Category Archives: Shaping Bonsai

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 species^ref 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 pine^ref 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.

https://pubmed.ncbi.nlm.nih.gov/34500678/

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 polar^ref, 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.

Bonsai tool materials – carbon or stainless steel?

A fellow bonsai enthusiast at my bonsai club asked for advice from club members on their choice of tool materials, which prompted me to look into the physical differences between the materials on offer.

The bonsai tools most people have include a range of branch, root and knob cutters, pruning shears/secateurs, scissors and pruning saws. A quick surf of bonsai stores online show that these tend to be made from steel of one form or another. Steel is an alloy made from iron and carbon. Its properties are adjusted by steelmakers by changing the level of carbon in the steel, by heat treating it or by adding other materials to the alloy such as nickel or manganese.^ref Many different types of steel are made with different combinations of these factors and the manufacture of steel tools can be an artisanal enterprise, with specific alloys and processes resulting in tools with different properties.

The Japanese are known for their hard steel called tamahagane which was used for forging Samurai swords^ref1. Iron-rich sand from Shimane was smelted with charcoal in a furnace, resulting in a high carbon steel which was then folded over and over to create a very strong, sharp sword.^ref Japan still has a massive steel industry and they are 3rd largest producer in the world after India and China, they have a reputation for ‘higher quality’ or at least higher carbon steels than other countries.^ref

The properties that are important for bonsai cutting tools include (1) strength, (2) the ability to hold an edge (stay sharp), (3) resisting deformation under stress (ie. not bending), (4) longevity, (5) rust resistance and (6) price to manufacture/buy. Hardness is the term used by steel manufacturers to encompass 1-3 of the above properties. In general a harder material is stronger, less able to be deformed by stress and more resistant to abrasion.^ref Ease of sharpening is inversely associated with hardness, as resistance to deformation or abrasion means resistance to sharpening. The downside of hardness is that this type of steel may be more brittle, as it would break or snap instead of deforming under stress. Knives made from very hard steel can chip or break more easily.

So for a bonsai tool which is strong, holds its edge and stays true, you are looking for a high hardness number (in the knife industry the Rockwell scale of steel hardness is used – HRC). However the harder it gets, the more difficult it will be to sharpen.

The types of steel you might encounter when buying a bonsai tool include:

High carbon steel – higher carbon content is the main way that steel is made harder. Exactly how hard depends on how much carbon has been added. High carbon is usually 0.6% carbon content or more and a HRC rating of >60.
White steel #1 is 63+ HRC containing 1.25-1.35% carbon^ref (officially ‘high carbon’ steel) also known as Shirogami in Japan^ref
White steel #2 61-62 HRC, with 1.05-1.15% carbon^ref (officially ‘high carbon’ steel)
White steel #3 60 HRC with 0.80-0.90% carbon^ref (officially ‘high carbon’ steel)
Blue steel #1 & #2 also known as Aogami in Japan, are white steel #1 & #2 with added chrome and tungsten, and HRC 58-60 & 63-65 respectively^ref The chrome gives a stainless steel component with rust resistance.
Super blue steel is blue steel with added molybdenum and vanadium, HRC 63-64^ref
‘Powdered’ steel can be extremely hard at 64-68 HRC – a product known as ZDP-189 contains 3% carbon and 20% chromium so is extremely hard and rust resistance. But I can’t find any bonsai tools which use ZDP-189, it’s mainly used for knives.
Medium carbon steel has between 0.3-0.6% carbon^ref and has HRC <60
‘Black carbon’ steel isn’t a type of steel, it’s a brand name used by Chinese manufacturer Ryuga or a description of a colour (eg. a black coloured carbon steel tool)
Low carbon steel has 0.3% carbon or less^ref and you ideally don’t want tools made of this
Stainless steel is a steel with at least 11% chromium content, which reduces its vulnerability to rust. It’s not necessarily the case that stainless steel is less hard than carbon steel – they can both be manufactured to different hardness levels. Studies performed on steel knife blades by the Cutlery and Allied Trades Research Association (CATRA) in Sheffield, England found that at hardness 61 stainless steel had slightly superior cutting performance over carbon steel.^ref Ryuga’s stainless steel is HRC 55+-2 which qualifies as medium carbon steel.^ref

Looking at what is available out there on the internet, it’s hard to see any data on hardness or carbon content in any of the bonsai tool descriptions. Caveat emptor applies then, since it could be any old steel composition that you are buying. It could easily be low carbon steel, which will lose the edge quickly and not be very strong, and rust easily!

