Tag Archives: Meristems

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 trees mature – understanding development phases

It’s probably obvious that many plants, like humans, go through different developmental phases throughout their lifetimes. In plant biology, a developmental phase describes a period of time during which a stem produces a specific type or combination of organs, such as shoots & leaves (vegetative organs) or flowers & cones (reproductive organs). In fact for plants it is individual stems, not entire plants, which go through these phases and so a single plant can have stems which each are in a different phase.^ref

All trees start in a vegetative phase – the initial growth phase when the tree is establishing. Usually this means that only foliage is produced, and no cones, flowers or fruit – in fact the growing tip is not capable of producing flowers during this phase. The vegetative phase can have stages within it, for example juvenile foliage may be produced before adult foliage, however in vegetative phases, stems are programmed genetically to produce only shoots and leaves. Although we often refer to defined ‘juvenile’ and ‘adult’ foliage in trees, it is apparently a bit more complicated, with variation within the phases as well. In fact many different attributes are affected by the stem’s phase, including the size and shape of leaves (as seen in conifer needle and scale foliage), phyllotaxy (the arrangement of the leaves), plastochron (the time between leaf primordia emerging), internode length, adventitious root production, trichome distribution and cell size.^ref These are all – as you have probably worked out – genetically controlled, apparently by ‘microRNAs’^ref, specifically miR156 and miR172 as shown below.

https://journals.biologists.com/dev/article/138/19/4117/44565/The-control-of-developmental-phase-transitions-in

Examples of juvenile and adult foliage are shown below, these show stems which have undergone a vegetative phase change.

https://www.genomebc.ca/blog/comparative-transcriptomic-analysis-of-juvenile-and-adult-leaf-morphologies-in-conifers

A phase change is when a stem starts producing a different type of organ from its growing tip (meristem). For example, it starts to produce buds which will become flowers. Phases changes are usually – but not always – stable – that is they don’t tend to move in reverse order. Once a tree has reached the reproductive phase on a particular stem, it should retain that capability since the meristem has changed to the new phase. As an illustration of this is that when propagating cuttings and air layers, once they are successfully rooted, the stem will maintain the properties it had on the tree (until that stem goes through the next phase change). If it had flowers before, it will continue to flower. In some species – particularly conifers – if the stem was horizontally oriented it will continue to grow horizontally.^ref

One good example of phase differences on a tree is suckers. Suckers are shoots which emerge from the base of a tree, and as they are derived from buds which have not passed through the same growth process as the rest of the tree, they are usually juvenile vegetative shoots, even if the main branches of the tree have reached a flowering phase.

One study found a logical sequence of developmental phases based on biochemical factors which turned on certain genes.^ref They found that substances which are important for embryo development in the seed promote the initial vegetative phase. It’s then sugars – the product of photosynthesis – which contribute to an ‘adult’ vegetative phase change. So continued photosynthesis and the production of more sugars over time, promote phase change. Plant growth regulators (aka phytohormones) also play a role, with Gibberellin A3 shown to revert ivy back to juvenile foliage^ref, although the exact interplay with auxins and other substances is still not clear as of 2020.^ref A key finding from this study was that defoliation delays vegetative phase change – so don’t defoliate or prune if you’re trying to develop mature foliage!

When a stem moves into a reproductive phase, the structure of the growing tip changes so that floral organs (which become flowers) are produced instead of shoots & leaves. In woody perennials (ie. trees) which have reached the reproductive phase, stems can transition between vegetative and reproductive, allowing them to continue to grow, as well as reproduce.^ref For example, they may produce vegetative buds at the start of the new stem, reproductive buds in the middle and more vegetative buds at the end.^ref This is all regulated by genes. One study on poplar identified two genes which control this transition based on environmental conditions – illustrated in the diagram below. The gene FLOWERING LOCUS T1 drives reproductive onset – in experiments, FT1 caused vegetative meristems to transition to reproductive when it was expressed in response to winter temperatures. As a result, the organs developing inside the winter bud moved from vegetative (formed earlier) to reproductive (formed later when it was colder). This created a bud with both forms of stem waiting to emerge in spring. Its partner gene FLOWERING LOCUS T2 then took over during warmer weather and drove vegetative growth.

