by Dave Bogan

This article was published in the May 2006 issue of the Evansville Bonsai Society newsletter. It is used with permission - ED

Leaf pruning is a huge win-win situation for bonsai. Through its use, the health and maturity of your bonsai are greatly improved. The problem lies in the fact it is also (in my opinion) a misused or maybe a topic which is not totally understood. Most new practitioners of bonsai believe this technique is primarily used to obtain smaller leaves. Generally, leaf pruning does have this result but the smaller leaves are an added benefit and not the main goal or reason for practicing this technique.

Most, I'm sure have noticed that when a branch is pruned in spring it will generally cause a few dormant buds to appear. These buds of course become new branches. Thus by pruning a branch, you may add 2 or so more branches. Keeping this thought in mind, by removing leaves, we also cause dormant buds to activate and become new branches. If you prune your tree back to the desired shape and then you remove the leaves, stop and think how many additional branches you will obtain.

As an example, several of my trident maples were in full leaf and had hardened off by the end of February in my cold room. I leaf pruned them. Let's say that I cut off 200 leaves. If all went well, I probably added about 100 new small branches. By mid March, these new branches had all leafed out. By the end of March, all these new leaves had hardened off and I leaf pruned again. Now, let's say I removed 400 leaves (after pruning again); I should now expect to have around at least 200 new branches with x4 or so new leaves. In a short 3 month period, I have more than doubled my branch structure and probably increased the number of leaves 6 fold. Keep in mind, not every leaf you remove will result in a new branch. Typically on most deciduous species, the very first and some times the second leaves do not have dormant buds. These leaves can usually be identified due to their smaller size than the leaves farther out the branch. Plus, occasionally some of the others may not bud. But in general, you will realize at least double the number of branches you previously had. Now, as these all leaf out, not all of them will survive. As they leaf out, some of the new branches will be shaded and some will either not bud or will die back.

Now, with all the new branches, leaves and growth in general, the tree has redistributed a lot of its energy. It has also (in some way?) determined that since it has, let's say, doubled the number of leaves, the new resulting leaves will not need to be as large as the previous leaves. A plant determines leaf size required based on several factors. First, it determines the total surface area of all the leaves. Based on the new quantity, the leaves can now be smaller and still have the same amount of surface area. This total amount of surface area results from the need of available light and moisture transpiration. This is why a tree in the shade will have larger leaves than one grown in full sun.

So, again, leaf pruning results in smaller leaves, but it definitely in not the primary objective. Also, in reference to smaller leaves, some trees even of the same species can have different size leaves. Some of this is caused by genetics and by the age of the tree. As trees age and the root system gets finer (also a benefit of leaf pruning), many times the upper areas will also get finer or smaller. A condition that increases leaf size is of course low light. The other condition is fertilizer. Fertilizer at this point should be a balancing factor. Once a tree has reached the final design stages, slow down on the feeding. Remember high doses of nutrients will make everything grow faster and sometimes larger.

Now, I should mention some of the techniques I use to leaf prune a tree.

1. Most important, the tree MUST be healthy.
2. Feed the tree about two weeks prior to the pruning then hold off on the fertilizer.

Now we're ready to remove the leaves. Leaves of course will change from species to species. Let's first look at the length of the leaf stem. This will dictate how you remove the leaves as you must not damage the dormant buds. Examples are trees of the elm species which all have very short leaf stems. These must be cut with scissors. In the case of most maples and tropicals, the leaf stems are long. On these, I simply grasp the leaf and pull it off. The best approach is to test a few leaves first. Grasp the leaf and pull it off. If it leaves part of the leaf stem, then you don't need to cut them. Simply pull them all off. This of course will save a lot of time.

Once you have all the leaves off, you need to go back and trim your tree. Trim it for shape. Keep in mind that as the new growth appears, it will add to the length of the existing branches, so you may want to trim back a little further. Next, make sure you trim off the tip of every branch. This tip is the strongest growing point. By removing this terminal end, you re-direct the strength to the remaining new buds.

After leaf pruning, I always place the tree in full sun. Watch the moisture closer. With the lack of leaves, the tree will use less water, but this will change as the new leaves grow out.

One more fact. Some species, no matter what you do, will not reduce leaf size very quickly. A good example is oak. It may take you several years to get smaller (maybe 1/2 original) leaves. Others, say a trident maple, generally will re-bud with leaves half the original size. Again, keep your feeding schedule and especially the age of the tree in mind.

EDITOR NOTE: When I asked Dave what was the latest leaf defoliation could be done, he replied the latest was the end of June. The biggest issue is that the trees need to re-leaf as soon as possible. A lot of trees (example Trident maples) go semi dormant in early July due to the high temperatures. For some trees, defoliation can be done in early September with decent results. The key to this time of year is to ensure the tree can re-foliate prior to fall. This means the tree must be very healthy as defoliation saps the strength of the tree.