Given this I would personally prefer to buy a tool which either has a specified hardness rating, or is labelled with the type of steel it’s made from so you can work out its hardness. Anything labelled high carbon, white or blue steel should work well. If you’re buying a high carbon steel tool, you do need to prevent rust, so make sure it’s cleaned and oiled after use, and stored out of the damp.

If you prefer to have some rust resistance, stainless steel is a perfectly good option and again look for something with a specified hardness in the high carbon range. Tian Bonsai has a 62 level hardness stainless steel range. Kiku appears to have 62-63 hardness stainless steel in their Gold range. Kaneshin have a range of blue steel tools (mostly scissors), and UK-based Niwaki have secateurs made by Tobisho from blue steel #2 and some blue steel scissors. I’d also consider the slightly more affordable ARS – a Japanese brand with a European distributor who produce high carbon steel secateurs and scissors and are available online.

Be prepared to fork out some significant cash for your high carbon and/or stainless steel tools, they are what one might call ‘investment pieces’. But since steel production contributes 8% of global carbon dioxide emissions, buying once and using forever is probably the best strategy.

Update: if you’re wondering what you can use to clean your tools, you can check the compatibility of carbon steel with various substances here, and stainless steel here. With carbon steel avoid anything acidic, according to these charts it can be damaged by buttermilk, gelatin, malic acid (apples), citric acid (lemons), lactic acid (milk), mustard, salt water, tomato juice, water, petroleum jelly, wine and whiskey! Stainless steel is much less sensitive.

Root-Shoot Connections (aka sectional growth) – when will pruning one kill the other?

Sometimes in a bonsai context it’s said that specific branches are connected to specific roots – often in discussions about pruning and carving. For example it may be suggested that pruning a specific branch will kill an associated root, or vice versa.

As I’ve learned over the last 6 months researching this site, when it comes to trees – ‘it depends’.

The effect of pruning branches or roots on the rest of the tree comes down to its ‘plumbing’ – that is, the way in which the xylem (water) sap and the phloem (sugar) sap flow around the tree. That plumbing is laid down as new shoots and other organs develop – each new organ has a connection to a vascular bundle with xylem and phloem ‘pipes’. These pipes (in reality different types of cells which connect to each other), then connect to the vascular system in a branch, then in the trunk, then to the roots.

Trees can have what is called ‘sectorial’ growth in one or both of these systems. Phloem appears to be more sectorial than xylem – there is less of it, it only runs around the outside of the trunk in a thin layer and it has fewer connections between cells than xylem. Since roots are dependent on phloem from leaves, this would suggest that roots might be more likely to die from a branch being cut than the other way around.

Xylem is a different system, and the way the xylem cells of a particular species are structured determines how sectorial that species is – trees with more connections between their xylem cells are less so (because water has more routes it can travel to reach an organ).^ref

If you’ve read the post on xylem, you’ll know that all gymnosperms/conifers (and some angiosperms) accumulate water-conducting xylem rings over time and have many layers conducting at once. This type of wood is called diffuse porous. Some angiosperms have a different strategy – they regrow their conducting xylem every year and only use that one outer layer for water transport. These trees are called ring porous.

It may then be obvious to you that ring porous species are more sectored than diffuse porous species. This was confirmed in one study using dye injections into xylem vessels – diffuse porous Acer saccharum, Betula papyrifera, and Liriodendron tulipifera had dye show up in more leaves than ring porous species Castanea dentata, Fraxinus americana, and Quercus rubra.^ref This is presumably because in diffuse porous trees there are more water conducting cells and more options for water to travel – it is less likely to get cut off.

