Tag Archives: Xylem

Air layering – an excellent technique for creating new bonsai

Of all the propagation techniques available for bonsai, air layering is surely one of the best. There are so many advantages to this practice! The main one is that it can be used on mature trees with large branches, so that your bonsai ends up with a large trunk from the beginning. If you air layer at an angle to the trunk, and/or at a junction with two or more branches, you can make it even wider again. If you select the right position for your air layer, you won’t need the trunk of the eventual bonsai to grow any more than it already has, which gives you a massive head start compared to other techniques.

We had a surplus-to-requirements magenta crabapple on our allotment which I have air-layered over the last two years – in the first year I did 20 air layers and in the second I did 10. Of these around 15 have survived. Some examples are below – the largest one I did was from a main branch and has a 12cm trunk. You can also see one where I did the cut at an angle to change the orientation of the resulting tree, and make the trunk wider.

Another advantage of air layering is that roots form from callus at the edge of the air layer, which thickens the trunk right down at the base and also creates nice radial roots for good nebari (assuming you’ve properly prepared the air layer).

And there’s more! An air layer is as old as it was on the original tree. If it’s old (or large) enough to have mature foliage, flowers and fruit, it will continue to do so once separated. As far as I can see, this is by far the most accelerated technique for creating bonsai with flowers/fruit and thick trunks (as opposed to cuttings, which also retain the age of the source material, but are usually not as wide).

So what is air layering and why does it work?

An air layer involves ringbarking the tree at the place where you want to separate it into two. By wrapping the bare strip of branch in growing medium, roots are encouraged to grow at this point, and after a period of time, the branch can be sawn through and removed – the section above the air layer will have grown its own roots and can be planted out just like any normal tree. It’s called an air layer because the roots are literally being grown in the air.

Below are some images from my crab apple air layer. On the left is the ringbarked section of trunk prior to applying the growing medium. You can see the matt texture of the wood – all the living cambium has been scraped off (I use the blade from a pair of scissors). In the middle is the separated air layer with its plastic wrap on, and the pot still in place around the roots. On the right is an example after it has been separated and the plastic covering removed (an old compost bag). You can see the roots have developed nicely, circling the pot which was being used to hold the growing medium in place (sphagnum moss).

How the heck does this work?

Air layering takes advantage of the ‘plasticity’ of plant cells. The meristems within plants can become all sorts of different cells depending on the hormonal signals they receive. In the case of air layering, the passage of phloem (the liquid which flows from the leaves and carries the sugars which are the products of photosynthesis) is interrupted. This causes the hormone auxin, which is produced by stems and leaves, to accumulate at the site of the cut. And where auxin accumulates, callus material develops, and roots grow.ref

What types of trees and branches work with air layering?

The positioning of an air layer is more important than the size of branch. Personally I think if you are going to the effort of air layering you may as well choose the fattest, chunkiest branch you can. But it’s important to know that some leaves need to remain on the section of tree above the layer, to drive the air layer root growth, and some need to remain elsewhere with an unimpeded path to deliver sugars to the roots of the main tree. In the image above left you can see there is another branch on the lower left of the air layer, which can supply the roots. If one layer is being placed above another, each needs to have their own source of sugars (ie. leaves with a connection to the layer). The year I did 20 air layers on the same tree, I made sure there were enough branches to go around, so each layer as well as the roots would have an energy source.

That’s the sugar supply, but what about water? Water can still flow to all the leaves on the tree via the xylem, as the xylem layers remain in the outermost part of the trunk & branches. These are not removed when the cambium is removed, so they continue to transport water around the tree.

Now – you may have read elsewhere on my site about ring-porous and diffuse-porous trees. Ring-porous trees only use a small range of xylem cells around the outside of the trunk just below the bark – some grow a completely new layer every year before they let their leaves bud out (eg. oak and beech). I have a hunch that it may be harder to air these species as they are reliant on this narrower xylem band which might be damaged by the layering process. There is some evidence that this is the case – one study could not successfully air layer several American oak speciesref and a quick search of bonsai forums suggests similar anecdotal evidence.

