About removing leaves
In my part of the world temperatures are falling. Days are shortening and many trees are dropping their leaves. Time therefor to look at our bonsai and consider the winter. Next to providing some form of protection agains the effects of cold wind on bonsai, the effect of sun in winter on bonsai, and in general the reasons why bonsai die in winter, some people remove the leaves from deciduous bonsai in late fall.
Why remove leaves from deciduous bonsai in Fall?
In nature, trees drop their leaves by themselves, at the time that the trees are ready. So why would one want to remove the leaves from bonsai before the tree indicates that the leaves are ready to drop? If you look into the subject a little bit, there are a number of good reasons to do this, but also some things to keep in mind. The benefits can roughly be divided in 4 categories. Aestetics, Maintenance, fysiological and health. The first two actually partially overlap.
Bonsai leaf removal in fall: Aestetics
Obvious to some, a revelation to others.. good deciduous bonsai look their best in winter. So to better and longer enjoy the winter image: Remove the leaves.
Bonsai leaf removal in fall: Maintenance
In line with the aestetics.. In fall, when the tree is still active you have a window to wire and trim your trees, without too much risk. If your fall does not turn into deep frost directly (An incidental night time sprinkling of fairy dust is not deep frost) your trees will recover quickly from wiring and trimming. After all, the tree is in prepare-for-winter mode, and is actively sealing all entrances to the tree (This is actually what causes senesence: The tree putting little layers of cork at the attachment of the leaves, closing of that entrance to the tree).
So fall is a good time for many deciduous to do your basic wiring and trimming. If you do this after the leaves have dropped, the trees are withdrawing sap from the branches, making them more brittle on the one hand. On the other hand, it means that any microbreaks in the branch are not dealt with as quickly, increasing the risk of dieback on that branch (in my personal experience Oak and Carpinus are really sensitive here; These I avoid wiring from full leaf drop to bud swelling).
Bonsai leaf removal in fall: Fysiological
Leaves contain a lot of nutrients. These are moved from the leaves into the roots, trunk, branches and buds for use in the next growing season. This leads to a lot of movement of fluids in the tree. And as we know that sap transport is related to thickness growth in bonsai, the relocation of nutrients pushes a cycle of branch thickening. In more refined bonsai this is something we would like to avoid. As such, the leaves are sometimes removed in fall.
Bonsai leaf removal in fall: Winter health
In order to keep your trees as healthy as possible throughout winter, we like to have our plants and post cleared of debris. Between leaves insects will overwinter. The foliage keeps moisture around the trunk enhancing moss and fungal growth. All things we ideally do not have around our trees. We certainly do not need them during times that bonsai are dormant, and thus not actively repairing damage done to bark and trunk. In order to assist the plant, removing foliage can help the plant stay healthy. Naturally, that does not mean you have to pick the leaves from the plant. Once they drop, you could also collect them.
Concerns regarding leaf removal in fall
When leaves turn color and eventually turn brown, the plant extracts valuable nutrients and trace elements from these leaves. By removing them early, you are removing part of these nutrients. This means that you weaken the plant by removing the leaves. Considering that for certain elements up to 50% may be recoved from the leaves, this can be a considerable effect. Here it is important to keep in mind that through a good broad fall fertilizing, this effect could be countered.
Furthermore, some deciduous trees do not drop their leaves untill spring time. Best known is the European Beech. This has a good reason. By keeping the leaves, the effect winter wind has on the canopy is reduced. As such the buds and fine twigs have extra winter protection. Naturally, you would never put your bonsai directly in the cold winter wind, so this is not really a big concern. But it is something to keep in mind.