Trees which have more isolated root – leaf paths include Quercus, Fraxinus, Prunus, Ulmus, Cotoneaster, Crataegus, Sorbus, Populus, Salix, some Acer and Olive.^{ref1 ref2} If you prune their roots, there is a higher risk of removing a xylem sap flow path to certain leaves and vice versa. Interestingly, if you look at anecdotal reports of ‘summer branch drop’ where trees drop their branches for no obvious reason, the species most susceptible to it appear to be these trees – Quercus, Fraxinus, Populus, Salix and Ulmus are all known for this phenomenon.^ref This implies that a sector has died – perhaps due to embolism (air gaps in the xylem cells) – and the branch has dropped off as a result. The same could happen to your bonsai trees of this species, either by root pruning or by underwatering. Fellow bonsai enthusiasts have reported this in Salix (willow) and Morus (mulberry – also a ring porous species). The upside of this behaviour is the survival of the tree – since the death of one part of it doesn’t cause the death of the rest of it.

Trees which are more integrated include all gymnosperms/conifers and these have more uniform water distribution.^ref Therefore they should be less susceptible to losing sectors due to root pruning or uneven watering. But once you’ve reached the point where they aren’t getting enough water overall (due to overly aggressive root pruning) or energy overall (due to overly aggressive leaf pruning), the tree is more likely to die since it is less able to keep one part alive separately to the others.

Note that trees may also drop branches for ‘economic’ reasons, when they don’t get enough return on investment from that branch, but that’s a post for another day.

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.

https://scholarlypublications.universiteitleiden.nl/access/item%3A2951200/view

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.

Carving Trunks and Branches

Most bonsai enthusiasts I know love a bit of Dremel action – a great way to add interest to a tree and to make it look like an old tree is to create deadwood like jin, shari & uro, or more informal natural-looking deadwood forms. For the art and craft of this you should look elsewhere – Will Baddeley at Wildwood Bonsai runs workshops on carving (and has a good example on a Prunus mume on his website). Let’s be clear that any carving you do to a tree is creating or shaping deadwood. You can’t carve a live branch and have it stay alive, at least not the part you carved.

But what does this physically do to a tree? That very much depends on where you are creating deadwood. If the part of tree you are carving is already dead, then carving it will not affect the tree (although, you then have deadwood to manage, see the end of this post). If you are carving live wood, there are some considerations.

Firstly, go back and remind yourself how xylem and phloem work. They transport water, nutrients and dissolved substances like plant growth regulators around the tree. Leaves ‘load’ photosynthates (sugars) into the phloem and roots load water into the xylem. The point here is that movement through these vascular tissues is required in order for water, photosynthates and nutrients to travel around the tree. If you remove these tissues by carving, it will affect at least some parts of the tree.

Xylem and phloem vessels are not usually just one layer wide and they don’t flow end to end like a pipe – there is movement between adjacent vessels and different ways for xylem and phloem sap to flow if areas are damaged. But if you carve away the entire phloem layer – which most likely you will when creating deadwood – that path for phloem sap is closed. Similarly for xylem – if you carve away active xylem vessels then water can’t flow that way any more. You need to understand what the effect of this will be for branches and foliage which you want to keep. If an area of the tree has its water source shut off – it will die. If an area of root has its photosynthate source cut off the same will happen.

Whilst its not true that trees have ‘veins’ exactly since they have multiple connected cells (more like a bundle) and not just one vessel like a vein, the bonsai parlance which refers to ‘live veins’ is approximately correct. If you can imagine a vascular bundle passing between leaves, trunk and roots, you’ll be able to work out what consequences any carving will have.

Apart from anything else, carving live wood will result in a wound, you can read about how trees deal with these in Repairing (?) damage.