https://www.pnas.org/doi/pdf/10.1073/pnas.1104713108

If you are working with material which has not yet flowered, you would probably like to know how long it will be before it does, and what you can do to encourage your tree to flower. This is where horticulturalists use the concept of ‘growing degree days’. Growing degree days is a measure of the amount of heat that a plant has received over its lifetime (this would also be associated with the amount of light, which as we read above drives sugar development which in turn encourages phase change). Growing degree days (“GDD”) are used when planning crops and flowering annuals & perennials. They are calculated as follows:

GDD = t (days) x ( (T_MAX+T_MIN)/2 −T_BASE)) ; where T_MAX and T_MIN are daily maximum and minimum air temperature, and T_BASE is a known baseline temperature.

For example, I have recently been trying to grow the Australian plant Sturt’s Desert Pea (Swainsona formosa). Studies have shown that this species requires 874 GDD for 100% of axillary branches to flower and 988 GDD for 100% of main stems to flower.^ref This is an extremely high light and temperature requirement for what is effectively an annual, so I have a heat lamp (and a grow lamp) providing daytime temperatures of 28^oC and evening temperatures of 18^oC. The base temperature for the calculation is 5^oC.

So the number of days theoretically required to achieve 100% flowering on axillary stems using my setup will be a minimum of 874 / ((28 + 18)/2 – 5)) = 874 / 18 = 49 days.

You can see that if this relationship is true then global warming will shorten the flowering time of plants since plants will achieve their GDD faster. And this is what has been observed; in the UK researchers found that plants are flowering a month earlier due to climate change.^ref

For trees which have to first achieve reproductive maturity, then generate floral organs, it’s likely that both growing degree days and other environmental accumulations (such as a cold period known as vernalisation, light levels and total rainfall) are involved.^ref The key point is that these are accumulations of the factor in question, which implies that time is needed, as well as the correct conditions.

What does this all mean for bonsai?

Firstly if you are obtaining material for bonsai, consider what type of phase you want for the tree. If you want a flowering tree straight away, you need to take a cutting or air layer from a stem which has reached the reproductive phase. A sucker, or seed, will start from scratch right at the beginning of the tree’s development – and depending on the species it may never flower or fruit the entire time that you own it! As has been noted elsewhere in this blog, if you have a tree with juvenile foliage and you keep pruning it back, it may never reach an adult foliage or reproductive phase, because it may not have accumulated the amount of sugar or growing degree days to move to that phase. So when sourcing a new tree, if you want fruit or flowers you should make sure that it has produced these already.

Also, the environmental conditions which your tree is naturally used to are important for its phase transitions. Using the above example, if you put a poplar indoors where it never gets the cold temperature signal to activate FT1, it won’t create flowers or seeds. When you have a non-native tree in your collection, it’s a good idea to research its usual climate and to try to replicate it as much as possible.

Bark (Cork Cambium)

Weirdly the definition of bark seems to be variable depending on what book or article you read. As my main reference for this post I’m using Romero’s “Bark: Structure and Functional Ecology” accessible via a free account on JSTOR here.

According to Romero, bark is all the tissues on a tree outside the vascular cambium – that is everything from (and including) the phloem outwards. The inner bark is simply the phloem (both the conducting layer and the non-conducting layer). The outer bark collectively is known as the ‘ritidome’. This is where a diagram is needed! This is the best one I could find (from the University of Vigo website).

