Category Archives: Keeping Bonsai Healthy

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.

Repairing damage (or not)

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

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

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

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

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

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

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

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

The Microbiome and Symbiotic Microbes

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

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

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

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

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

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

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

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

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

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

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

Repotting Tips

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

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

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

Bonsai enthusiasts repot to avoid their trees becoming pot-bound – ie. the roots filling the pot. Why? There aren’t many research papers on this subject but luckily the eminent Australian research organisation CSIRO performed one 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)!

Bonsai growing medium

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 nutrients^ref. 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

https://royalsociety.org/-/media/policy/projects/soil-structures/soil-structure-evidence-synthesis-report.pdf

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.

Watering bonsai trees

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 study^ref 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 requirements^ref

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 geographies^ref – “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.

The water system of a tree

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).

Artificial light for bonsai

It is possible to keep a bonsai alive with artificial light such as a lamp as long as the lamp emits the spectrums of light needed by plants to grow. This starts with their need to perform photosynthesis. Chloroplasts are the organelles within plant cells which are responsible for photosynthesis, and they do this using photosynthetic pigments – substances which absorb photons of light.

You may be surprised to learn that there are actually multiple photosynthetic pigments in chloroplasts and that land plants always have two forms of chlorophyll – chlorophyll a (C₅₅H₇₂MgN₄O₅) and chlorophyll b (C₅₅H₇₀MgN₄O₆)^ref, and six carotenoids – neoxanthin, lutein, β-carotene, violaxanthin, antheraxanthin and zea. The role of chlorophylls are to be “light collectors, whereas carotenoids, apart from participating in light harvesting, are also involved in photoprotection”.^ref Different levels of these pigments are contained in angiosperms vs gymnosperms – to see the differences you can review the data in this article (figure 4) but in summary gymnosperms have more chlorophyll (a + b) and deciduous angiosperms have the least chlorophyll b. The different chlorophylls and carotenoids absorb different spectrums of light as shown in the chart at the bottom of this post.

But it’s not just the process of photosynthesis which uses light in plants. Actually many of their growth responses depend on light – including seed germination, flowering, leaf senescence, stomatal and chloroplast development, cell differentiation and root growth. Plants also have a circadian clock which controls when processes happen – such as stomatal closing & opening, flower closing & opening, leaf position changes and so on. The circadian clock uses light sensors to keep the clock in time with the world.

Plants use phototropins, phytochromes, cryptochromes and UV detectors to perform different light sensing tasks and to trigger signals throughout the plant.

Phytochromes are red and far-red light photoreceptors, whose role in plants is to detect favourable conditions for growth and to signal within the plant accordingly – five have been identified, known as phytochromes A to E.^ref They function not just as light detectors but as temperature sensors since they are able to detect infrared light which is associated with heat.^ref I was interested to read that phytochromes A, B and E stimulate lateral root production while phytochrome D inhibits it.

Phototropins are responsible for the phenomenon of phototropism whereby a plant grows towards the light – they contain blue light photoreceptors.^ref Cryptochromes are another form of blue light photoreceptor only recently discovered, they’ve been shown to inhibit elongation of the germinating stem and to form an input to the circadian clock.^ref

Several plant growth processes have been found to be initiated by UV-B light, summarised in this table but including stomatal closing and the production of compounds such as anthocyanins which protect the plant from damaging UV levels. UV-B light has been found to be detected in plants by a receptor known as UVR8^ref.

