many trees – such as this Linden tree near my house:
The majority of species which are known to be prolific producers of epicormic shoots fall into the external clustering strategy. I often see Oak trees in Richmond Park with rounded protuberances on their trunks – these are epicormic complexes under the bark.
The isolated bud strategy is the “initial production of larger external epicormic buds, mainly high buds, which are less persistent and less likely to form large clusters.” These buds are buried in the bark, or in the case of gymnosperms a meristematic ‘bud base’ is left in the bark.
The detached meristem strategy is also observed in conifers and involves “the maintenance of minimally developed meristems hidden in leaf axils” which require some trigger (like fire) to become active. These meristems are not connected to the vascular system but can connect later when they create buds. Members of the Araucariaceae family have been found to use this strategy, such as the Hoop and Wollemi pines.Ref1, Ref2
The final strategy is epicormic strands “characterized by the presence of extensive meristematic strands within the bark that are capable of producing a continuous series of ephemeral epicormic buds” – this is observed in Eucalyptus.
One key point is to understand why a tree develops epicormic buds into shoots – and the answer to this is that is a response to stress – stressors can include insect defoliation, fire, frost, wind damage, disease, drought , intense competition, low site quality, bole orientation, vascular embolisms and heavy pruning.ref The bad news is that epicormic branches have a reputation for being weaker and not very long-lived. In his book the Wild Trees Richard Preston references epicormic branches on Coast Redwoods, noting that the people who climb these trees avoid putting weight on epicormic branches since they are liable to shear off the tree.
To work out where epicormic buds might appear on a tree, go back and read Buds, as preventitious buds in particular will develop in places where buds could have formed in previous growth period.
Gymnosperms relevant for bonsai include ginkgo and the Pinales order (Araucariaceae, Cephalotaxaceae, Cupressaceae, Pinaceae, Phyllocladaceae, Podocarpaceae, Sciadopityaceae & Taxaceae – this is explained in The kingdom Plantae and where trees fit in). Ginkgo is a special case described separately at the end of this post.
So what we’re interested in in bonsai is where lateral buds appear, and in particular whether they can develop adventitiously (or backbud). Angiosperms (flowering plants) are relatively easy to understand in terms of their lateral budding, as many species reliably produce a bud in each leaf axil (the axil is the place on the stem where the leaf is/was connected). In gymnosperms though, this is not as predictable and it’s not the case that each needle contains a bud – at least not in every species and not detectably. And looking at the different foliage forms below, you can see that different bud types must be involved to generate all these different leaf models.
Many conifers have a terminal bud at the end of each long shoot/branch surrounded by a number of close lateral buds in what’s called a ‘whorl’. These include pines, spruce, fir, and the Auracaria family. The whorl in the picture is a Scot’s Pine, with a vegetative bud in the middle and reproductive buds around it. This will usually be the apical or strongest bud, receiving the majority of the sugars from photosynthesis.
When the vegetative bud extends, it is called a ‘candle’ because it is a long thin structure – which looks like a candle. Below you can see a Pinus Thunbergii (Japanese Black Pine). Some candles are extending and some have extended and formed cones from the lateral buds around the main bud. No branching will occur from reproductive buds as they terminate the shoot.
https://www.conifers.org/pi/pi/t/thunbergii02.jpg
Bonsai enthusiasts commonly prune the candles to maintain a short needle length, this has the effect of arresting the needle growth; it is also possible to completely remove the candle, to force bud break at the base of the candle which results in smaller and more buds. In pines there are usually short shoot buds at the base of the candle – these will produce needle clusters in the future but no stem elongation. Breaking or pruning the top of the candle will activate these buds, which is good for ramification. If you want to continue developing the structure of the tree, you need a long shoot with a terminal vegetative bud as this won’t fall off.
As well as the terminal buds, pines sometimes have buds on their lateral shoots, between the needles, as well as internodal buds, which appear along the stem and not just at the end. These usually appear at the axil of the individual leaves on a long shoot/stem (Dörken, 2012).
Other conifers such as those in the Cupressaceae family (Thuja, Juniperus, Cypress) do not have whorls or needles, they have scale-type leaves in ‘branchlets’ (and needle-like leaves when juvenile). You can see below some examples of these which show the lateral buds forming from inside the lateral leaves (the leaves on the sides of the shoots). Since these branchlets squeeze a lot more leaves in, they have more potential for budding than do individually-leaved species such as Abies (fir) and Picea (spruce).
