Tag Archives: Thigmomorphogenesis

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!


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 longref (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.


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).


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 shootsref – 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 articleref 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 articleref, “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 touchedref. 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 studyref 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.