So below there is a nice diagram put together by a producer of LED grow lamps (they’ve left the UV receptor off but everything else is there). As is clear to see, there is a wide range of spectra required by the different photoreceptor apparatus in a plant. So back to the whole point of this post – can you use artificial light? Well yes but for full functioning of your tree it should provide the spectra needed – which is basically most of the range of light from UV to infrared excluding green and yellow. The LED grow lamp seller has an article on finding lights which meet the requirements below (I do not know them, I just think their website makes sense).

https://www.ledgrowlightshq.co.uk/chlorophyll-plant-pigments/

The other factor to consider is that leaves are adapted to respond to the parallel rays of light from the sun. Their internal cell structure, for example, is organised to encourage the scattering of light to the spongy mesophyll cells which are underneath the palisade cells.^ref In general a light bulb or LED light is not going to produce parallel rays of light so the photosynthetic efficiency of the light will be reduced (since less light will make it into the spongy mesophyll cells) and more power will be needed from the light.^ref

Personally I’ve never used artificial light for a bonsai – the one indoor bonsai I have (a Ficus retusa) lives quite happily on a table near a window in my kitchen where it gets natural light throughout the day and copes with the large variations in photoperiod experienced in the UK. But theoretically it should be possible to use a light instead, assuming you have one which meets the requirements outlined in this post in terms of spectrum and intensity.

If you end up with a light which doesn’t reflect the spectra needed, this may impact your tree’s growth. For example “low blue light from warm white LEDs increased stem elongation and leaf expansion, whereas the high blue light from cool white LEDs resulted in more compact plants”^ref – you can end up with ‘leggy’ plants if you don’t have enough light.

What each nutrient does (x17)

Understanding the exact role of individual plant nutrients is actually quite complicated. This is because many nutrients have multiple roles, and research hasn’t always clearly identified each role. It’s also because nutrients don’t act on their own – they usually form part of a larger molecule, such as an enzyme, and they usually interact with other nutrients, molecules and environmental factors such as pH. Then it requires diving deep to the molecular level to work out how chemical reactions take place and even how individual electrons move in the presence of the enzyme. I’ve read a bunch of papers and articles about the nutrients below but there is sure to be more out there – consider this a taster.

Macronutrients

Macronutrients are those inputs which plants require in substantial volumes in order to grow and metabolise. There are six macronutrients, of which three are metals.

1. Nitrogen: nitrogen is needed by plants for various reasons but one of the most important is for photosynthesis. Nitrogen is a component of chlorophyll, the green pigment in leaves which absorbs the energy from sunlight to break apart water in the photosynthesis reaction. The chemical formula of chlorophyll is C₅₅H₇₂O₅N₄Mg – as you can see both nitrogen and magnesium are needed in addition to carbon, hydrogen and oxygen which is obtained from air and water.

Secondly, nitrogen is a key component of amino acids, which are used in all living things to make proteins. There are 21 amino acids in plants which are connected together in a myriad of different sequences and shapes to make the different proteins. Examples of plant proteins include the storage material in seeds and tubers, plant cell membranes and the enzyme known as RuBisCO. RuBisCO is present in leaf cells alongside chlorophyll, and enables carbon fixation during photosynthesis – it is said to be the most abundant protein on Earth and represents 20-30% of total leaf nitrogen^ref. So photosynthesis requires chlorophyll and RuBisCO, both of which rely on nitrogen as a component element.

Finally nitrogen is present in the four bases used to create DNA (adenine, thymine, guanine & cytosine) and RNA (adenine, urasil, guanine & cytosine) which enables cell replication^ref.

Nitrogen needs to be in soluble form to be used by plants, either as nitrate or ammonium, which requires the presence of specific bacteria either in the soil or in the plant’s roots. As I mentioned in my first post about nutrients, Hallé states that obtaining nitrogen is a constant challenge for plants. This is why the fertiliser industry is so prosperous – but unfortunately also so damaging for the environment. The Haber-Bosch process used to make Ammonia (NH₃) is “one of the largest global energy consumers and greenhouse gas emitters, responsible for 1.2% of the global anthropogenic CO₂ emissions”^ref and nitrate runoff from highly fertilised fields damages rivers and ecosystems.^ref The natural world has some clever ways to make nitrogen accessible – some plants incorporate bacteria into their roots which can do this, and in old growth Douglas fir forests, lichen in the canopy which contain cyanobacteria capture nitrogen from the atmosphere and when they fall to the forest floor they rot into components which trees can access (Preston, ‘The Wild Trees’). Similarly a relatively sustainable source of nitrogen for plants is rotting organic matter (along with the necessary microbes) – compost and manure are good options.

