Category Archives: Shaping Bonsai

Ramification of Roots (lateral root development)

Lateral roots are ones which branch off from the main root – just like lateral or axillary buds aboveground. Lateral root development is how roots branch and ramify – similarly to stems, which ramify through bud initiation and stem growth. Encouraging strong lateral root development is a goal in bonsai, because we want to create lots of fine root mass to provide maximum exposure to water, nutrients and enabling symbiotic partners (fungi & bacteria); it also helps to stabilise the tree in the pot.

The below image is a nice one illustrating the development of lateral roots. The root grows from the tip – known as the root apical meristem (RAM), where new cells are created and the root tip is constantly extending. Above the apical meristem are pericycle cells which are preparing to initiate lateral roots, and above that these have been initiated and are starting to grow. At the top a lateral root emerges. The pale beige section in the centre labelled the ‘central cylinder’ is the location of the vascular bundle containing xylem and phloem.

https://images.app.goo.gl/E112RndafnE5jdNz5

I have been astonished while researching this post to discover just how many things can affect lateral root growth. My post on Ramification of Branches and Foliage had just three ways to improve ramification – dividing the apical meristem, pruning (in various ways) and applying cytokinins. In this post there are no less than nineteen different mechanism for encouraging root ramification! I’ll list them (x) as I go.

Before we look at each one of these, let me tell you about the ‘root clock’. The root clock is an oscillating cycle in roots which determines where lateral roots are formed – the spacing between them is dependent on the cycle time of the clock.^ref The way this works is through the oscillating expression of genes in a region close to the tip of the primary root, called the oscillation zone.

As with anything growth-related, our friends the plant growth regulators (along with genes) are involved. Auxin (1) promotes the development of branching lateral roots as well as adventitious roots (such as on air layers) whereas cytokinin (2) opposes these effects.^ref The formation of lateral roots involves both shoot- and root-derived auxin with the root tip responsible for lateral root initiation (the first step of creating a new root), and auxin from the shoot responsible for lateral root emergence (the elongation and growth of the initiated root).^ref The root clock is involved here because the back and forth gene expression causes programmed cell death at the root tip to happen periodically, which releases auxins back up the root and initiates a lateral root.

Ethylene (3) inhibits root growth, and brassinosteroid (4) and abscisic acid (ABA – some species only and in small amounts (5)) stimulate lateral root growth and elongation.^ref1 ^ref2 In fact there are complicated interactions between genes and plant growth regulators when it comes to roots with different hormone levels detected in different parts of the root, based on the differing roles they are playing in each stage of root growth. For more check out this article about tap roots which has a good diagram showing plant growth regulators (fig 2).

Other substances produced within plants which promote lateral root development include salicylic acid (6) and melatonin (7).^ref

Aside from plant hormones, the nutrients in the soil also affect the level of lateral root development – for example nitrogen (8): “in low-nitrate soils, patches of high nitrate have a localized stimulatory effect on lateral root development in many species, however where nitrate levels are globally high (i.e. not growth limiting), lateral growth is inhibited”^ref A phosphorus (9) deficit “favours a redistribution of growth from the primary roots to lateral roots”.^ref A sulphur (10) defiency “leads to the development of a prolific root system, usually at the expense of shoot growth…roots elongate faster than those with sufficient sulphate, with lateral roots developing earlier, closer to the root tip and at a greater density.”^ref

Lateral root formation is restricted when water availability is low (11), and somewhat surprisingly, when there is a lot of salt (12) in the soil more lateral roots form.^ref

You might not think that roots need light (13), but in fact light above ground is necessary for maintaining the oscillating signal of the root clock and for the formation of sites where lateral roots can branch off; an absence of light has a strong inhibiting effect on root elongation and branching.^ref This may be related to the point below about sucrose – light drives photosynthesis and the creation of photosynthates like sucrose.^ref

This intriguing study found that drenching roots in sucrose solution (14) “significantly increased lateral root branching and root formation compared with non-sugar supplemented controls.”^ref This may be because of effects on the rhizosphere vs the roots themselves. Just beware what you might attract if doing this in your bonsai garden as a sucrose drench sounds like an insect’s dream come true.

