Leaf size reduction

[stymie]

Here is an article which I prepared for people over here on the other side of the world. Some of the points may help in Oz and some may not. please feel free to add constructive critisism. I'm thick skinned and you may be able to improve on my thoughts.

REDUCING LEAF SIZE

The roots and top growth are in constant motion to stay balanced.
In spring, the roots pump most of their stored energy to produce a full canopy of new leaves and later shoots for more leaves.
In summer the process reverses and the leaves re-supply the roots with food and provide energy for increased top growth.
In autumn 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 60F, using some of their stored food to increase their mass, and yet retaining enough reserves to start the process again in spring.

If you top prune a tree while it is dormant, you remove some of the buds, the result is the over stimulation of the remaining buds that are released from apical dominance (no more restricting hormones from the now cut off terminal buds). This results in long internodes and huge leaves.

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 remaining 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. The physiology is vague but I have seen it enough times to know that it works.
Root pruning succeeds in reducing top growth up to the point where 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.
New roots growing in fresh medium easily absorb water and nutrients and increase their mass very quickly, upsetting the balance in favour 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 it accumulates, the leaves get smaller and the internodes shorter. This is one reason why our trees are in tiny pots, aside from the aesthetic value. 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 movement 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.

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. Plants growing in poor soil with the maximum of light that they can easily tolerate will have the smallest leaves and the shortest internodes like mountain yamadori.

Now you know why I fertilise mature trees very little and then with something like a tomato formula.

[PeterW]

Thanks Stymie, interesting read. Defoliation is a well used procedure here in Australia. Your article will help with understanding the reasoning behind defoliation as well as the importance of timing. Thanks for sharing.
Peter

[Jon Chown]

Thank you Stymie for this valuable information. Every beginner should read this - then read it again, it sure goes a long way in explaining the why and when pruning and repotting should be done.

Jon

[kcpoole]

I have just added this thread as a link to the Wiki, but thought I might bump it as many members will have not seen it.

Ken

[Pearcy001]

I have just added this thread as a link to the Wiki, but thought I might bump it as many members will have not seen it.

Ken

Great work on the bump Ken, much appreciated.

Cheers,
Pearcy.