However one key attribute of species in Cupressaceae like these scale-leaved ones above is that just like pines they do still have differentiated short shoots and long shoots (Dörken, 2012). The short shoots are the individual branchlets, which abscise as a unit after a few years (detach from the long shoot and fall off). At the base of this short shoot is another bud waiting to generate a new shoot once the branchlet falls off. So new foliage will come from the leaves on the branchlet while it is active, and then from where the branchlet was connected to the stem when the whole branchlet falls off.
Conifers with individual needles such as firs and spruce, and needle-leaved junipers, have buds at the base of each leaf, but tend to bud towards the end of the most recent growth. Last year we dug up a Christmas tree from our allotment and I pruned the ends of most of the branches because it was too wide to fit into the house. The effect of this has been to stimulate the subordinate branches to bud – but again this has only happened at the ends of the branches (see below). Something about firs & spruces keeps the active budding zone at the end of branches.
As well as understanding the budding pattern, a key question for bonsai afficionados is whether or not a particular tree will backbud. That is, will it be possible to increase ramification and foliage density by encouraging axillary or adventitious buds to form.
Gymnosperms were traditionally believed not to resprout, with research in the past finding that buds are not present in leaf axils of conifers. Despite that, there are quite a few gymnosperms species which do, including the following. Some of these “do not have distinct buds at all; they produce new growth from meristematic tissue hidden under the skin of the twig” (Thomas, 2018) – this is known as an epicormic bud. This may be a false distinction since the meristematic tissue may just be early buds which are not developed enough to be visible.
Some Abies (fir)ref including Abies nordmannianaref
Araucaria & Agatha species including including Hoop Pineref and Wollemi pine ref1, ref2
Some Pinus (pines)ref – but pines are notorious for losing their ability to bud anywhere other than on the most recent 1-2 years old stems. Brent Walston at Evergreen Gardenworks says with Pinus thunbergii that as long as there is still a living needle on a stem, if you cut the stem above it, that will force a bud at the needle axil.ref This lines up with the idea that buds in pines are present under the leaf axil of long shoot leaves.
Thujopsis dolabrata (a Japanese species similar to Thuja)
So actually there are quite a few!
Some studies have indicated that “cytokinin sprays on conifers growing in the field can increase the number of visible axillary buds“ref and as a result this study concludes that “conifer leaf axils might not be as blank or empty, at least in recently initiated shoots, as they might appear to be. Cells in the leaf axils, while not forming buds, can maintain a meristematic potential and if they lose meristematic appearance, they may be preferentially able to dedifferentiate into bud forming structures.”ref
In ginkosref, axillary buds are present in the nodes of long shoots only, and these trees can backbud – below is an example of a ginkgo at the Seattle Japanese Garden – you can see new leaves sprouting from the bark of a well-established tree (from the longest long-shoot of all – the trunk).
I’ve also spotted this tree around the corner from my house in London – it was quite tall with all the foliage at the top of the tree – when I saw it cut back so severely I was sure it would die. There were only the tiniest of shoots here are there on the trunk. But in a matter of a few weeks it grew back profusely, which makes me think it must be a Thuja of some kind – perhaps Thuja occidentalis ‘Golden Smaragd’.
Finally another lovely example of conifer resprouting are the amazing dai sugi in Japan – these are Cryptomeria japonica which are cultivated for forestry purposes. The tree is encouraged into a multi-stem form with horizontal branches, which sprout new vertical stems. These are harvested over and over, and new stems grow. In this way the same tree has been used for forestry for hundreds of years without killing the tree. The technique is explained in Jake Hobson’s book Niwaki, which also includes a brilliant photo of bonsai dai sugi, which I think look bizarre but amazing. I have several Cryptomeria japonica at my allotment in the hope of creating something similar (although realistically the ones in this image are probably air-layered).
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.
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.
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.
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.
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:
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)!
Now here’s a topic to generate some internet debate! This is really a subject that every bonsai enthusiast has an opinion about – whether akadama is worth the money, whether cat litter is a legitimate medium, whether to add organic material, there is a ton of disagreement on this subject. So how might we take a scientific approach?