It might be important with these trees to create the air layer after they have leafed out, to be sure they have xylem there for water transport before you remove the cambium. And to be extra careful when scraping off the cambium, to avoid removing the water-conducting layer as well. This won’t be relevant for conifers, which are all diffuse porous and should be air-layerable. I have successfully air-layered cypress as well as juniper and you can see both in the image below (cypress in front, juniper behind on the left):

For angiosperms, you can check whether they have diffuse or ring porous xylem on this website. From experience I can tell you that Acer japonicum and Malus air layer relatively easily.

I have found that on an older section of tree (where the cells may be less plastic and less amenable to becoming root cells), you can increase your chances of success by air layering at a junction with a younger branch. Layering at a junction results in a multi-stemmed tree, as well as larger more interesting nebari, but it also seems from the ones I have done that the presence of the younger branch encourages more vigorous roots.

How do you do a successful air layer?

The basic practice for creating an air layer is to remove a strip of bark around the trunk, with the top of the strip aligned to where you want the base of the trunk of your bonsai tree to be. The strip of bark needs to be completely removed – all the way around the tree – and the cambium layer which sits just underneath the bark needs to be scraped off (sometimes this layer is not very visible but once you start scraping, you will see it coming off). In effect this creates a ‘phloem dead zone’ by removing the cells in the tree which transport photosynthates (the sugars produced by photosynthesis). It’s important that there are no stray cambium cells left, and that the gap is wide enough that it cannot be bridged by any callus which grows.

Once this has been done, the cut at the top of the strip needs to be packed with moist growing material and sealed. Many people will use sphagnum moss, but I have also successfully used half-moss/half-soil, and half-coco coir/half-soil, usually in a plastic pot which I have cut to fit the branch. The medium needs to be quite moist, and thickly packed above, below and around the cut. It has been demonstrated that adding IBA (Indole-3-butyric acid also known as auxin – found in rooting gels) can improve root growth speed and quantity.ref

Once you have packed the cut with moist growing medium, it can be sealed in a plastic bag, or in plastic wrap (I also use a plastic pot under this). I have found it best to attempt to seal the wrap as best as possible, as this maintains the moisture within the air layer throughout the entire period. Moisture is critical for root development. Some people advocate leaving a hole for watering, but I think this just risks the layer drying out and is unnecessary extra effort. I use cable ties to secure a plastic bag around the base of the cut (on the bare trunk) and then wrap it several times around the layering medium before securing it around the top, leaving no gaps. If needed you can also tape up any loose edges with duct tape or similar.

It may be possible to do away with the growing medium altogether and to use a strip of aluminium foil instead. One study found that the reason why this exceeded the performance of moss/plastic on air layered radiata pineref was that the moss absorbed some of the auxin, taking it away from the plant and slowing down callus formation

People often ask how long an air layer will take to grow roots, but it’s very hard to answer this question. I would suggest give it a growing season – in the UK that could be creating it in March/April and separating it at the end of August or in September. If you unwrap it and the roots are not developed enough, it can be rewrapped and left for another season.

The obvious downside of using air layering is that it’s a lot more effort than taking a cutting or growing a seed, and you have to have access to good source material. Also that nobody will mind the presence of plastic bags and cable ties in the tree for the growing season! But the effort really is worth it when you consider the quality of material that can be created – here’s one of my favourites from the crab apple batch, only one year after separation:

To see a video of all the layers that succeeded, in their bonsai pots, please check out my Instagram @londonbotanica.

How to get that conifer resin off your hands and tools

When working with conifers it can get extremely sticky as these trees exude resins from cut stems as well as other organs such as seed cones and needles. We can use our understanding of the chemistry of these resins to work out the best way to dissolve them so we can clean our hands and tools (read on).