Several studies have determined that root pruning (15) encourages lateral root formation ^ref1, ^ref2 and that this happens most likely due to a surge in auxins after the root is cut. Similarly, synthetic auxins applied to roots of scarlet oak resulted in six times the number of adventitious roots compared to a control, and resulted in longer roots as well.^ref

Another way to increase lateral root formation is via encouraging arbuscular mycorrhizal fungi (16). Interestingly, the mechanism for this is that the plant root detects the presence of chitin in the fungi cell walls.^ref Chitin (C₈H₁₃O₅N) is “the most abundant aminopolysaccharide polymer occurring in nature, and is the building material that gives strength to the exoskeletons of crustaceans, insects, and the cell walls of fungi.”^refResearchers found that any source of chitin (17) had the same effect – including chitin derived from shrimp shells^ref Chitins can be found in many living creatures, including crustacean shells, insect shells (beetles, grasshoppers, cockroaches, blowflies), bat guano, resting eggs of Daphnia species, spiders, green algae, zooplankton (krill), phytoplankton and fungi.^ref1 ^ref2 ^ref3

On a similar theme (and maybe a repeat of 14) the presence of Pseudomonads bacteria has been shown to increase adventitious root development in tobacco.^ref

While reading a study about plant growth substances recently I came across (18) – apparently you can physically bisect a root apical meristem and it will become two autonomous RAMs.^ref

And lastly let’s look at two types of pots which affect root ramification: (19) if you use a white pot instead of a black, green or other dark colour, roots will be up to 2.5 times denser^ref, and if you use an air pot (20) they will have fewer circling or malformed roots but also less root mass overall.^ref

So let’s summarise all the different ways you can practically encourage root ramification in your bonsai:

Root pruning
Encourage auxin & sucrose production via photosynthesis (leave some leaves and give the tree good light)
Apply exogenous (from outside the plant) auxins – for example from compost or compost leachate
Drench roots in sucrose solution
Add and nurture mycorrhizal fungi and friendly bacteria
Add a source of chitin (as a vegetarian I can’t recommend any of the animal sources, but some other ideas include scooping the algae from your garden pond (or similar) or adding some mushrooms to your compost and adding that (or its leachate) to your pots
Bisect the root apical meristems (ie. cut them down the centre with a clean sharp blade)
- Use a white pot

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

Reducing leaf size

In bonsai a small leaf is preferred, because this give the impression of the proper scale of the tree. But how small do leaves need to be? Let’s take the beech tree out the back of my house. It’s about 25m tall, with a 75cm diameter trunk, and its leaves are 8-10cm long. If you were to actually scale this down to a generous bonsai size of 50cm tall, it would have a trunk of only 1.5cm and leaves of 2mm long!! Which is obviously ridiculous. But even if we can’t get a bonsai tree down to the precise scale of its full-sized siblings, we do want to reduce the leaf size to make the tree look more realistic.

The first thing to say, if it isn’t already obvious, is that you can’t shrink a tree’s leaves – they have to grow small in the first place, or be prevented from growing as large as they could.

Achieving the former is all about selecting a small-leaved variety of tree. Many species have small-leaved varieties which lend themselves much better to bonsai than their large-leaved siblings. Unfortunately if you are selecting a variety with small leaves (vs a species) you will need to use a vegetative form of reproduction to obtain your tree – a graft, a cutting or an air layer. I’ve had some success collecting seeds from small-leaved Japanese maples, which sometimes pass their diminutive leaves to their progeny.

If you happen to have the opportunity to analyse a prospective bonsai tree’s genome, you’ll prefer to choose haploid trees (with just one set of chromosomes) and avoid polyploid trees (with more than two sets of chromosomes) – as can be seen in this image of different ploidy ginkgos, the leaves are much larger for trees with more replicated genetic material. Unfortunately determining ploidy requires a sample of your tree, a flow cytometer and some lab skills most of us lack!

https://www.nature.com/articles/s41438-018-0055-9/figures/2

Achieving the latter (preventing the leaves from growing large) basically involves disrupting the leaves as they are growing to stunt them before they grow to their full size.

