How to Reduce the Size of Leaves

by Brent Walston

Introduction

Reducing the size of leaves and the distance between them (the internodes) is the primary focus of the final training of bonsai. This comes after years of developing the trunk, the nebari, and the branches. I have been studying this phenomenon for many years. This is what I have concluded thus far:
The size of leaves, the length of internodes, and the thickness of twigs are related to the:

• Balance of the canopy to the root mass,
• Age and density of the roots,
• Level of fertility,
• Presence or absence of hormones,
• Level of light.

Of these five factors the first and second have the greatest importance for bonsai, but the other three also play a role.

The Balance of the Canopy to the Root Mass

The roots and top growth are in constant cyclical motion to stay balanced. In spring, the roots pump most of their stored energy to produce a full canopy of new leaves, and then shoots for more leaves. In summer the process reverses and the leaves resupply the roots with food, and provide energy for increased top growth. In fall the leaves stop producing food, but food continues to move down the stem from tissues. The roots continue to grow until the soil temperature falls below 60 degrees, using some of their stored food to increase their mass, and yet retain enough reserves to start the process again in spring.
To manipulate growth in bonsai it is essential to be thoroughly familiar with this cycle and all of its ramifications.
If you top prune a tree while it is dormant, you remove outlets, that is buds, for receiving food in spring, the result is the over stimulation of the existing buds, that are released from apical dominance (no more 'stop' hormones from the now cut off terminal buds). This results in long internodes and huge leaves, gigantic whips on some plants like apples and plums.
If you root prune a dormant plant without top pruning, you remove part of the food supply for stimulating buds and new growth. The result is that released buds will form smaller leaves and drastically shorter internodes. Plants seem to know how many buds to stimulate and how much new growth can be supported by the roots. I don't know the physiology, but I have seen it enough times to know that it works. Root pruning succeeds in reducing top growth up to the point that so many roots are removed that water transport becomes critical. At this point, the plant cannot support any new growth and dies. This is called overdoing it.
If you top prune a tree that has just leafed out, you remove the food factory that has just been created with most of the stored energy of the roots. The roots will have to use whatever reserve is left to issue a new set of leaves. This severely taxes the roots, and the new growth will have shorter internodes and smaller leaves. We use this principle in defoliation, although some time is usually allowed to restock food in the roots first. Multiple defoliation will result in ever decreasing leaf size. Liquidambar can be defoliated three times a season if they are in good condition, and are in a region with a long enough growing season.
The manipulations based on this principle allow one to do all sorts of things to control the speed and character of growth. This is the basis for most of the pruning practices used in bonsai.

The Age and Density of the Roots

New roots growing in fresh medium easily absorb water and nutrients and increase their mass very quickly, upsetting the balance in favor of larger leaves and longer internodes due to the excess food stored.
Roots confined in a space tend to get woody and begin to lose their ability to store food readily. One of the first symptoms of a root bound plant is chlorosis resulting from the inability of the aged root system to take up essential nutrients. As the plant stores less and less food relative to the amount of top growth that accumulates, the leaves get smaller and the internodes shorter. This is one reason why these trees are in tiny pots, aside from the aesthetic value.

The Level of Fertility

The level of fertility is somewhat obvious. Plants with good nutrition have normal size leaves and internodes. Virtually no one recommends not feeding bonsai to reduce the size of the leaves and the internodes. However, it is sometimes done to plants in training, particularly seedlings in the first year or two to get a series of close internodes low on the trunk.
Keeping a seedling a little on the hungry side and a little root bound can dramatically shorten the first internodes. This becomes valuable later on in the training process when the plant is trunk chopped to introduce a low curve on the trunk and attain taper. The places where the internodes formed will be dense with adventitious buds which will break easily on most deciduous plants. This is particularly important for the Maple genus, Acer. Most strongly affected by this phenomenon is Japanese Maple, Acer palmatum.
Of all the nutrients, the one affecting leaf and internode size the most dramatically is nitrogen, so it is best to keep the level of nitrogen balanced in the feeding program.

Presence of Absence of hormones

Some hormones can affect leaf size and, particularly, internode length. The most important of these hormones is probably gibberellic acid, which can dramatically increase internode length. These affects have little importance for bonsai.

Light Levels

Plants with good nutrition growing in the maximum of light that they can easily tolerate will have the smallest leaves and the shortest internodes.

And finally

Spend some time reviewing the first two growth principles, and thinking about their ramifications. The answers to most questions about the timing of pruning, and the correct procedure to achieve design objectives can be deduced from analysis of these two principles.
For more information about manipulation of growth in bonsai see the companion article "Using Principles of Growth to Manipulate Plants for Bonsai".