https://mmegias.webs.uvigo.es/02-english/2-organos-v/guiada_o_v_tsecundario.php

The ritidome contains another meristem within the tree – the cork cambium. The cork cambium (called the phellogen) works similarly to the vascular cambium – it has a layer of stem cells which create layers of differentiated cells. In the top diagram there is one phellogen, a pale beige line. On its left is the phelloderm – this layer is not always created depending on the species but if present it contains living cells. On its right is the phellem, or cork, this is the thickest layer and these cells become suberised/lignified (impregnated with suberin or lignin) so they become the corky bark texture we are familiar with. All of these ‘phello’ layers together are known as the periderm. A microscope image of these layers in an old Pinus sylvestris are shown in the image below:

https://link.springer.com/book/10.1007/978-3-319-73524-5

What’s interesting is that multiple periderms can develop over the life of the tree. A new periderm will develop on the inside of the old one, pushing that periderm layer to the outside. These aren’t always continuous either, and are affected by the structure of rays and growth rings within the phloem, which is why old bark has more character. Periderms can be shed, or retained, depending on the species. The pattern of a tree’s bark is genetically determined by the structure of the phellem cells which are produced, and by the location of successive periderms. Smooth bark can come from a single periderm and continuous shedding, while rough bark is created when the periderm has structural fractures or constraints – for example due to the development of rays (radial lines of cells in the phloem).

See this image of old bark from ‘The Plant Stem – A Microscopic Aspect’ by Schweingruber & Börner. It shows how the bark splits apart as the xylem and sapwood layers expands from the inside of the tree.

Bark is made up of quite different materials from the wood or foliage of the tree, with considerably more mineral compounds (such as ash). Both the inner and outer bark contain so-called ‘extractives’ (organic substances which can be dissolved in solvents, such as polyphenols, alcohols, resin acids, vitamins, alkaloids, pigments including flavonoids, terpenes, steroids and essential oils) as well as suberin, lignin and cellulose. Bark chemistry in general is poisonous and indigestible, representing a good barrier to herbivores or insects. As the inner bark is living tissue, it can produce its own metabolites as a defence mechanism, whereas outer bark is dead tissue and relies on its physical structure and the substances impregnated into its cells to repel invaders.

Bark helps trees reduce water loss, prevents pathogens entering and provides a protective layer to protect the living tissue underneath from mechanical and heat/cold damage. It provides a flexible covering for the tree which can absorb the stresses of bending and twisting, and prevent cracking of the trunk.

As bonsai enthusiasts, bark is a key part of the look of our trees and we want to encourage interesting bark with good character. Since the cells of bark are renewing from the inside, the only way to modify the appearance and texture of bark in a natural way is to manipulate the periderm – as mentioned above, this causes fractures and divergence of the growth habit of the phellem. Harry Harrington has a video showing exactly this on a young black pine – he wires the tree so that the wire interrupts the shape of the periderm and forces the phellem to grow in a twisted habit.

https://www.youtube.com/watch?v=f0fe6v7X0MQ

I’m a bit nervous by the suggestion to leave the wire in as this seems like it would then cut through the phloem and ultimately the xylem. Whilst the twisted shape should leave continuous conducting cells for both, I’d be concerned at how much water and photosynthate conducting would be reduced. If possible to remove the wire I think I would.

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 growth^ref. 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 thick^ref 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 regulators^ref. 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.

Growth Types Table

Fixed Growth (determinate) (leaves formed inside the bud before opening)	Free Growth (indeterminate) (leaves and buds continue forming throughout season)	Rhythmic Growth (a bit of both)
Ash Beech Hornbeam Oak Hickory Walnut Horse chestnut Pine Spruce Ginkgo short shoots	Elm Lime/linden Cherry Birch Poplar Willow Sweet gum/ liquidambar Alder Apple Larch Juniper Western Red Cedar Coastal Redwood Ginkgo long shoots Maple	Loblolly pine Shortleaf pine Monterey pine Caribbean pine Cocoa Rubber tree Avocado Mango Tea Lychee Citrus Olive Pinus radiata

From Thomas (2018) and RNET^R

Plant Growth Regulators (or Phytohormones)

You’ve probably heard of rooting hormone powder, or auxin, or gibberellins – these are all ‘Plant Growth Regulators’. Plant Growth Regulators used to be known as ‘phytohormones’ which means plant hormones. This has been quite a contentious topic among plant biologists.