Well the starting point is that the growing medium needs to enable the supply of everything that the tree via its roots requires – specifically water, oxygen (for respiration) and nutrientsref. Now, you may add nutrients via fertiliser, but the medium needs to catch those nutrients so that the roots (or symbiotic bacteria) can absorb them, similarly with water – so one important characteristic is that the medium must hold water in a form which is accessible to roots.
Another super-important attribute of the medium should be that it helps establish and nourish a thriving rhizospere. This means providing a home for beneficial bacteria and fungi, enabling the roots to come into contact and to interact with them and for the roots to generate their exudates. The medium needs to hold and release the substances which are important to these microorganisms, and it needs to allow them to breathe.
We also want to have a medium in which roots grow freely, and ramify, to better support the tree in the pot and provide more surface area for nutrient and water absorption.
Wouldn’t it be amazing if there was such a medium out there? Oh, well actually there is – soil! The world over, the nutrient, rhizosphere and root growth requirements of trees are supplied by soil. According to the Royal Society, “‘well-structured soil’ will have a continuous network of pore spaces to allow drainage of water, free movement of air and unrestricted growth of roots…typically, a ‘good’ agricultural soil is thought to consist of around 50% solids, 25% air and 25% water,”ref
They also say that “bacterial diversity is affected by soil particle size, with a higher percentage of larger sand particles (ie coarser soil) causing a significant increase in bacterial species richness” and “the ability of soil structure to hold moisture is linked to a high microbial diversity and more robust populations of soil mesofauna and macrofauna”ref
This study found that bacterial and fungal abundance was positively associated with high phosphate, high pH, a lower Carbon:Nitrogen ratio, sandiness of soil texture and soil moisture. It was negatively associated with the presence of Chromium, Zinc, silt, a high Carbon:Nitrogen ratio or clay soils.ref
So what can we conclude from all of this? In terms of structure we want the right ratios of soil/water/air (50% soil particles, 25% water, 25% air) and the soil to have a higher percentage of larger, sandy particles (not clay or silt). The question for bonsai comes down to water retention since a pot with a hole is much more draining than soil. Options for water retaining elements in bonsai medium include bark, compost, biochar, perlite or vermiculite. Clay also has high water retention but perhaps too much, as it can cause anaerobic conditions which results in nasty gases being produced by bacteria. Different components such as akadama, lava rock, pumice and so on can provide the structural part of the mix which create air spaces.
Some media have so-called pores – tiny holes which hold water which is accessible by roots. “The higher the large pore (macropore) density, the more the soil can be exploitable by plant roots… the presence of continuous macropores significantly benefits root growth.”ref An example would be biochar which has a huge surface area thanks to many tiny tubes and pores throughout its structure.
What you want to avoid in your bonsai medium is anything which is too acidic (except if you have an acid-preferring tree) as this would reduce the microbes, or anything with anti-fungal or anti-bacterial properties (such as – ahem – cat litter or diatomaceous earth). You also want to avoid (per the above) anything which reduces the roots’ access to air & water by getting overly compacted or wet, or having overly draining components which don’t hold water.
Bonsai wisdom says that adding ‘organic’ components such as compost or leaf litter is bad for various reasons – they break down and reduce drainage, they run out of nutrients too quickly, they aren’t controllable. But personally I think adding organic matter of some kind is a good thing, as it mimics the natural world, has all sorts of beneficial compounds (such as those included in some non-nutrient additives) and provides some small particle sizes as part of an overall mix.
As it happens, I finally found a bonsai-specific research study! These are extremely rare. In the Journal of American Bonsai Society this article showed the results of an experiment measuring the water retention of different bonsai soil components. See below:
Based on this, if you were using the 25% air 25% water rule of thumb, most of these would be fine as bonsai soil with just a bit of added water retention. Interesting that pine bark is actually quite similar to akadama – I have recently been wondering whether you could grow trees entirely in bark if it was the right size. Maybe it’s time to try!
Another study looked at particle size, finding that “media components that differ significantly in particle size have lower total porosity, water-holding capacity and air-filled porosity than media composed of similar particle sizes.”ref
One final word on different mediums for different trees. Obviously, different trees come from different habitats and happily grow on soils native to that habitat. I have a tiny olive in a tiny pot with extremely coarse medium that dries out easily and it’s thriving (albeit, I live in London). Angiosperms transpire more than gymnosperms so in theory need more a more moisture-retaining medium. A tree with a very high foliage ratio relative to the size of the tree will also need a lot of moisture. So think about the ‘natural’ habitat of your tree and what the soil conditions likely are, and try to adjust accordingly.