Ennos (2016) reports that ‘thigmomorphogenesis’ – mechanical perturbation by the wind, results in smaller leaves. This study on Ulmus americana seedlings found that total leaf size was reduced by 40% – but only when they were exposed to the highest level of ‘flexures’ (a proxy for wind).^ref Another study which I can’t access behind a paywall is summarised as finding “in needle-shaped leaves the elongation of the leaves is inhibited”^ref. Researchers think that mechanical perturbation of plants triggers the production of ethylene, and its cross-talk with auxin, both plant growth regulators. So putting your trees in a windy position may result in smaller leaves (and shorter internodes). But be aware this will also increase transpiration so they will need more water.

The other mode for leaf size reduction is to starve the tree of resources when it is making leaves, in one form or another. This leaves less energy available for leaf production leading to smaller leaves. Various forms of defoliation achieve this, such as:

partial or full foliage removal, forcing the tree to use up resources growing a new flush of leaves
bud pinching, which is personally the best way I’ve seen to reduce leaf size on deciduous trees
maintenance pruning – cutting off leaves when they exceed a certain size – so that new leaves are grown and only the ones below a certain size remain
note – the above should not be used on conifers

On conifers, pruning back the candles to a few needles at the base will apparently trigger another flush of budding, and due to depleted resources the needles will not grow as long^ref (since leaf size is apparently not very interesting commercially, there really is little research on how to achieve it).

Another technique is to deprive the tree of fertiliser until it has leafed out. I think this might weaken the plant over the long term but it’s apparently popular for Japanese maple enthusiasts (for their trees, not for them!)

Finally, it’s important to balance leaf size reduction techniques with the tree’s energy requirements because reduced leaf area will reduce photosynthesis.

Should I remove flower buds or fruit?

That depends what tree you have and what you are trying to achieve. Obviously if you have satsuki azalea, you probably want to leave the flowers on the tree! If you have a crabapple, personally I don’t think there is much point if you don’t let a few fruit form. And I am really partial to rose-coloured larch cones. All trees form some kind of reproductive organs, whether they be conifers with their strobili (cones, either pollen or seed forming), ginkgo with their ovules, or angiosperms with their flowers and fruit. Some are almost unnoticeable and others are right in your face. Bonsai wisdom sometimes says these should be culled or removed entirely in order to avoid draining the tree of its energy.

When considering this question we need to understand the idea of resource ‘sources’ and ‘sinks’ in plants. A source is a material producer and exporter, and a sink is a material importer and consumer.^ref See the below table for sources and sinks in trees. As you’d imagine, leaves are a major source of carbon and a sink of inorganic nitrogen (nitrogen as a macronutrient). Roots are a source of inorganic nitrogen and leaves are a sink. So what about fruit, seeds, and flowers, which supposedly drain the tree? As you can see they are major sink organs – but not only sink organs…they are also source organs!

https://academic.oup.com/jxb/article/68/16/4417/3002648

Let’s have an interesting little diversion – did you know that it’s not only leaves which photosynthesise? This fascinating study^ref looked at the photosynthetic activity of (a) ears of wheat (b) sycamore seed pods (c) a green tomato (d) unripe and ripe strawberries (e) a greengage (f) unripe cherries; and (g) a green apple. The images below were taken using fluorescence imaging and anything with a colour indicates that there is photosynthesis taking place – with the red and orange areas the strongest. Check out the sycamore seed pods!

https://onlinelibrary.wiley.com/doi/full/10.1111/tpj.14633

How the heck can this happen – well there are various theories about the mechanism (including recycling CO₂ from respiration, and the presence of stomata on fruit) but the point is that maybe seeds and fruit, particularly if they have periods when they are green, don’t act as such as sink as we might think, and for a period are acting as a source and not a sink.