A hormone in an animal is a chemical messenger, a substance which acts as a signalling or control molecule to cause an action to take place. “Hormones carry out their functions by evoking responses from specific organs or tissues that are adapted to react to minute quantities of them”^ref. In animals, hormones are produced at a specific site (a gland, like the pancreas), work at low concentrations, and have a predictable dose-response. That is, an increase in hormone will result in the more of the related action (eg. more insulin leads to more sugar being taken up by the liver).

There *are* substances synthesised by plants which are involved in regulating growth – plant growth regulators – but they don’t work in the same way as animal hormones. It’s said that the assumption that they did sidetracked plant researchers for decades.^ref Plant ‘hormones’ are synthesised in multiple sites in a plant (and potentially in every cell^ref), have multiple actions on different cells (they don’t act on just one organ or tissue), don’t exhibit predictable dose-response behaviour like animal hormones and are involved in significant interactions or feedback loops with each other.^ref

What this means is that it’s quite hard to unpick what they do and how they work. The roles and mechanisms of plant growth regulators are still very much current research topics, as can be seen at two of the research groups at Cambridge University’s Sainsbury lab^ref1,^ref2 Some of the early theories about them were comprehensively demolished in a seminal article by Anthony Trewavas^ref (in particular the theory of auxin-derived apical dominance which was later proven wrong as explained below).

We now know that plant growth regulators act in concert with genes and the proteins they express, and not as an independently acting substance (one of the genes involved in cytokinin synthesis is known as LONELY GUY…).

So what are the plant growth regulators, where and how are they made? There are nine main plant growth regulators you may come across in your reading:

Auxin^ref – classically called ‘the growth hormone’ and a signal for division, expansion, and differentiation throughout the plant life cycle – involved in root formation, branching, the tropic responses, fruit development, shoot meristem function, the development of cotyledons and senescence. The most common form is Indole-3-acetic acid (IAA). Auxin acts in a ‘ying-yang’ relationship with cytokinin (see below)^ref as well as with gibberellins. More about auxin below.
Cytokinins (CK)^ref1 ^ref2 – involved in cell division, shoot initiation and growth (including maintaining the stem cell niche), nutritional signaling, root proliferation, phyllotaxis, vascular bundles, leaf senescence, branching and nodulation, seed germination, nutrient uptake, and biotic and abiotic stress responses. 6-BAP or 6-Benzylaminopurine is a synthetic cytokinin which is used in micropropagation and agriculture. Coconut water (not milk) has been found to be a natural source of cytokinins.^ref
Brassinosteroids^ref – involved in a wide spectrum of physiological effects, including promotion of cell elongation and division, enhancement of tracheary element differentiation, retardation of abscission, enhancement of gravitropic-induced bending, promotion of ethylene biosynthesis, and enhancement of stress resistance.
Gibberellins (GA)^ref – involved in multiple processes including seed germination, stem elongation, leaf expansion and flower and fruit development.
Strigolactones (SL)^ref – induce hyphal branching of arbuscular mycorrhizal fungi and are shoot branching inhibitors.
Abscisic Acid (ABA)^ref – involved in the induction and maintenance of seed dormancy, stomatal closure, and response to biotic and abiotic stresses.
Jasmonates (JA)^ref – shown to be inhibitors of growth but also involved in development of flowers and defense responses against herbivores and fungal pathogens
Salicylic Acid (SA)^ref – associated with disease resistance
Ethylene^ref – multiple effects on plant development including leaf and flower senescence, fruit ripening, leaf abscission, and root hair growth.

Slightly maddeningly none of these substances do just one thing – they’re all involved throughout the plant!

So how and where they are made in a plant? This isn’t simple either. In fact local biosynthesis is thought to be critical for plants, whereby plant growth regulators are made at the site where they are needed. For example both auxin and cytokinin are synthesised in leaves *and* in roots^ref, and can be made by chloroplasts and mitochondria, organelles which occur throughout the plant.^ref Chloroplasts can make precursors to auxin, abscisic acid, jasmonates and salicylic acid.^ref

Even though plant growth regulators don’t act in a predictable dose-response way like animal hormones, they still have a role in shaping plant growth in tissues which are sensitised to respond to them. Theoretically by understanding these responses we can manipulate a tree’s growth. And this is what they do in plant tissue culture (more on that below).