Conifer resin helps a tree resist microbial attack, particularly when it is cut, and also acts as a deterrent to herbivory.ref So you can understand why it might need to stick to the stem and cover a wound. Some of the active components of resin which defend against microbes are volatile organic compounds, or VOCs, which evaporate under normal atmospheric conditions.ref This wouldn’t be much use to a plant, so the VOCs are dissolved in non-volatile substances, and resin is this combination of both substances.

One study assessed the composition of resin from 13 species of conifers grown in Taiwan and found that the main non-volatile components were ‘diterpenoids’ – these are organic molecules in the terpene family, shown below. You can also see the volatiles they found in this table – α-Pinene was a common one across species.


To work out how to dissolve such a molecule, we need to know what kind of solvent works against it. The rule is ‘like with like’ – you need a similar molecule to dissolve a substance, specifically as it relates to the electrical charge across that molecule – or its polarity.ref

Water is a great solvent, but only for polar substances – those molecules which have a different electrical charge at one end versus the other.ref Not only are terpenes including the diterpenoids above not polarref, but we already know that water won’t dissolve resin otherwise you wouldn’t be reading this post. For similar reasons soap and water won’t work either, because resin is just too hydrophobic (resisting water).

So we need a non-polar solvent. Unfortunately many of these are nasty substances such as benzene and carbon tetrachloride, which are toxic to varying degrees. They also tend to be produced from crude oil, not exactly a sustainable approach.ref1,ref2

But another non-polar solvent turns out to be plain old vegetable oil.ref

This came to me after remembering my year 12 chemistry teacher explaining how soap works. It stuck in my head that soap is able to dissolve oil because the soap molecule has one end which is attracted to water, and another end which is attracted to oil, which it then disrupts so it can be washed away. So if you can dissolve something in oil first, then you should be able to use soap to wash it away.

And this in fact works really well! Put a decent sized drop of cheap vegetable oil on your hands (you don’t need extra virgin olive oil for this one). Rub the oil thoroughly into the resin and over your hands, and you will quickly see it start to dissolve. Step 2 is to add some hand soap, lather well and rinse. One or two rounds of this will remove even the stickiest, blackest, most persistent of conifer resins. And for tools, you can just use the oil and wipe it off versus washing with soap and water, particularly when you have carbon steel which rusts easily.

Live Veins on Bonsai – do they exist?

Most bonsai enthusiasts will have come across the term ‘live veins’ in the context of bonsai. Live veins are areas of living bark surrounded by deadwood. They are often seen on juniper bonsai, where a section of bark twists around the tree in a dramatic contrast to the white deadwood (Sierra juniper are particularly amazing). But how does this actually work and is it a ‘vein’?

The bark layer on a tree contains the phloem, which is responsible for transporting photosynthates (sugars) and other molecules around the tree – it sits just at the base of the bark next to the sapwood. As new plant organs develop, a connected line of phloem cells is created so that sugars can be transported from these organs (if they are leaves) or to them (if they are sugar consuming organs like roots).

Phloem cells in the leaves connect to phloem cells in the branch, then to phloem cells in the trunk. They are long tubular cells with the main connection point for sap flow at the end of the tube, and minor connections in the sides. Below is an vertical image of sieve tubes of Cercidiphyllum japonicum (Katsura tree) – you can see the sieve tubes in blue, and their connections at a diagonal in dark blue. The brown cells are companion cells which help the sieve tubes to function. In this example there are some connections between the sides of tubes, but most of the connections are end to end.


Phloem & sugars preferentially flow along this natural end-to-end route. One research study looked at what happens when you block a phloem path by girdling. It was observed that initially sugar flow to roots from that branch stopped, then resumed partially by finding another route (probably laterally through the sides of the phloem cells), then the tree grew new phloem and resumed sap flow.ref

So what does this mean for bonsai? Basically – live veins (or more accurately, ‘live strips’) of bark can supply sugars to roots as long as they have phloem connections to sugar producers (leaves) and to sugar consumers (roots). What is really important is that we work with the orientation of the phloem cells when creating deadwood. Cutting across the grain of the phloem would sever the sap connections and be a form of ringbarking. Instead leaving a strip which goes along the grain of the phloem will provide a leaf to root connection. The phloem tubes will always be aligned lengthwise along a branch or on the trunk – ie. heading down to the roots. The variation you might see is that some phloem & bark spirals around the trunk and some goes straight down. This should be obvious from the bark pattern.