Once you have deadwood, what does it mean for your tree? Well, dead wood which is exposed to the environment decomposes over time, through the action of so-called sapotrophic organisms (those that feed off dead organic matter). The decomposition of deadwood worldwide is a critical component of the global ecosystem, releasing nutrients and carbon into the soil and atmosphere.^ref In the forest, fungi, bacteria, invertebrates (like beetles) and nemotodes are the organisms which decompose dead wood. Basidiomycota is the only type of fungi which is know to degrade lignin, a major component of wood^ref (the dreaded Honey Fungus or Armillaria is a member of the Basidiomycota family). Below is the mix of fungi and bacteria involved in decomposition of a European beech (Fagus sylvatica)-dominated temperate forest.^ref

The rate of decay of deadwood in the forest is determined by environmental and genetic factors. Gymnosperms (conifers) resist decomposition due to the volatile compounds in their wood.^ref Angiosperms which have distinct heartwood, including oak, take longer to decay for a similar reason – heartwood often contains substances which deter fungi and bacteria.^ref A fun fact is that plants don’t excrete like animals do. Instead they store away toxic compounds in their vacuoles (fluid-filled spaces within plant cells which occupy up to 90% of the cell volume) (Hallé). Just some of the compounds stored in vacuoles include pigments in flower petals, latex, digitalis in foxglove, resins, alkaloids such as opium and the chemicals in garlic.^ref So these compounds can have the effect of slowing down decomposition, by being extremely unpalatable to microbes.

From a bonsai point of view, you want to avoid your deadwood being colonised by sapotrophic organisms – or at least you want to slow this process down as much as possible.

One approach for keeping your deadwood fungi/bacteria load down is to apply some ‘extractives’ to the wood – extractives are volatile compounds found in heartwood and bark, which have anti-bacterial/fungal properties. There’s quite a good thesis online which identifies many extractives from a range of different trees – you could try turpentine for example, which is extracted from pine tree resin. I’d avoid putting this into the soil though, for fear of harming beneficial microbes in the rhizosphere.

One of the main accelerators of decay in young stumps is moisture content.^ref This is a another key control you have to minimise decay in deadwood on your bonsai – keep it dry – or in scientific terms reduce its ‘wettability’. This can be achieved by applying something like linseed oil. Other substances I have heard applied to deadwood are superglue (it reduces wettability by creating an impermeable layer on the wood), and wood preservatives but most of these have chemicals I wouldn’t want washing into my bonsai soil.

So in summary – before you carve, work out what you’ll be doing to the phloem and xylem flows to avoid damaging areas of the tree which you want to keep alive, and once you have deadwood, keep it dry and repellent to microorganisms. And in order to help with wound healing, carving in warmer weather when the tree is in active growth gives it the best chance of defending against pathogens which try to enter via the wound.

Adding New Branches or Roots by Grafting

Grafting is the practice of splicing one plant onto another, so that they fuse together to become one plant – the new plant is known as a ‘chimera’^ref. Most of the grafting you’ll see out in the horticultural industry is putting a different root stock together with a named variety above ground, as a form of clonal propagation of the above ground plant.

But the same principle is used in bonsai to add new branches or roots to an existing tree. According to Garner (see references) two main forms of grafting exist – approach grafting and scion grafting. Approach grafting is when two plants (or two parts from the same plant) are held together for long enough that they fuse – but neither are detached from their parent until the union is made. Scion grafting is when the stem to be added is removed from its donor plant before the union takes. Bottle grafting appears to be halfway between the two, where the scion (the stem being added) is sustained by standing it in a bottle of water until the union is made.

I am certainly not the person to be informing you about the techniques for good grafting (check out The Grafter’s Handbook by Garner for that), but I do want to look into the science behind grafting, how and why it works, and what you can do to make it more successful. First some terminology – the plant which is being grafted onto is the ‘stock’, and the piece of plant which is being grafted onto the stock is the ‘scion’.^ref

The basic idea behind grafting is that the vascular systems (the xylem & phloem) of both plants become connected – this is needed so the scion can obtain the water and nutrients it needs to survive, having been separated from its parent plant.