This study states that “reproduction in Beech does not deplete stored carbohydrates, but it does change the amount of nitrogen stored” and this study found that “fruiting is independent from old carbon reserves in masting trees”^ref which basically means that fruit uses current year photosynthates/energy and doesn’t actually deplete reserves.

On the other hand this study found that Douglas fir tree rings were narrower in years when they bore many seed-cones^ref and this one mentions that “experiments with apple trees have shown that roots can die from lack of carbohydrate supply when they are over cropped”^ref

All living things have processes for managing and balancing resource allocation^ref and this is likely an evolutionary differentiator. In trees, resource availability limits the amount of fruit which is allowed to develop – even pollinated flowers may not develop into fruit if the tree does not have enough resources available – these could include energy, or nutrients.^ref So to an extent the plant itself manages the resource allocation.

To complicate matters further many trees use a ‘masting’ strategy for reproduction, which means they have years where many more seeds are produced, often synchronised with other trees of the same species. One theory for how this happens is that the weather influences how pollen is distributed – in beech windy conditions lead to mast years whereas in oak short pollen seasons do.^ref Temperature and precipitation also affect pollen production and distribution (high temperature increases pollen production but high precipitation washes it away).^ref In this study on Japanese oak, “high seed production never occurred in two successive years, but successive years of low abundance were observed several times between 1980 and 2000.”^ref

Overall there are a lot of factors interacting when it comes to reproduction. Photosynthetic seeds or fruit can contribute to carbon production, and may use only current year photosynthates, so the tax may not be as high as thought, but there is some evidence that reproduction can divert energy from roots and foliage.

If you are really focused on trunk growth, branch structure or foliage development on your bonsai tree, you might want to divert the energy from reproduction to these areas by removing some or all reproductive organs, until you are happy with the trunk/foliage. At this point then you could then let the tree reproduce (noting that removing cones one year will cause more cones to develop the following year)^ref.

Ramification of Branches and Foliage

After establishing trunk and branch structure, ramification (a fancy word for ‘branching’) of branches and foliage (as well as roots) is a key goal of bonsai. This makes a tree look older and more sophisticated, and gives the bonsai enthusiast options for continued development of the tree.

Ramification is created by branching the stems. Stem branching usually* requires buds, as a new bud creates a new stem. The pattern of stem branching for a particular species will depend on its ‘phyllotaxy’ (leaf morphology) and pattern of buds.

Usually in bonsai we don’t want more than two stems from the same location, the general guidance is to fork into two at any given junction. This is because strong growth of multiple branches at a junction leads to a bulging area on the trunk which bonsai judges don’t like. In the real world, many trees have reverse taper and bulging branch junctions though, so it’s your call. To avoid this situation, remove buds which are in places you don’t want by rubbing or cutting them off.

To improve ramification, you need to encourage as much budding as quickly as possible, then select the buds you want to develop. Pruning the growing tip is the main way to encourage budding, because pruning removes the apical bud (the dominant bud at the end of the stem), diverts resources into buds lower down the stem and sensitises those buds to respond to auxins and develop into shoots. In deciduous trees this should result in at least two buds generating from the stem instead of the one which was there. Another great way to create ramification on deciduous trees is through bud pinching – see Harry Harrington’s detailed explanation of how to do this. Bud pinching removes the entire primary meristem except for two outer leaves, this encourages the buds at those leaf axils to grow, along with two new buds at their bases.

Different species have differing abilities to respond to pruning, so try to get a sense by observing your tree of how well it will cope. Deciduous trees are designed for regeneration so in general they take pruning reasonably well, although if you take it too far they might send out suckers instead of new buds from the branches. With evergreen conifers you want to ensure there is some foliage and at least some buds remaining after you prune, otherwise it may not regenerate (unless it’s a thuja, or a yew, these guys are refoliating machines). I have cedrus seedlings in my collection and by cutting back the apical leader from not long after they germinated, and every year since, they have become extremely bushy and well-ramified (although, at the cost of developing a think trunk).

