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 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.