You may have heard the theory of auxin-controlled ‘apical dominance’, which holds that auxin produced by leaves at the apex inhibits lateral buds. This theory was strongly criticised by Trewavas in 1981: “The only hypothesis of apical dominance which has retained some measure of support is the nutritional one. A number of plants placed under conditions of reduced nutritional status adopt a growth pattern of strict apical dominance.” His point of view was further supported by a 2014 study which found that “sugar demand, not auxin, is the initial regulator of apical dominance”.^ref The researchers found that after removal of the shoot tip, sugars were rapidly redistributed over large distances and accumulated in axillary buds within a timeframe that correlated with those buds releasing. But auxin didn’t travel fast enough to be responsible for bud release. So basically they found that apical dominance arises because the main shoot is greedy for sugars, and due to its position at the end of the vascular system it can prevent lateral buds from taking the sugars needed to release and grow.

Auxin does play some role though, and the theory is that its role related to the fact that it’s the only plant growth regulator which displays polar transport. That is – it moves from the apex to the base of the plant, via the phloem, and can travel the entire length of the plant, ending up in the roots. This gives auxin a special role related to the spatial aspects of growth, and auxin ‘maxima’ (locations where auxin accumulates) are sites where new buds, flowers or lateral roots emerge. In fact, auxin and cytokinin work in concert throughout the plant, from shoots to roots, with apparently opposite effects in each location “like yin and yang”.^ref

An excellent reference point for this subject is the world of plant tissue culturing. This is where small pieces of plant tissue are sterilised and cultured in a medium containing plant growth regulators, which cause the tissue to grow into a ‘plantlet’ (sometimes in a test tube, if the source material is small). Further steps multiply the plantlet into several plantlets, which are then encouraged to create roots, multiplied again and/or planted out as seedlings to harden off. This process is used for industrial plant cloning where large numbers are required, and in the aquarium trade to avoid contamination with snails and other microbes (see Tropica’s website).

In plant tissue culturing, plant growth regulators are used to induce the relevant growth stage, which ones work for each species in which stage is documented in the ‘protocol’ for that species. In all cases specific ratios of cytokinin:auxin (and sometimes gibberellins) lead to different developmental stages – shoot growth, lateral shoot growth and root growth.^ref1,^ref2 To give you a bonsai-oriented example, one study determined a protocol for the micropropagation of Prunus Mume^ref. They were able to multiply fresh prunus mume shoots in a petri dish using a 4:1 ratio of cytokinins to auxins, and then root them using auxin.

So – apologies for the rather long read, it is quite a complicated subject! What can we take from all this for our bonsai practice? Firstly we can stop the brain-bending trying to understand how auxin controls apical dominance because it doesn’t – access to sugars does this instead.

Also we can use the yin-yang rule – high cytokinin:auxin encourages buds & shoots, and high auxin:cytokinin encourages roots. So I’m going to start adding some auxin rooting gel into my air layers and cuttings. Cuttings have never worked for me in the past so maybe this will be the secret sauce I need. I’m also going to try some cytokinin gel to encourage lateral budding on my trees.

If you are looking for products to give this a try, make sure the product actually contains the plant growth regulator you want. For example, Clonex contain auxin, and some of the orchid budding pastes such as Keiki paste contain Kinetin (a cytokinin). Many other ‘rooting hormones’ or plant hormones products on the market have no ingredient list at all, so avoid those. You can also find these products online in shops dedicated to hydroponics, where cloning and plant tissue culturing is a technique used by practitioners, or in lab supply shops such as microscience or Phillip Harris in the UK. You can even make your own hormone gels following these instructions.