It’s also important to ensure there is enough foliage at the top of the live vein to meet the needs of the tree (or scope to grow more foliage). It’s useful to know that sugar from a leaf is prioritised for use local to that leaf. Leaves provide sugars for the developing shoot apex nearest to them, and flowers or fruit on the same branch; so from an energy perspective, as soon as it can be, a branch is self-sustaining. Lower leaves on a branch typically are the ones exporting sugars to the roots.ref This might be useful when thinking about deadwood creation and possible options for live veins.

If you are aggressive with your live vein creation, and remove a lot of bark, it’s likely that some roots will die. One way to minimise this is to maintain a reasonable bark/phloem coverage around the base of the tree, and to start the deadwood further up.

One final word – there isn’t really any such thing as ‘finding’ a live vein. All phloem/bark is live until you create deadwood above or on it. It’s more about creating the deadwood and leaving the live vein (or ‘live strip’) behind.


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

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

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

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

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

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

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

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

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

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

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

Vascular Cambium

The cambium – or more precisely the vascular cambium – is a layer of cells underneath the outer and inner bark and outside the wood of a tree. It’s officially defined as a ‘meristem’ – that is, a region of cells capable of division and growthref. You may recall the ‘shoot apical meristem’ in the post about How Trees Grow – this is the part of the shoot which is actively dividing and creating new cells at the tip of the shoot (known as primary growth). The vascular cambium does something similar – it divides to create the vascular system – a layer of xylem cells on one side and a layer of phloem cells on the other. The vascular cambium is where part of the secondary thickening of a tree takes place, as the xylem layers become the wood of the tree, and the phloem layers become the inner bark (the outer bark has another meristerm – more here). Always in between there is a single-cell thickref layer of vascular cambium. See below for an image of the cambial zone, phloem and xylem cells.

From The Plant Stem: A Microscopic Aspect 1st ed. 2018 Edition, Kindle Edition
by Fritz H. Schweingruber & Annett Börner

Two types of cells exist in the cambium – vertically elongated ‘fusiform initials’ and horizontally oriented ‘ray cell initials’. The fusiform initials produce xylem and phloem cells and the ray cell initials produce rays (which cut across the tree connecting xylem and phloem).ref The ray cells can create gum and resin channels, which can also be activated when the cambium is wounded. You can see a resin duct cavity in the image above, as well as a ray.

Like a lot of growth in a plant, the activity of the cambium meristem involves plant growth regulatorsref. Auxin levels peak in the middle of the cambial zone, where cells are dividing, cytokinin peaks in the developing phloem cells and gibberellin peaks in the developing xylem cells. A study in Populus found that increased local biosynthesis of cytokinin led to increased trunk biomass and radial size (width).ref ‘Local’ biosynthesis in this case meant a tree which had been transgenically modified to produce more cytokinin.

The vascular cambium slows down or stops completely during winter in temperate zones, this depends on the tree’s phenology. More detailed information about the vascular cambium can be found in this book.

From a bonsai point of view the main takeaway is that the vascular cambium tissue underneath the bark is critical for your tree’s growth so avoid damaging it.

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).


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 airref. 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 belowref:

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,


The xylem and the phloem are the vascular system of a tree; they transport fluid to and from every cell in the plant, via structures that branch, divide and become very small – small enough to be able to reach every cell. But xylem and phloem don’t transport the same substance, like our blood vessels do. Instead the xylem carries water and dissolved nutrients up from the roots, and the phloem carries the products of photosynthesis (‘photosynthates’) from the leaves to other parts of the tree. This post is about xylem, I’ve also written one about phloem.