Gratuitous image of one of my cedar bonsai

Anything which stops or prevents tip extension will drive bud activation and ramification further back on the tree. In the case of conifers, the presence of flowers on the growth tips (as you see in juniper) has this effect as well, and can cause back budding. Lammas growth (a second flush in summer) can give you another round of ramification as long as you’ve pruned beforehand (otherwise it will just add to the existing stems).

Research has found that bud outgrowth is “controlled by plant hormones, including auxin, strigolactones, and cytokinins (CKs); nutrients (sugars, nitrogen, phosphates) and external cues”.^ref In particular the sugar sucrose has been identified as a key driver for promoting bud outgrowth and accumulating cytokinins – this is generated by photosynthesis.

In one study on apple trees, foliar application of a synthetically produced cytokinin 6-benzylaminopurine (BA) was found to generate three times the lateral bud growth on currently growing shoots compared to controls (but not on old growth)^ref and at the same time reduced the length of the main stem. BA was used to encourage better growth of bean sprouts in China before being banned^ref and has been shown to increase the number of leaves (ramification!) on melaleuca alternifolia trees^ref (melaleuca is the source of tea tree oil), and on some conifers^ref. Could BA (also known as 6 BAP) be useful in bonsai? You can (like most things) buy this product in foil bags on ebay, but there is a product in the orchid world called Keiki paste which also contains 6 BAP – so maybe some judicious use of ‘crazy keiki cloning paste‘ might also help ramification and shoot development in your trees?^ref You can also purchase BAP (as its also known) from vendors involved in hydroponics and suchlike as it’s used in in-vitro plant micropropagation.

If you baulk at paying £18 for 7ml of keiki paste, there is one other source of cytokinins which is a lot cheaper, more sustainable and clearer in its provenance – compost. This study found that compost created particularly from waste collected throughout spring^ref contained 6 BAP. Frustratingly there weren’t any free to read articles analysing compost leachate for cytokinin content, but if it’s in solid compost it’s a fair assumption there are cytokinins in leachate as well. Which makes me feel a lot better about the £300 I recently spent on a Hotbin composter! Which incidentally, produces gallons of leachate, which can be diluted and added as a liquid fertiliser.

* I’ve recently read a study which states that “apical meristems can be surgically divided into at least six parts and these then become autonomous apical meristems.”^ref What this suggests is that you could slice growing tips into 6 (or better, two since we don’t want more than two stems from a node) and they would become two stems instead of one! One to try next spring.

** By the way – it’s not auxins which cause apical dominance! Check out page 215 of this book, it’s nutritional status and phyllotaxy which determine the apical stem’s sensitivity to auxin which is present.

Shoots

This is a rewrite of my original post on shoots, now I know a *lot* more…

So what are shoots? They are the vegetative growth which comes from buds, extending to create new stems. Since stems create the architecture of a tree, shoots are really important when it comes to bonsai.

There are three key concepts to know about when it comes to shoots. The first is the existence of long and short shoots, the second is the way in which different shoots are formed and the third is the concept of the internode.

I had never heard of long and short shoots before researching this site, and I have since found that many articles and books don’t really talk about the fact that many species of tree possess two types of shoots. Shoot differentiation (as it’s known) is present on the vast majority of deciduous angiosperms (flowering trees), all deciduous gymnosperms, and quite a few (around 25%) of evergreen gymnosperms as well, particularly conifers.^ref

In these trees, two different types of shoot develop – long shoots and short shoots. Long shoots are exactly as described – they have a terminal bud which continues to build up the length of the shoot over time so it becomes (relatively) long. Short shoots meanwhile don’t persist beyond a limited number of years, they are much shorter than long shoots and have many fewer nodes. Both types of shoots can have leaves, flowers, cones and fruit, but only long shoots can create the long-term architecture of the tree. Importantly, aside from their structural trunk and branches older trees mostly grow short shoots, which is why they look more ramified.^ref

In some species (such as pines), short shoots – otherwise known as fascicles – are a feature of the mature vegetative phase of the tree, and don’t appear in the juvenile phase nor with juvenile foliage. An interesting side note is that fascicles can be used to propagate trees with needle leaves, the fascicle is treated like a cutting and placed in rooting hormone and well drained medium – the reason this works is because the fascicle is actually a short shoot and not a leaf.^ref