The first requirement for this is to have genetically compatible plants. If you are using the one plant to graft to itself, obviously this will be compatible. If you are using two plants of the same species, known as ‘homografts’, they will be compatible also. Otherwise rootstock and scion belonging to the same botanical species are nearly always compatible, rootstock and scion belonging to different species of the same genus are usually compatible, intrafamilial (within the same family) grafts are rarely compatible, and interfamilial (between different families) grafts are essentially always incompatible.^ref

To find out the genus of a particular plant, you can search on http://www.theplantlist.org/ – for example Pinus is a genus and Pinus sylvestris is a species. So in theory you should be able to graft any pinus onto another one. The Pinus genus is a member of the Pinaceae family and there are examples of intrafamilial grafting working in this family – for example grafting Cedrus atlantica scions onto Pinus strobus stock.^ref

In order to create connections between the two vascular systems, each stem needs to be cut to expose the vascular tissue, then the vascular cambiums of both plants are aligned as closely as possible and held tightly together with tape or a rubber band (or similar). Since the vascular cambium can be extremely narrow (depending on the species, but 3-10 cells if you look at the images in The Plant Stem by Schweingruber & Börner – see References for details) – it can be extremely challenging to get the positioning correct. After this the graft is ‘sealed’ to the extent possible – beeswax has traditionally been used.

Below is a diagram showing the sequence of events in a successful graft. It’s not the case that the vascular systems just line up and start working, like you’ve connected pipes together. When plant cells are wounded they die (see repairing damage), so these can’t just reconnect to another plant. Initially there is “a necrotic layer of one or two damaged cells” between the wound sites. When the two wound sites are placed together, the plant activates a process known as autophagy^ref – incidentally this is a similar process which is invoked when humans fast – it clears away and recycles dead or damaged cell material. Although their vascular systems are not connected, there is some communication between cells at the graft boundary, otherwise they would not detect each other and activate autophagy (which isn’t activated if another plant is not present at the wound site).

At the same time auxins and sugars start to accumulate at the wound site (since there is nowhere for them to go) and callus tissue starts to form – this is what ultimately joins the two plants together. Callus tissue is a mass of unorganized cells that forms in response to wounding – this can then regenerate the entire plant based on the plant growth regulators which are present.^ref The callus tissue differentiates into vascular tissues which act as a bridge between the two plants.

https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.14383

A key point with grafts is that even after the graft is completed, you will always have two genetically distinct individuals with a joining layer between them (which apparently includes transfer of DNA between the individuals, but only for a short distance)^ref. The upward supply of water and mineral nutrients as well as the downward flow of photosynthates are modified and so is the root–top interchange of hormonal signals.^ref This can result in graft failure many years afterwards, due to more long-term genetic incompatibilities. The best way to reduce this risk is use the closest genetic match as possible – the same plant (best), variety (good), species (good but not if very different varieties) or genus (OK but risky).

To optimise the chance of success of a graft there are a few factors which contribute to better outcomes (aside from compatibility), according to studies^ref:

Grafting technique – some types of graft work best with specific plants – for example in conifers terminal fissuring and lateral plating are used.
Use vigorous stock and scions
Use younger stock and scions, unless you are bud grafting, which seems to be also successful from older plants
In some species winter grafting is more successful
Temperature can affect success – depending on the species – you don’t want it too hot or cold
The graft union needs to be held together, and protected as best as possible from drying out or from pathogens. One study found that paraffin wax was effective.^ref This might be one situation when the slightly dodgy tree ‘wound sealants’ would actually be useful.

So bonsai nerds, what does it all mean? If you are considering using grafting techniques, my first piece of advice is to find someone or a book which has proper detail in it about the process. As noted I have the Grafter’s Handbook by Garner. Brent Walston also has instructions for grafting pines on his website.

Do some research about the species you are looking to graft, to find out the most successful techniques for that species.

Consider using bud or chip grafting – it’s supposed to be one of the easier forms, doesn’t disfigure your tree and allows you to use the same plant as the donor, reducing incompatibility issues.

Repositioning Branches

A major part of bonsai practice involves moving branches into more desirable positions to meet a particular vision for the look of the tree. This is done using a wide range of different tools and techniques, which are not really the focus for this website. What I want to look at it is how some of these techniques affect the tree from a physical and biological perspective.

The first and most commonly used technique is to use flexible wire which is wound around a branch and then bent into position. To learn how to do this you can look on just about every bonsai website out there. But how does this work, how far can you go, how long does it take to work, and what do you have to watch out for?

Basically the way this works is that it forces a branch into a different position, it’s as simple as that. As long as the branch doesn’t break, you leave it there until the branch ‘hardens’ in the new position, and then you take the wire off. So two questions arise – how does it harden into this new position, and how long does it take before the wire starts to cut into the bark?