As mentioned, the function of xylem is to transport water (and nutrients) throughout the tree. Although as has been the case throughout the creation of this website, I’ve learned that plants are never as simple as they seem! Recent research has found many other substances in xylem sap as well as water and nutrients, including plant growth regulators, sugars and proteins. This studyref into poplar xylem sap found six plant growth regulators, 124 plant metabolites including salicinoids, coumarins and benzoates, and 289 proteins with major groupings including proteins related to defence, cell wall-related processes and catabolic processes (breaking down sugars).

There is evidence that xylem is used for translocating sugars from storage organs such as roots particularly when photosynthesis is not operating (eg. for deciduous species before they break bud in the early spring). For example “birch, which blooms in late winter, clearly transports hexoses in the xylem with a potential of providing nutrients to the developing tissues at rates that equal or exceed those provided through the phloem”. ref

The majority of water-conducting xylem in a tree is in fact mainly made up of dead cells. This makes sense because living cells have cell membranes, vacuoules and other organelles which aren’t needed for the transport of water – instead what’s needed is an open space which can store water and pass it to the next storage space. In fact xylem vessels do not work for water transportation until they are deadref.

Water moves through the xylem from the roots and up throughout the tree, in a continuous stream, evaporating from the leaves via the stomata. This evaporation is what pulls water up against the force of gravity. Since the process relies on continuity, if air bubbles form in the xylem this can be a problem, as explained in this post: embolisms. You can read more about this in: trees’ water system.

When new xylem cells are born, they go through a process of strengthening the cell walls with substances such as lignin, hemicellulose and pectin among others, ref, then the cell dies (often because the vacuoule bursts) and its contents are cleared out (digested by enzymes)ref. This leaves a space for water to enter and occupy. In angiosperms, which use vessels for their xylem, death occurs within a couple of days, while in gymnosperms, which have tracheids instead, this happens a lot later – for trembling aspen and Norway spruce around a month.ref

For trees – which undergo secondary thickening – there are two producers of xylem cells – the vascular cambium (located under the bark) and the procambium (located in growing tips).

The vascular cambium is a single layer of cells responsible for producing the xylem which becomes the trunk and wood of the tree. It produces new xylem cells throughout the growing season – the cells deposited at the beginning of a growing season are the ‘earlywood’ and towards the end the ‘latewood’ (and yes if you want to know, there is also ‘transitionwood’). These make up the rings in the trunk of a tree, and are what thickens the trunk year after year. You’re probably aware that these rings can be used to understand historic climate variation and for the dating of all sorts of things – covered in the excellent book by Valerie Trouet “Tree Story, The History of the World Written in Rings”.

Although technically all xylem cells are ‘dead’ in the sense they don’t contain cytoplasm or the usual organelles like a nucleus, some are more dead that others! That is, after time some xylem cells aren’t used for water transport any more either, and their main role becomes a structural one, of holding up the tree. The ‘sapwood’ is the section of xylem still actively transporting water, and the ‘heartwood’ is the section which no longer does. For example in Picea Abies (what we use in the UK for a Christmas tree), once a tree reaches a certain age, “the width of the sapwood band remains more or less constant (on average 7.8 cm for dominant and 2.0 cm for suppressed trees)”ref. In certain angiosperms which regrow their active xylem every year (known as ‘ring porous’) there is only one active ring, and the rest is heartwood. In trees with less light which are ‘suppressed’ there is less need for water transport so the sapwood is thinner.

The structure of xylem is different between gymnosperms (conifers) and angiosperms – in fact this is one of the main differences between them. Conifers have only one type of xylem – ‘tracheids’ which are “are overlapping single-celled hollow conduits, closed at both end”ref. These cells are relatively small – ranging between 4-80μm in diameter, no longer than 5mm in length. For water to reach the top of the tree, it enters a tracheid, travels to the top and passes out through a pit connected to its pit pair in an adjacent tracheid, in this way it zig-zags its way up the tree. Here’s an image of a pit from pinus contorta, the open area around the outside is where the water flows through and the central area can be used to close the tracheid off if an embolism occurs.

https://www.srs.fs.usda.gov/pubs/chap/chap_2015_domec_001.pdf (fig 2.5)

As you can probably imagine, this isn’t a super-efficient way to transport water – conifers can’t transport water as quickly as angiosperms and can’t support the high levels of transpiration that angiosperms can.