Another trick you can use is that gibberellic acid can be used to break dormancy in seeds, if you really don’t have the patience to wait for natural dormancy to break. Or give coconut water a try, this has been found to have a similar effect in a range of species. For hard coated seeds in particular, usually it’s best to search Google Scholar for a researcher who has experimented with different approaches, since what works is very species-dependent.

Epicormic Buds

There are a few different terms bandied about to describe buds which pop up in unexpected positions on a tree – ‘adventitious’, ‘dormant’, ‘suppressed’ ‘preventitious’ ‘proventitious’. Epicormic growth is actually just growth which forms on old-growth units of the tree – not in the current season’s growth.^ref It’s great for bonsai because it helps keep the foliage condensed and well-ramified and allows you to develop branches which are closer to the trunk, to keep the profile compact.

This study proposes standardising on the terms ‘adventitious’ and ‘preventitious’. The key difference between the two types is how they develop – preventitious buds originate exogenously (due to an external trigger) and descend from a shoot apical meristem, while adventitious buds develop endogenously (due to internal triggers) from previously non-meristematic tissue.^ref

A key concept here is that of the meristem which we encountered back in How trees grow. A meristem is an area in a plant containing stem cells – cells which can become any other type of cell. Trees maintain four active meristems which are continuously producing new stem cells as well as differentiated cells used to build the plant (the shoot apical meristem, the vascular cambium, the cork cambium and the root apical meristem). These allow the plant to respond to damage by growing new organs. The different types of epicormic buds arise from epicormic meristems, which are traces of meristematic tissue which are not located in the active portions of the above-ground meristems.

Preventitious buds arise from meristematic traces which are triggered to become full buds and sprout at some point in the future, for example if the tree is wounded or damaged and needs to generate more foliage for photosynthesis. In fact these preventitious meristems may come from axillary buds which aren’t activated during their growth season. Particularly in angiosperms, many more buds are generated than are activated in a growth season, and these buds may stay dormant until needed. Not only that, but mature buds approaching bud burst have tiny buds inside them as well, which become next season’s buds if a bud extends. If it doesn’t extend, there are 2 years of dormant bud tissue available for future activation.^ref

Adventitious buds happen when a callus or other wound response creates meristematic tissue which connects to vascular growth (eg. the cambium) and establishes a trace similar to preventitious buds. In the future this trace can become a shoot.

This study identified four different strategies for epicormic bud development – external clustering, isolated buds, detached meristem and epicormic strands.

External clustering is where “trees produce relatively small, persistent axillary buds, which develop into epicormic complexes consisting of numerous buds and shoots”. Even though they are not visible, every year they extend with annual growth creating more meristematic tissue and/or leaf primordia (embyonic leaves), and sometimes shoots as well. These bulges on tree trunks are a familiar sight on many trees – such as this Linden tree near my house:

The majority of species which are known to be prolific producers of epicormic shoots fall into the external clustering strategy. I often see Oak trees in Richmond Park with rounded protuberances on their trunks – these are epicormic complexes under the bark.

The isolated bud strategy is the “initial production of larger external epicormic buds, mainly high buds, which are less persistent and less likely to form large clusters.” These buds are buried in the bark, or in the case of gymnosperms a meristematic ‘bud base’ is left in the bark.

The detached meristem strategy is also observed in conifers and involves “the maintenance of minimally developed meristems hidden in leaf axils” which require some trigger (like fire) to become active. These meristems are not connected to the vascular system but can connect later when they create buds. Members of the Araucariaceae family have been found to use this strategy, such as the Hoop and Wollemi pines.^{Ref1, Ref2}

The final strategy is epicormic strands “characterized by the presence of extensive meristematic strands within the bark that are capable of producing a continuous series of ephemeral epicormic buds” – this is observed in Eucalyptus.

One key point is to understand why a tree develops epicormic buds into shoots – and the answer to this is that is a response to stress – stressors can include insect defoliation, fire, frost, wind damage, disease, drought , intense competition, low site quality, bole orientation, vascular embolisms and heavy pruning.^ref The bad news is that epicormic branches have a reputation for being weaker and not very long-lived. In his book the Wild Trees Richard Preston references epicormic branches on Coast Redwoods, noting that the people who climb these trees avoid putting weight on epicormic branches since they are liable to shear off the tree.