The first question is interesting. Basically what you are doing by moving a branch out of position is that you are creating a new shape with the branch, and when the new layer of xylem, phloem and bark grow and the xylem lignifies, the branch will be set in that new shape.

The majority of the structural strength of a branch or trunk comes from the xylem^ref (sapwood & heartwood). Xylem cells are dead cells impregnated with lignin, a polymer produced by plants which strengthens these dead cells. Lignification happens after cell death is achieved – one study found that clearing out a new xylem cell took 96 hours^ref and that lignification continued for several hours after that.^ref

In order to set a branch in a new position the xylem needs to grow enough new layers of cells to hold the branch. The amount of new xylem needed to achieve this probably depends on the bending force exerted within the branch – the higher this is, the more xylem will be needed to hold it. So repositioning a branch which is easy to move might need less than one season of xylem growth, a branch requiring more force might need more than one.

The force that can be applied using wire wound around a branch is limited by the structure and strength of the wire and this is dissipated by the winding which distributes the force along the wire. Hence a heavier branch or a harder bend will require heavier wire or might not be possible at all using this method. An alternative is to attach a branch to something else (like the pot, or a piece of deadwood on the same tree). Wire, cable or any strong material can be used. In my experience, you can achieve a lot more force with less wire in this way, because you are using the resistance of the tree itself as a counterweight. Branch bending tools available for bonsai operate on a similar principle.

Trees create something called reaction wood to counteract strong forces – for example a branch which grows out horizontally from a trunk exerts a lot of force on the trunk.^ref To handle this force a gymnosperm creates compression wood – specially structured xylem cells at the bottom of the branch’s join to the tree. This has the effect of providing extra support to hold the branch up. Angiosperms create tension wood, which is above the join. See below for examples from the Bushman’s Friend blog. Once you remove the wire or whatever is holding your branch in place, depending on its position and weight you might expect reaction wood to form.

**Gymnosperm compression wood under the branch**
http://bushmansfriend.blogspot.com/2010/12/reaction-wood-compression-wood-and.html

**Angiosperm tension wood above the branch**
http://bushmansfriend.blogspot.com/2010/12/reaction-wood-compression-wood-and.html

Obviously when bending into a new shape, you don’t want to break the branch. In general, younger branches are easier to bend than older ones because they have less lignified xylem holding them in place. The ability for wood to bend is measured by the ‘modulus of elasticity’ – a low MOE means it doesn’t resist bending. MOE is correlated to density – the higher the density the higher the MOE^ref, so in general denser woods will be harder to bend – usually it is angiosperms which have denser wood than gymnosperms. Conifers are easier to bend on average although there are some exceptions. You can see some species below.

Now we come to the question of how to avoid wire (if used) cutting into a branch it is repositioning. There are two meristems producing secondary growth on branches – the vascular cambium which adds biomass to xylem and phloem, and the cork cambium which adds biomass to the outer bark. Note however that they are both doing this in layers underneath the outside bark layer, so the bark itself isn’t growing up over your wire, it is being pushed from underneath by new cells. The problem with this from a bonsai perspective is that any damage to the outer bark layer may be permanent (unless you have a species which sheds its outer bark).

The rate at which secondary growth happens depends on the species of tree (it’s genetic) but usually there is a growing season (based on the tree’s phenology). During this season is when the branch and bark will expand and this is when you risk getting wire marks. Two ways to avoid marks on the bark due to wire are (1) leave some slack in it so the branch can expand underneath it and (2) keep an eye on it and remove or adjust the wire when you see it getting tight. Obviously if you have a species which puts on a lot of xylem & bark every growing season it is harder to keep wire on for longer periods – this is one of the reasons why I avoid bonsai wire wherever possible. Using the attachment method described above allows you to keep a branch held in position without wire marks becoming an issue, while wire can be used for smaller branches which set quickly and can have the wire removed within one growing season.

Pruning

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

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

Minor Pruning

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

Major Pruning

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

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

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

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

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

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

Root Food Storage (or, can I root prune before bud break?)