Angiosperms have two types of xylem cells – narrow fibres which are used for strength, and wide, thin-walled vessels, which are used for water transport (Ennos). The wide vessels allow for much faster water transport, which enabled the larger leaves of angiosperms (which have higher transpiration rates) to evolve.

To see the difference between conifer tracheids and angiosperm vessels, this chart is from a studyref comparing the two. Note that both axes have a logarithmic scale, so the vessels are clearly a lot (100x or more) longer than the tracheids, and also 2-8 times the diameter.

The procambium (or ‘primary cambium’) is responsible for generating the xylem in leaves, roots and shoots. The initial xylem cells created by the procambium is called protoxylem and this in turn creates metaxylem.

In roots, the metaxylem is in the centre and the protoxylem next to it, with the phloem on the outside of the root: see the image below showing red protoxylem vessels, blue metaxylem vessels, orange procambial cells and green phloem cells (diagram from this paper). You can read more about roots and how they work in roots.

So what does it all mean for bonsai? Well the basic principle is that the tree needs water, nutrients and other substances like plant growth regulators, in every living cell and the xylem gets some of these substances there. Interrupting the xylem will slow down or stop cells accessing the inputs they need for growth, and potentially cause an embolism.

Bonsai activities which affect the xylem include wiring, carving, pruning, repotting and feeding.

Wiring too tightly may destroy the xylem on branches but there are two other layers which will be destroyed first – the bark and the phloem. Carving the sapwood will kill anything above the tree which is dependent on that sapwood. Pruning roots *may* kill the branches above if none of the rest of the root ball feeds those branches.

In general ring-porous trees are most vulnerable to xylem damage from the above because they have a small amount of sapwood, and rely on fewer wide vessels for their water transport. Popular ring-porous trees in bonsai include Oak, Ash, Black Locust, Catalpa, Chestnut, Hickory, Mulberry. Conifers aren’t ring porous, so are more tolerant of xylem cuts and interference. You can see what your tree is (diffuse or ring porous) on this website.

Because they are dead cells and no longer have a cell membrane as a barrier or the ability to create metabolites which can defend the cell, xylem cells can be (and are) populated by communities of microbesref. In one study of wild and cultivated olives, they found 5 phyla, 8 classes, 17 orders, 23 families, and 31 genera of bacteria, including Methylobacterium, Sphingomonas Frigoribacterium and Hymenobacter.ref

There is actually a xylem microbiome – just like there is for roots (the rhizosphere) – it is part of the endosphere. Xylem microbes include bacteria, fungi and oomycete organisms, some of which can be beneficial and others pathogenic. On the beneficial side some species of Methylobacterium are known to assist nitrogen uptake and to produce auxins which support plant growth. Other the other side, at least ten different microbes are know to cause vascular wilt – a destructive disease which targets the xylem – detailed in this article – including Verticillium (a fungus), Ralstonia solanacaerum (a bacteria) and Pythium ultimium (an oomycete).

Thickening the Trunk

The first quality of a good bonsai is a thick trunk with movement and mature bark. So what actually contributes to the growth of a tree trunk?

Two processes are involved. The first is the creation of new sapwood. Sapwood is the living wood towards the outside of a trunk which conducts water (Ennos, 2016). Sapwood formed in spring is called ‘earlywood’ and is optimized for water & nutrient transport to help the tree with its growth spurt. Latewood is designed for structural support and carbon storage.