Below is an example of Cedrus libani where the clusters of needles (N1) are on the short shoot (S), and occasionally along the long shoot (L) there are individual needles (N2).

https://www.researchgate.net/figure/Shoot-and-needle-characteristics-of-Cedrus-libani-A-Approximately-6-year-old-spur-shoot_fig1_303469784

A fascinating – and useful for bonsai – attribute of short shoots is that they almost always have more leaves than the equivalent long shoot. In angiosperms, short shoots have multiple smaller leaves with an almost identical leaf area to a single leaf grown from a long shoot (see example A below).^ref And in gymnosperms short shoots have many more leaves and leaf area than long shoots – examples D and E below show the leaves on a short shoot (right hand side) compared to the individual long shoot leaves (left hand side) on larch and dawn redwood.

(source: https://beckassets.blob.core.windows.net/product/readingsample/10943560/9783510480326_excerpt_001.pdf)

So hopefully you can see that short shoots are fantastic for ramification! But not so fantastic for building the structure of the tree, since they don’t persist. So how can you tell which is which? Very simply short shoots are smaller, have a lot more leaves, and fall off when their time is up. Often in gymnosperms they will have cones at the end of their leaves.

You may not have realised that whilst the ‘leaflets’ on Cupressaceae species such as dawn redwood, cypress and juniper may appear to be compound leaves, instead they are actually short shoots. When their life comes to an end, the entire short shoot abscises (falls off) along with its leaves. Similarly for pines, what you might know as ‘fascicles’ are actually the short shoots, and on pines only the short shoots bear photosynthesising leaves (needles). Eventually they will fall off.

In angiosperms, a short shoot usually develops from the bud in the leaf axil of the long-shoot leaf, arriving the next season. In gymnosperms, it depends on the species. In Cupressaceae a bud will be sitting at the base of the short shoot so another one should grow once it falls off. In Pinus short shoot buds sit in the long shoot leaves towards the base of the long shoot, and they are positioned at the base of the long shoot bud.^ref In Ginkgo both short shoots and long shoots can come from any bud on any type of shoot.

Below is a picture of some interesting behaviour I’d never seen before – this Japanese larch belonging to a member of my bonsai club produced buds and new stems right through the middle of its cones. Pollen and seed cones on larch are terminal organs growing only on short shoots^ref – which means they aren’t supposed to extend. But Larix is known to be able to change the type of shoot from short to long if damaged (which may have been triggered by the hard pruning it received).^ref So in this case what had been a short shoot destined to eventually fall off, instead turned into a long shoot.

So what does it all mean? From a bonsai point of view, the first thing is to work out if a tree has shoot differentiation. If it is deciduous, it will, and if it is a gymnosperm, it still may even if evergreen – gymnosperms which have shoot differentiation include Pseudolarix, Taxodium, Sequoia, Cedrus, Larix, Ginkgo, Pinus & Metasequoia. Understanding the difference between short and long shoots will allow you to understand where foliage will ramify, and where the long-term structure of the tree can come from. On trees which don’t have shoot differentiation, any stem which has a vegetative bud can be used to develop the shape of the tree.

So now we know that long and short shoots exist in many trees, let’s turn to how those shoots form. According to Thomas (2018) , there are three options.

Option 1 is ‘fixed’ or ‘determinate’ growth. These trees preform every part of the shoot in the bud, so they extend very quickly (a few weeks) and then stop. If they are young (less than 10-15 years old) and have the right conditions, they may do this a second time around the start of August (in the Northern hemisphere), this is known as Lammas growth. The shoots from these trees developed based on the conditions at the end of *last year’s* growing season.

Option 2 is ‘free’ or ‘indeterminate’ growth. These trees have only some preformed leaves. Once extended the shoot will continue to produce other leaves from scratch in a continuous fashion. Often these are found in the tropics or warmer climes (my potted Eucalyptus never seem to stop producing leaves even during winter).