The nice thing about the scientific method is that it’s not all theory – observation and experiment is an integral part. If you start with a general medium, you can adjust it to be more water-retaining by adding compost or bark, or less by adding more akadama/pumice or increasing the particle size. See how things go and adjust when you repot.
They say that a lack of watering is the number one reason that newbies kill their bonsai trees. It is quite a surprise when you first learn about the hobby to find out that you need to water your trees *every day* and sometimes multiple times a day! It suddenly feels like more of a serious commitment than you might have been expecting. Taking a more zenlike attitude and instead learning to enjoy the time with your trees when they are being watered is just one of the delightful things you discover as you become more obsessed with bonsai.
As you’ve read elsewhere on this site, water is essential for bonsai trees. Water is essential for plants in general, including trees. It’s a key ingredient in the process of photosynthesis, along with CO2 and sunlight, it’s a component of plant cells’ protoplasm, it’s essential for the structural support of leaves and stems (water creates ‘turgor’ ie. the water pressure which helps plant cells keep their shape), and it transports nutrients and photosynthates in the xylem and phloem sap. Water is estimated to comprise over 50% of the weight of woody plants.ref
Surprisingly, the majority of water taken up by a tree (90% or more) is actually lost through transpiration (which means evaporation from the leaves)ref. This is partly a by-product of having open stomata on leaves to enable the entry of CO2, but also performs a useful function for the tree, pulling water and nutrients up from the roots by hydrostatic pressure – as the evaporating water causes a pressure differential in the xylem which pulls more water up.
What this all means is that trees need a LOT of water. They also store water for times when water is low – in this studyref they found that Cryptomeria japonica can store 91.4 ml of water per kg of mass, distributed among leaves, sapwood and elastic tissue. For the first 2 hours of transpiration when photosynthesis started in the morning, they found that the water transpired was supplied exclusively from the tree’s leaves – it wasn’t until later in the day when stored water was low that the tree started to take up water from its roots.
OK so bonsai trees are small, they will need less than a full-sized tree of the same species, but sufficient water is necessary not just for photosynthesis but to maintain turgor in the cells, to allow the stomata to open and close, to resupply the water lost through transpiration, to bring nutrients up to its cells and sugars away from leaves, to build new cells and to avoid embolisms.
Trees in nature will spread their roots out to access water sources deep in the ground, but your bonsai doesn’t have that option. Trees in pots – such as bonsai – depend on their humans for water.
Furthermore, the water requirement of your tree (and thus how much watering is needed) will depend on several factors. In general, a tree will need more water if:
It has a lot of foliage, since the level of foliage determines the level of photosynthesis *and* the level of transpiration, both of which require more water (but the latter being the largest driver)
It gets a lot of sun, since sun exposure drives increased photosynthesis and transpiration (assuming foliage is present)
The weather is hot, dry or windy – all of these increase transpiration
Its growing medium is very open, free-draining or lacking moisture retaining components (such as bark). A more open, draining medium will lose water more quickly.
Its pot is very shallow, as this means the water quickly drains out.
It’s going through a growth spurt – making fruit, flowers or seed, or pushing sap up to push out embolisms
It’s in a low-CO2 environment – conversely if you have your bonsai tree indoors where there are lots of people, it may benefit from the increased CO2 by reducing its water requirementsref
When and how should bonsai trees be watered? The unscientific answer is – whenever their owner is most likely to be available and remember to do it! Convenience is important, since missing a watering could damage the trees.
But from a scientific point of view…the latest time when watering is needed is when the tree is approaching the point of running out of water. Obviously you don’t want it to actually run out for the reasons explained above. Bonsai lore is actually well-founded in this case – look at the growing medium and check how dry it is, this gives you a good indication of whether the tree needs watering.
Trees don’t need a lot of water at night, because many/most of them close their stomata which reduces transpiration – except when they are getting ready for sunrise – this article says they open their stomata up during the night in order to get water up into the leaves to be able to photosynthesis immediately that the sun comes up: they “can calculate the time of sunrise in advance”ref This is a one-time occurrence prior to sunrise though, and a lot less than the continuous transpiration that happens during the day. According to another article trees actually do the majority of their growing overnight (that is, creating new cells), due to the increased water availability and humidity during this time (due to the lack of transpiration)ref These points have two implications for bonsai enthusiasts – 1. if you want your tree to grow, make sure it has enough water at night but 2. it’s not going to be at its highest water usage rate overnight, so this is likely not the time when it requires watering.