2. Phosphorus is needed in plants – and in fact in most living things – because it’s a key component of the molecule used to store, transport and release energy in cells. That molecule is called adenosine triphosphate (“ATP”) and it’s created during photosynthesis by adding a phosphorus atom to adenosine diphosphate. The ATP can then be transported throughout the organism to be used where energy is required, at which point it’s converted to ADP (adenosine diphosphate) which releases energy back to the cell. Phosphorus also provides the structural support for DNA and RNA molecules, needed for cell replication.

For plants phosphorus is fundamental to photosynthesis, as a phosphorus-containing substance called RuBP (C₅H₁₂O₁₁P₂) is used to regulate the action of RuBisCO in fixing carbon from carbon dioxide.

Phosphorus is a key part of most fertilizers and manure is a good source^ref.

3. Sulphur is a constituent of two amino acids – methionine and cysteine. Cysteine is used to create methionine as well as glutathione, an anti-oxidant which helps plants defend themselves against environmental stress^ref. Methionine is involved in cell metabolism and the majority of its use in the cell is to synthesise ‘AdoMet’ which is used in methylation reactions (such as DNA methylation which regulates gene expression) and to create the plant growth regulator ethylene^ref.

Manure is a source of sulphur for plants.

Metals

In addition to the top three outlined above, it turns out that plants need some atoms of various metals as well. In order to understand why, we need to know that plant cells are full of chemical reactions which perform all the different functions required for metabolism. Helping these chemical reactions along are a type of protein known as enzymes. Enzymes act as a catalyst to chemical reactions in cells, speeding them up without consuming the enzyme material itself. Some enzymes have metal ions at their heart which assist the reaction (called ‘co-factors’) and these play a critical role in the enzyme’s function (for example magnesium is used in RuBisCO).

Three of the six macronutrients needed by plants are metals – potassium, calcium and magnesium.

4. Potassium is one of the three major macronutrients usually found in NPK garden fertilisers (K is the chemical symbol for potassium), reflecting its important role across several different dimensions of plant growth. One important role is in the production of proteins. As outlined above, proteins make up a large part of plant biomass, and the key protein RuBisCO is necessary for photosynthesis. Proteins are manufactured on structures called ribosomes within the cell – and ribosomes need potassium in order to do their job^ref.

The other important role for potassium is in providing the pressure within plant cells which keep them stiff and ‘turgid’ – without it they would become flaccid and the plant would fall over. The manipulation of turgidity within leaves is one way a plant controls the level of air coming into the leaf – to close off the air holes (stomata) the guard cells around the stomata are made more turgid which closes the gap between them. Potassium is involved in this process.

In nature the main source of potassium is the weathering of rocks, but it’s also contained in organic matter, particularly in seaweed.

5. Magnesium is a component of the chlorophyll molecule and is key to the operation of RuBisCO, both of which enable photosynthesis. It is found in soil from the weathering of magnesium-containing minerals – the University of Minnesota recommends dolomitic limestone or tap water^ref.

6. Calcium isn’t just used for human skeletons, it’s also used for plant structure as it strengthens the cell walls in plants. Its presence (or absence) is also used for signalling of stresses to the plant, allowing it to activate defences against pathogens. Sources of calcium for gardening include lime or shells.^ref

Micronutrients

Micronutrients are required in much smaller amounts than macronutrients, although they are still required.