Water and nutrients are conducted from the roots through xylem vessels. The mechanism by which they work is explained in xylem but for the purpose of this section it’s important to understand that the reason why trees add new xylem vessels is because as it adds biomass – new branches and leaves – more water is required. So – the more biomass is added in a given growing period – the more water is needed – the more xylem vessels are added to the trunk. Xylem vessels also become non-functional for reasons explained in embolisms, so trees need to replace them as well as adding to them due to new growth. 

New sapwood (with xylem vessels) is added around the previous sapwood, encircling the tree. How much of the girth of a tree increases each year is determined by the tree’s food supply (Trouet, 2020); this is a combination of the amount of rainfall and the energy from the sun during that year.

This studyref found that “low precipitation at the start or during the growing season was found to be a significant factor limiting radial growth” for a range of urban trees in the UK. According to Trouet, “alternating wet & dry years create wide and narrow rings respectively.” So low water levels lead to small rings and high water levels lead to large ones. The earlywood creates a larger ring than the latewood, since the xylem vessels are larger in earlywood (for water transport) and smaller in latewood (for structural strength) (Ennos, 2016).

What this means for bonsai is that watering your tree well is important while developing its trunk, whilst ensuring you have a well-drained growing medium to avoid creating anoxic conditions (lacking oxygen). If your medium is well-drained and you water thoroughly throughout the tree’s growing season (but particularly during earlywood development), you’ll boost your tree’s girth by creating wide ‘good times’ sapwood rings.

The other factor mentioned is energy from the sun. Energy from the sun is used by the tree in photosynthesis, which converts energy into a form that the tree can use to respire and grow. If there is more sun, more energy is available and the tree is able to create more xylem, buds, leaves and biomass. This isn’t a straightforward linear relationship however, as photosynthesis reaches a saturation point based on a number of limiting factors (more in the post about photosynthesis).

The key point here is that reducing the ability of the tree to capture and convert energy will affect its growth. If you reduce the foliage on your tree or cut it back in spring, you reduce its biomass, it can’t generate as much energy, and doesn’t need as much water, so doesn’t add as many xylem vessels as it would have nor as wide a ring of sapwood. This reduces the trunk thickening you can achieve in a given time period. 

It’s worth noting that the roots of a tree need to be capable of delivering the amount of water that its foliage and branches require. Optimising trunk thickness requires a dense canopy of leaves and branches, matched by roots capable of delivering the amount of water that they need. This is why many bonsai enthusiasts will start a tree off in the ground or in a large pot, allowing growth to drive the trunk size until it’s at the level required.

Attempting to restrict the roots and size of the tree too early (e.g. by putting it in a bonsai pot) will restrict trunk growth by reducing the water available to the tree and reducing the energy it can create by reducing its foliage.

Like people, trees are genetically programmed to have different maximum heights and lifespans. Some trees are slow-growing (such as Yew) and some are fast (such as Eucalyptus) so to an extent the amount of trunk thickening that is possible also depends on the species of tree.

Trees grow most vigorously when they are free from environmental stressors – such as drought, extreme cold, loss of leaves due to high winds, attack by insects or animals.  A stressed tree will grow a narrow ring. BUT stress in the form of wind can foster positive qualities in a trunk. Ennos (2016) says that trees exposed to high winds without a prevailing wind direction grow shorter, with thicker trunks & roots, and adjust their wood cells to spiral around the tree creating a twisting effect. It’s not just the trunk that is affected – apparently this results in smaller leaves and shoots as well. Get your bonsai a wind tunnel!

Another way to thicken a trunk is to grow a ground-level branch, as layers of xylem will be added around this branch as well as the truck, or to have a multi-stem tree, which operates on the same principle. You want to avoid having one too much above the ground though, as it might cause the dreaded reverse taper.

I mentioned two processes involved in secondary thickening – the second process is the effect of an increasing bark layer. In most cases this will be dwarfed by sapwood increases but nevertheless biomass is added as bark via the cork cambium, another secondary meristem on trees. Some trees which retain multiple periderms (layers of cork with their meristems) can develop very thick bark which does contribute to the overall trunk girth as well.