To work out where epicormic buds might appear on a tree, go back and read Buds, as preventitious buds in particular will develop in places where buds could have formed in previous growth period.

Gymnosperm (Conifer) Budding

Gymnosperms relevant for bonsai include ginkgo and the Pinales order (Araucariaceae, Cephalotaxaceae, Cupressaceae, Pinaceae, Phyllocladaceae, Podocarpaceae, Sciadopityaceae & Taxaceae – this is explained in The kingdom Plantae and where trees fit in). Ginkgo is a special case described separately at the end of this post.

So what we’re interested in in bonsai is where lateral buds appear, and in particular whether they can develop adventitiously (or backbud). Angiosperms (flowering plants) are relatively easy to understand in terms of their lateral budding, as many species reliably produce a bud in each leaf axil (the axil is the place on the stem where the leaf is/was connected). In gymnosperms though, this is not as predictable and it’s not the case that each needle contains a bud – at least not in every species and not detectably. And looking at the different foliage forms below, you can see that different bud types must be involved to generate all these different leaf models.

https://cmg.extension.colostate.edu/Gardennotes/134.pdf

Many conifers have a terminal bud at the end of each long shoot/branch surrounded by a number of close lateral buds in what’s called a ‘whorl’. These include pines, spruce, fir, and the Auracaria family. The whorl in the picture is a Scot’s Pine, with a vegetative bud in the middle and reproductive buds around it. This will usually be the apical or strongest bud, receiving the majority of the sugars from photosynthesis.

https://joshfecteau.com/meet-the-pines-scotch-pine/#jp-carousel-8472

When the vegetative bud extends, it is called a ‘candle’ because it is a long thin structure – which looks like a candle. Below you can see a Pinus Thunbergii (Japanese Black Pine). Some candles are extending and some have extended and formed cones from the lateral buds around the main bud. No branching will occur from reproductive buds as they terminate the shoot.

https://www.conifers.org/pi/pi/t/thunbergii02.jpg

Bonsai enthusiasts commonly prune the candles to maintain a short needle length, this has the effect of arresting the needle growth; it is also possible to completely remove the candle, to force bud break at the base of the candle which results in smaller and more buds. In pines there are usually short shoot buds at the base of the candle – these will produce needle clusters in the future but no stem elongation. Breaking or pruning the top of the candle will activate these buds, which is good for ramification. If you want to continue developing the structure of the tree, you need a long shoot with a terminal vegetative bud as this won’t fall off.

As well as the terminal buds, pines sometimes have buds on their lateral shoots, between the needles, as well as internodal buds, which appear along the stem and not just at the end. These usually appear at the axil of the individual leaves on a long shoot/stem (Dörken, 2012).

Other conifers such as those in the Cupressaceae family (Thuja, Juniperus, Cypress) do not have whorls or needles, they have scale-type leaves in ‘branchlets’ (and needle-like leaves when juvenile). You can see below some examples of these which show the lateral buds forming from inside the lateral leaves (the leaves on the sides of the shoots). Since these branchlets squeeze a lot more leaves in, they have more potential for budding than do individually-leaved species such as Abies (fir) and Picea (spruce).

https://craven.ces.ncsu.edu/2022/03/conifers-with-scale-like-leaves-what-makes-a-leaf/

However one key attribute of species in Cupressaceae like these scale-leaved ones above is that just like pines they do still have differentiated short shoots and long shoots (Dörken, 2012). The short shoots are the individual branchlets, which abscise as a unit after a few years (detach from the long shoot and fall off). At the base of this short shoot is another bud waiting to generate a new shoot once the branchlet falls off. So new foliage will come from the leaves on the branchlet while it is active, and then from where the branchlet was connected to the stem when the whole branchlet falls off.