7. Boron appears to have been identified as a plant micronutrient mainly due to the effects of not having enough of it, with researchers having observed that boron deficiency leads to root, leaf, flower, and meristem defects. The precise role that boron plays is not easy to find in the literature, other than it being generally agreed to be important as a structural component of the plant cell wall, helping to provide rigidity^ref. Boron also appears to be required for reproduction, and a lack of it can affect pollen germination, flowering and fruiting. Because boron has poor mobility in a plant and cannot be moved from one part of the plant to another, continued exposure to it is needed in order to avoid a deficiency – but the exact levels vary widely by plant.

Sources of boron include borax (sodium borate) but since boron toxicity is apparently possible at high levels be very careful with this approach. Obtaining boron from organic matter or liquid seaweed instead may be less risky.

8. Chlorine was added to the plant micronutrient list in 1946 since it was found that chlorine molecules are required as part of the machinery of photosynthesis – particularly relating to the water-splitting system. In addition to this essential function, chlorine has also been found to be beneficial at macronutrient levels, enabling “increased fresh and dry biomass, greater leaf expansion, increased elongation of leaf and root cells”^ref.

Since most people will be using tap water for watering their bonsai, getting adequate chlorine to your trees should not be a problem, but if you are using rainwater perhaps consider tap water every now and then.

9. Copper is essential for plants because along with iron it makes up part of an enzyme called cytochrome oxidase which performs the last of a sequence of steps in respiration^ref. Cytochrome oxidase is present in the mitochondria of plant cells – these are separate organelles with their own unique DNA which are dedicated to energy production. Copper is also found in plastocyanin, a protein which is responsible for electron transfer in the thykaloid – the light-dependent part of the chloroplast^ref. This protein is key to the conversion of light energy to chemical energy in the cell during photosynthesis. Copper is found in the soil but apparently is deficient in soils with high amounts of organic matter^ref.

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

Iron can be toxic if too much is present, so plants have evolved mechanisms to remove it when it gets too high. ^ref1 ^ref2 ^ref3

11. Manganese is also a co-factor to enzymes involved in photosynthesis, as part of the ‘oxygen-evolving complex’ in photosystem II, which contains 4 manganese ions. Photosystem II “is the part of the photosynthetic apparatus that uses light energy to split water releasing oxygen, protons and electrons”^ref.

12. Molybdenum is used by certain enzymes to carry out redox reactions (reactions where electrons are gained or lost from a molecule); these include nitrate reductase, xanthine dehydrogenase, aldehyde oxidase and sulfite oxidase^ref. Its role facilitating the nitrogen pathway in a plant is important since nitrogen is the nutrient plants require the most of, and the presence (or absence) of molybdenum can make a big difference to the efficiency of nitrogen uptake which in turn affects growth rates and plant health.

13. Nickel is a co-factor to the enzyme urease which breaks down potentially toxic urea (a product of metabolism) into ammonia which can then be used as a source of nitrogen for the plant^ref.

14. Zinc is the final micronutrient alphabetically but not in terms of importance. It is associated with 10% of all proteins in eukaryotic cells including those of plants, assisting as a co-factor to many enzymes and so enabling many different biological processes including transcription, translation, photosynthesis, and the metabolism of reactive oxygen species. Zinc also plays a role in ensuring the correct folding pattern for proteins as they are created.^ref

Interestingly, zinc deficiency is associated with smaller leaves and internodes, which you might consider a benefit for bonsai, but perhaps not at the expense of your trees’ core biological processes!

Wait – I only got to 14 and there are supposed to be 17 – what the heck? Oh yeah – here they are (15) carbon and (16) oxygen from carbon dioxide, and (17) hydrogen from water (H2O). Did you know that the oxygen emitted from plant photosynthesis actually comes from the water and not from the carbon dioxide? More in Photosynthesis.

Embolisms

You may have heard of embolisms in the context of humans – this is when an artery is blocked by something like a blood clot or a bubble of air^ref. A similar process can happen with trees and their xylem (water carrying) vessels.