Option 3 is ‘rhythmic’ growth. These trees extend in recurrent flushes, with multiple cycles of growth and bud formation during the season.

Outside of the tropics, towards the end of the growing season all trees will stop shoot and leaf growth according to their phenology, in order to complete the formation of buds for next year. If conditions are not good, these buds will be fewer and contain fewer leaves. To see a list of which trees have which types of growth see the Growth Types Table. The relevance to bonsai is that trees with determinate growth are only going to give you one or at most two cracks of the whip in a given season. Those with indeterminate growth might be easier to develop since they will keep extending as long as the conditions are suitable.

Interestingly one study on lammas growth (second flushing) found that 73% of this occurred from lateral buds. We’d obviously love to have this in bonsai as it helps ramification within the same growth season.^ref This article^ref summarising lammas growth factors says that it can be encouraged by warmer temperatures (Pinus densiflora), extra watering (Pinus sylvestris), nitrogen fertiliser (Pinus sylvestris, Pseudotsuga menziesii) and applying a blackout treatment for less than 2 weeks early in the summer (Picea abies). So from a bonsai perspective see if you can encourage second flushing to generate those lateral buds.

And finally we come to internodes – these are the length of the shoot between each successive leaf. In general bonsai afficionados are looking for short internodes so they can achieve compact, dense foliage. The factors which affect internode length when a tree grows are the same as for any other type of growth – genetics, plant growth regulators and availability of nutrients. Shorter internodes can be achieved by (1) shoot pruning, (2) thigmomorphogenesis and (3) starvation.

If you allow a shoot to extend naturally (and it has no competing stressors), it will prioritise resources into growing as long as it can and the growing tip will suppress the growth of any lateral shoots below it – because the driving force for a tree is to grow large and establish the biggest exposure it can to sunlight. An angiosperm will grow a series of internodes with leaves at each point. What I have observed from looking in my garden is that the internode length on an angiosperm tends to start small (or in some cases leaves are grown directly at the node as well), then increase in size, then reduce again.

To get the smallest internodes, you should prune off the growing tip once the first pair of leaves and the first internode has grown. If leaves have grown at the node, you could remove the shoot altogether (there will be no internode in this case). New shoots will grow from buds in the leaf axils, and if you keep doing this, you will always retain the short first internode and increased ramification.

You could also make use of thigmomorphogenesis which is “the response of plants to mechanically induced flexing, including the brushing or movement of animals against plants, or the flexing of above ground portions of a plant by wind, ice, or snow loading”^ref According to this article^ref, “the most consistent thigmomorphogenetic effects are a reduction in shoot elongation and an increase in radial growth in response to a flexing stimulus resulting in a plant of shorter stature and thicker, stiffer stem.” i.e shorter internodes and thicker stems.

Thigmomorphogensis is thought to be triggered by plant growth regulators or other substances within the plant signalling when it has been touched^ref. To trigger thigmomorphogenesis in your tree, you could expose it to wind while the buds are developing, rub the internodes for 10s daily (seriously, this is what they did in the original study^ref which identified the phenomenon), touch the leaves regularly or manhandle the growing shoots.

Another way that bonsai enthusiasts encourage small internodes is by starving the tree. Fertiliser helps the tree grow and this will lead to longer internodes and larger leaves. Holding back fertiliser may result in the desired effect – but also could impact the tree’s health negatively – so it is a balancing act.

So there you have it – shoots turn out to be surprisingly interesting. For your bonsai, try to work out if your tree is shoot differentiated, and if it is, aim to use long shoots for structure and short shoots for foliage ramification. If it has determinate shoot growth, you need to work with the one or two shoot extensions that you get per year, and to get that second flush with lots of lateral buds try using one of the techniques above (warmer temperatures, extra watering, nitrogen fertiliser). Finally keep internodes small with judicious pruning, foliage fondling and holding back fertiliser.

Buds

How big should a bonsai trunk be?