Conifers with individual needles such as firs and spruce, and needle-leaved junipers, have buds at the base of each leaf, but tend to bud towards the end of the most recent growth. Last year we dug up a Christmas tree from our allotment and I pruned the ends of most of the branches because it was too wide to fit into the house. The effect of this has been to stimulate the subordinate branches to bud – but again this has only happened at the ends of the branches (see below). Something about firs & spruces keeps the active budding zone at the end of branches.

As well as understanding the budding pattern, a key question for bonsai afficionados is whether or not a particular tree will backbud. That is, will it be possible to increase ramification and foliage density by encouraging axillary or adventitious buds to form.

Gymnosperms were traditionally believed not to resprout, with research in the past finding that buds are not present in leaf axils of conifers. Despite that, there are quite a few gymnosperms species which do, including the following. Some of these “do not have distinct buds at all; they produce new growth from meristematic tissue hidden under the skin of the twig” (Thomas, 2018) – this is known as an epicormic bud. This may be a false distinction since the meristematic tissue may just be early buds which are not developed enough to be visible.

Some Abies (fir)^ref including Abies nordmanniana^ref
Araucaria & Agatha species including including Hoop Pine^ref and Wollemi pine ^ref1, ^ref2
Cedrus (true cedar)^ref
Cryptomeria japonica (Japanese cedar)
Ginkgo
Juniperus^ref
Larix (larch)^ref
Metasequoia glyptostroboides (dawn redwood)
Pseudotsuga (Douglas fir)
Some Pinus (pines)^ref – but pines are notorious for losing their ability to bud anywhere other than on the most recent 1-2 years old stems. Brent Walston at Evergreen Gardenworks says with Pinus thunbergii that as long as there is still a living needle on a stem, if you cut the stem above it, that will force a bud at the needle axil.^ref This lines up with the idea that buds in pines are present under the leaf axil of long shoot leaves.
Taxus baccata (yew)
Sequoia sempervirens (coast redwood)
Sequoiadendron giganteum (giant redwood)
Taxodium distichum (swamp cypress – deciduous)^ref
Thuja occidentalis (sometimes called White cedar)
Thujopsis dolabrata (a Japanese species similar to Thuja)

So actually there are quite a few!

Some studies have indicated that “cytokinin sprays on conifers growing in the field can
increase the number of visible axillary buds^“ref and as a result this study concludes that “conifer leaf axils might not be as blank or empty, at least in recently initiated shoots, as they might appear to be. Cells in the leaf axils, while not forming buds, can maintain a meristematic potential and if they lose meristematic appearance, they may be
preferentially able to dedifferentiate into bud forming structures.”^ref

In ginkos^ref, axillary buds are present in the nodes of long shoots only, and these trees can backbud – below is an example of a ginkgo at the Seattle Japanese Garden – you can see new leaves sprouting from the bark of a well-established tree (from the longest long-shoot of all – the trunk).

I’ve also spotted this tree around the corner from my house in London – it was quite tall with all the foliage at the top of the tree – when I saw it cut back so severely I was sure it would die. There were only the tiniest of shoots here are there on the trunk. But in a matter of a few weeks it grew back profusely, which makes me think it must be a Thuja of some kind – perhaps Thuja occidentalis ‘Golden Smaragd’.

Finally another lovely example of conifer resprouting are the amazing dai sugi in Japan – these are Cryptomeria japonica which are cultivated for forestry purposes. The tree is encouraged into a multi-stem form with horizontal branches, which sprout new vertical stems. These are harvested over and over, and new stems grow. In this way the same tree has been used for forestry for hundreds of years without killing the tree. The technique is explained in Jake Hobson’s book Niwaki, which also includes a brilliant photo of bonsai dai sugi, which I think look bizarre but amazing. I have several Cryptomeria japonica at my allotment in the hope of creating something similar (although realistically the ones in this image are probably air-layered).

https://twitter.com/wabisabi_teien/status/1038034988841627648?lang=zh-Hant

Repairing damage (or not)

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

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

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

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

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

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

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

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