One piece of advice often given to bonsai enthusiasts is that root pruning should be avoided until bud break – usually the advice says you should wait until the buds are just about to burst and then you can repot to your heart’s content. But is there any scientific basis to this? The rationale for the advice is the belief that trees store energy for bud burst in their roots, which translocates prior to bud burst and is used to power bud swelling and opening.

Below is a chart showing non-structural root carbohydrate levels through the year for Prunus avium – these include sucrose, glucose, fructose, sorbitol, raffinose & inositol. FB indicates when the tree was in full bloom, and H was the fruit harvest. As can be seen, the root carbohydrates don’t deplete until after bloom has happened (this species flowers before leafing out) and then builds up again after leafing, is depleted at fruiting and then builds up again. So in this case the tree has used the majority of root carbohydrates after blooming, and they were built back up again once the leaves were out.

https://journals.ashs.org/downloadpdf/journals/hortsci/25/3/article-p274.pdf

Labelling studies use radioisotopes to track where carbon has moved over a period of time. These have shown evidence that carbohydrates from roots are translocated to the first formed leaves and flowers in apple, cherry, pecan & grape.^ref This study also confirms that “In broadleaf deciduous trees, non-structural carbohydrates are depleted during winter dormancy and at the onset of spring growth, then replenished during the growing season”, however “in evergreen conifers non-structural carbohydrates accumulate in the crown in late winter and gradually decrease during the growing season”.^ref In evergreen angiosperms (Eucalyptus in this case) it was found that root carbohydrates did vary somewhat between a peak in summer and a minimum in spring, with starch being the major storage molecule – not only that, the researchers also found a lot more starch in the roots than in the lignotuber which is commonly believed to be some kind of storage organ (but apparently isn’t).^ref

So in general it is correct that trees are using their root-stored carbohydrates to flower and leaf out – although it would appear that they use these for actual leafing and not just to get to the bud stage. So theoretically it may be better if you are doing a major root prune to do this once the leaves are out (taking care not to remove so many roots that the leaves can no longer access the water they need).

Another study looked at the age of sugars in the woody and fine roots of different tree species. They found a big difference between those of ring-porous vs diffuse-porous trees – remember that ring-porous trees have a smaller and more defined ring of conducting xylem – and in some of these trees the xylem completely seizes up during the winter and a new conducting layer is grown every year. In the chart below ring-porous trees are on the left and diffuse-porous (which includes all conifers) on the right.

https://academic.oup.com/treephys/article/40/10/1355/5861906

In both types of trees, the youngest sugars are in the smallest coarse roots, suggesting these are being used as a sugar supply within a season. The sugars in the larger roots are aging with the tree, suggesting that the tree has obtained enough carbohydrates by other means (from photosynthesis or other storage tissues such stemwood) and hasn’t needed to tap the coarse root food storage.

The obvious difference between the two is that ring porous trees have younger sugars in their fine roots as well. It looks like ring-porous trees, which probably have a higher energy requirement since they need to regrow conducting xylem as well as buds & leaves, are tapping the fine roots for energy as well as the small coarse roots. Diffuse porous trees on the other hand do not appear to be using fine roots for this purpose.

But how much are roots contributing relative to other storage tissues in the tree? One study looked at a range of different trees in Harvard Forest near Harvard University in Massachusetts in the USA.^ref See below for the data showing the change in total non-structural carbohydrates throughout the year starting at January and going through to December for five species. What’s obvious is that root storage plays a different role depending on the species – and is least important in the white pine.

https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.15462

What’s also interesting is that the only gymnosperm in the study (white pine), has a different peak – in June (midsummer when the sun is highest in the northern hemisphere). The other species peak in October after a season of photosynthesising.

Why do we care about this as bonsai enthusiasts? Well, stored energy helps to power processes within the tree, so whenever we prune storage tissues such as branches, stem & roots, we are removing energy reserves. So ideally we’d prune these when stores are lowest. When this is depends on the species but the above chart would indicate that actually August is a good time to remove roots – which goes against the advice often provided. Using the same chart would suggest that April pruning is best for branches. Which maybe suggests that bud break is being driven more by branch stored carbohydrates than root stored carbohydrates.