At night, there is also a water gain from dew, depending on location. This article shows how much net water loss happens overnight in different geographiesref – “in parts of the tropics and at high latitudes” dew is actually greater than nocturnal evaporation. But on average there is 8% net water loss on land overnight.
The point at which a bonsai tree is going to start running out of water will depend on all the criteria above – foliage mass, pot size, growing medium, dryness, heat and its stage of growth. In most cases this will happen at some point during the day, after the tree has been transpiring. Depending on these factors, it may require a top-up again during the day. So maybe mid-morning to noon is a good time, with a possible follow-up water later in the day if it’s excessively hot or dry.
Most bonsai enthusiasts dream of the perfect automatic watering system. Unfortunately this is quite hard to find, for a number of reasons.
Firstly, the amount of water needed for each tree varies based on pot size/growing medium/transpiration rate. The only way to achieve this is to have individually controlled watering devices for each tree. Secondly, you ideally want to avoid wasting water by watering outside the pot or when it’s not needed – this again requires individual control for each tree, plus a spray pattern which covers just the pot area and nothing else.
The final issue is that there is risk associated with relying on an automated system. This summer when I went on holidays I set up timed sprinklers and grouped my trees together for a twice daily watering. This worked great – until one of the hose connectors popped off the tap. I had quite a few losses but on reflection probably could have avoided these by setting up two independent systems. An enthusiast from Twickenham Bonsai Club which I attend has used mini soaker hose and a garden irrigation system for his holidays which he says has worked well – but it doesn’t look good enough for continual use due to soaker hose being coiled on top of every pot.
The compromise most bonsai nuts end up with is hand-watering the majority of the time and a sprinkler or similar system while they are away.
Can you use water sources other than the tap? Find out in this post.
One of the first topics you come across when starting to study trees is the question of how they manage to lift water all the way to the leaves at the top of the canopy.
Different organs play their part in this system, starting with the roots where water is absorbed into the xylem. Xylem is a network of interconnected cells, which die quickly after birth, so that the cell contents is eliminated leaving a large space for water to enter. New xylem is constantly being created in the roots, trunk, branches and leaves, and this is all connected so that water can pass from one to the other.
But what causes it to rise up towards the leaves? The phenomenon is well described in pretty much any tree biology book you care to pick up (see references page). The answer (as is beautifully described in Ennos’s book ‘Trees’) is that it is pulled from above.
The force which pulls up the water actually starts at the leaves. Cells in leaves need gases to photosynthesise and respire (carbon dioxide and oxygen), and the waxy epidermis (outer layer) is impermeable to gas. So, leaves have small holes called stomata which are pores in the epidermis allowing gas to enter the leaf interior. These holes also allow water vapour to escape from the leaf, and as this water vapour evaporates from the leaf it pulls up the water underneath it by hydrostatic force. Water is strongly attracted to its own molecules (a force known as cohesion), and when they move upwards by evaporation it creates tension pulling more water up. This is known as the ‘cohesion-tension’ theory (Smith et al) and the process is known as transpiration. This is why trees need far more water than their size would suggest – the majority is evaporated from the leaves during transpiration.
As most bonsai enthusiasts know, when you cut a branch, water does not spurt out. So it’s obviously not being pumped from the roots. But you can make water spurt out, if you put a cut branch in a pressure vessel and apply pressure which is equal to the tension that the water was under. Experimentally this has shows stretching forces of over 20 atmospheres (294 p.s.i) (Ennos), evidence which has supported the cohesion-tension theory. There are those who disagree with this as the exclusive mechanism for water movement against gravity – one paper argues that there is an “interplay of several forces including cohesion, tension, capillarity, cell osmotic pressure gradients, xylem-phloem re-circulation, and hydrogel-bound gradients of the chemical activity of water”.ref
Whatever the nuances of the forces involved, the transpiration flow is essential for other processes within the tree – it helps maintain cell turgor (stiffness), maintains solute levels in cells which are needed for metabolism, draws nutrients, plant growth regulators and metabolites up through the tree from the roots via the xylem sap, cools leaves via evaporative cooling, and supplies water to the top of the phloem for the transportation of photosynthates (Smith et al).