As outlined in trees’ water system, water enters the tree’s root cells and is pulled in a continuous stream up through the xylem by the negative pressure created by leaf transpiration in the canopy. The xylem is actually a “continuous water column that extends from the leaf to the roots”.^ref

If a bubble of air gets into the xylem, this breaks the water transport process and stops water from below the bubble being lifted any further. This stops water from reaching any parts of the tree dependent on the xylem cells which have been affected. Embolisms in trees are also known as ‘cavitation’. This process is even audible and apparently explains half of the sound heard from drying wood.^ref

Thomas explains this really well in chapter 3 of his book in the section ‘Air in the system’. He says embolisms can occur from water in the xylem being under too much tension (ie. the tree becoming too tall or too high a rate of water being transpired), from xylem damage, or from freezing.

Embolisms are relevant to bonsai enthusiasts for two main reasons. Firstly they explain why a tree may die if it isn’t watered. When in leaf (for deciduous trees) and all the time for evergreens, transpiration will occur as water evaporates from the leaves through their stomata. If transpiration happens faster than the tree can replace water through its roots, embolisms can occur. If too many embolisms occur, the tree might not have enough routes for water to reach the cells, or enough volume of water to meet their needs.

The second reason why embolisms are interesting from a bonsai perspective is that they also explain the two very different xylem structures which can be observed between trees and which have implications for their growth behaviour.

All conifers and some angiosperms called ‘diffuse porous’ trees, add to their xylem network each year, and have many smaller, narrower, interconnected xylem vessels which don’t allow large air bubbles to form. These trees have active xylem in multiple rings (Thomas says that conifers can have 30-40 years worth of rings still actively transporting water up the tree). In cross-section they have small ‘pores’ and a denser consistency.

The alternative approach is taken by ring porous trees, which regrow their xylem vessels every year, and only ever have a single ring of much larger, longer super-pipes of active xylem transporting water. These trees can grow much quicker because their water transport is more efficient in the right conditions (ie. not freezing). But they can’t break bud until they’ve constructed the new year’s xylem. This explains why some species such as oak leaf out relatively late – they have to spend time at the beginning of the season regrowing their xylem ring. An evergreen tree cannot be completely ring-porous, because it would not be able to supply water to its leaves through the winter.

Like everything in nature there are trees which are combination of the two as well. There are some nice microscope images of the xylem vessels of different types of wood here. The difference in size between coniferous species (whose xylem cells are called tracheids) and angiosperm species (whose xylem cells are called vessels) is show in the table below^ref:

As can be seen, conifers have tracheids which individually don’t get much longer than 2-3mm or wider that 10-12 μm (note that the table uses a logarithmic scale). Ring porous species on the other hand have vessels in the 1-12cm range in terms of length, and 20-80 μm wide.

The implication for this from a bonsai point of view is that even though our trees are small, for those species which are ring porous trees (mostly deciduous angiosperms) embolisms are perhaps even more of a risk. The scale at which we are working means there are only a small number of vessels available for water transport – a 20cm high tree might have single vessels all the way from root to crown. To mitigate this risk we should ensure these trees are well watered especially when it is hot or windy, and that they have the energy and nutrients to regrow their xylem every season.

I’m still trying to find a definitive source with a list of which species are ring porous and which are diffuse porous, but here’s what I’ve found so far (Ennos, Thomas, and some reference articles ref):

All conifers are diffuse porous, as they have tracheids (short & narrow) providing their water transport. Embolisms in these tracheids dissolve naturally due to their small size.
Diffuse porous angiosperms have narrower vessels – they are still vulnerable to embolism but less than ring porous trees, and have adapted mechanisms to resolve them, such as pumping water (sap) up from the roots to refill the vessels in spring (Ennos): poplars, beech, birch, maple,
Ring porous angiosperms have wide, fast flowing vessels and routinely get embolisms every winter, requiring a new set of vessels to be grown every year: oak, ash, black locust, catalpa, chestnut, hickory, mulberry,