It’s a how-long-is-a-piece-of-string question because the trunk on a bonsai doesn’t exist in isolation, it exists relative to the foliage, nebari and pot. Because trees undergo secondary thickening however, their trunks expand with every year. So, older trees have thicker trunks.

For another post I found this data below. It shows mass rather than volume, but you can see that as trees get older and bigger, their mass skews to the trunk, which ends up being 80%+ of the total mass of the tree. Whereas at the beginning of the tree’s life, on the left hand side of the chart, the leaf mass exceeds the stem mass.

https://nph.onlinelibrary.wiley.com/doi/full/10.1111/j.1469-8137.2011.03952.x

So in general if we want to emulate older trees, our bonsai needs to be weighted towards a fat trunk (and main branches). Note also that the root mass doesn’t go below 20% – the main contribution to mass in a root are the big structural roots which are largest within a metre or so of the trunk. So this gives an indication of how big a nebari should be.

But as mentioned above the trunk exists relative to the canopy so what do we know about the ratio between the two? One measure which is used in forestry is the live crown ratio which is used as an indicator of tree health. The live crown ratio is the vertical length of the foliage as a percentage of the total tree height.^ref Some studies have measured crown ratios for different species (usually in managed forests):

A stand of coast redwoods: between 30%-50%^ref
Douglas Fir: in the 80% range for 20y old trees, down to the 40% for 40y old trees and up to 60% range for 450y old trees
Turkey oak: between 20%-50%

Also interesting is the crown radius to trunk diameter. A study measured this for 22 different species including both angiosperms and gymnosperms and came up with equations that represent the ‘allometric types’ for each species – that is an equation that describes how a tree’s dimensions change over time.^ref For example for common beech (Fagus sylvatica) they found (see table 5) that the following equation could be used to calculate the crown radius given a particular trunk diameter:

ln(crown radius) = 0.0111 + 0.4710 x ln(trunk diameter) ; (note crown radius is in m and trunk diameter in cm)

if we have, for example, a 1m wide trunk, you could calculate the crown radius as follows: 0.0111 + 0.4710 x ln(100) = 2.180 so crown radius = e^2.180 = 8.85m – this actually then gives a crown diameter of 2 x 8.85 = 17.8m. So an old beech which has achieved a 1m wide trunk could have a nearly 18m crown diameter – which means the trunk is about 5.5% of the width of the crown.

Because I love a bit of excel, I took the data for the rest of the species to work out the trunk/crown diameter ratio for each of them based on a 1m trunk – and here is the answer:

So for most species a 1m trunk will be between 4% and 10% of the width of the canopy. I couldn’t resist looking up Auracaria cunninghamii to see why it was different – it looks like the canopy habit is quite narrow which increases the trunk/canopy ratio#.

If you have Douglas fir, this study found that “the vertical distribution of branch volume shifted toward the upper-crown with increasing tree age”^ref The mechanisms at work include self-pruning, branches dying and falling off and then adventitious branches growing in the spaces. As they included a picture you can see it makes quite an obvious difference to the look of the tree.

https://archives.evergreen.edu/webpages/projects/files/studycenter/ishii.pdf

That’s just one species though – the shape of old trees is going to be to a certain extent genetically determined so different species will have a different mature look in terms of their shape and branch distribution.

Conventional bonsai wisdom says a tree needs to have good taper in order to look old. This means it is thicker at the bottom than at the top. But tree-ring researcher and dendrologist Valerie Trouet in her book Tree Story says otherwise. She says “once height growth has stopped in an older tree, then the upper part of the stem will start to catch up, it’s girth increasing year after year, and the stem will gradually take on a more columnar, rather than tapered, look….the tree’s limbs also continue to thicken; branches and roots of old trees often are quite sizable.”

What we are trying to achieve with bonsai is small trees which look like mature, large ones in nature. So the size of the trunk, whether it has taper or not, needs to be in proportion with the canopy and the roots, and the branches should start anywhere from the 20% to the 50/60% of the total tree height mark and be in proportion to the trunk as in the table above.

There are more attributes which make a tree look old, to learn more check out this post: Old Trees.