AABS April 2004 Meeting

View from Here
by Jack Wikle

Options in “Leaf Pruning”:
Probably it can’t be overemphasized that removing leaves or needles – and reducing photosynthetic capacity in the process – reduces growth and weakens trees rather than strengthening them. Yes, new growth will be stimulated by defoliation, but the new material produced will never equal what would have accumulated if no foliage was removed.

Therefore, complete defoliation is basically a refining technique best saved for use on well developed, vigorous trees. Declining trees and very old bonsai should not have all their leaves removed. Even working with young, healthy trees, shoots that one wants to lengthen and thicken as rapidly as possible should not be defoliated. To expedite bonsai progress, wait until the trunk and main branches are almost as thick and long as you want them. Then and, only then, in the last stage of the bonsai process, will it be time to begin complete defoliation of the healthy tree.

Complete Defoliation:
The clipping off of every leaf of deciduous trees leaving only part of the leaf stem and the bud at its base (often referred to as “leaf pruning” in bonsai literature) is a much used technique for stimulating production of small leaves on an abundance of thin twigs. Fall color is often enhanced by this process too. Complete defoliation of needled evergreens typically results in severe dieback or death.

The tree defoliated after producing its first crop of leaves is forced to dip into its diminished store of carbohydrate reserves (both energy source and building material) in producing new shoots with new leaves. The result is a second flush of growth, weaker, with thinner twigs, shorter internodes and reduced leaf size. Another impact of defoliation is that some buds that would have remained dormant otherwise also become active. These effects are often likened to forcing two years of growth in one year. That is, the tree produces normal spring shoot growth then -- responding to defoliation -- a second flush of shoots putting even newer branches on the new branches already present, all in one year rather than two years.

Another advantage in removing all leaves is that the freshly defoliated tree, now with its structure easily seen, can be styled and even repotted with less caution and special follow-up care than required for a bonsai in full leaf. In other words, leaf pruning makes possible a second season for major styling and repotting activity with tree species that tolerate the process well.

Ideally the first crop of leaves is allowed to expand almost fully and to harden before being removed completely. This usually means late May or sometime in June in our cool temperate climate. Complete defoliation after the summer solstice (about June 21 when days begin getting shorter) will result in weaker and less predictable return growth. Late defoliation (July and beyond) may result in unacceptably weak growth or, at the extreme, no re-growth at all until the following year. Exceptions are some very resilient species such as Zelkova serrata when grown in Japan and Ficus species in many other countries, both of which are completely defoliated two or three times a year with no problems -- leaves becoming increasingly smaller with each re-growth.

Partial Defoliation:
In many situations, clipping off all leaves from areas of vigorous growth (large diameter shoots with long internodes and large leaves) and leaving foliage in weaker areas (thin twigs with short internodes and small leaves) untouched offers distinct advantages over complete defoliation.

This approach can be especially useful in restraining the strong growing parts of one’s bonsai and in strengthening weaker areas. And it will also help in encouraging survival and growth of buds and branches well inside the tree canopy. Note that this response is in contrast to the effect of complete defoliation which tends to favor growth from strong branch ends on strong branches at the expense of weaker branch ends on weaker branches. In short, unless growth is reasonably well balanced, a good practice is to defoliate the strongest branch ends only and leave weak branches undisturbed. Kathy Shaner has described defoliated branch ends in Japan looking like antlers rising out of a green ball of retained interior foliage.

Large Leaf Removal:
A less disruptive alternative to complete or partial defoliation is to remove just the most conspicuous leaves, those which stand out as being larger than the rest of the crop, whenever they become apparent. This not only makes the tree look better, it slows growth in the area where the leaf was removed (again balancing growth), and it also opens the interior of the canopy to light and air circulation. In Jerry Meislik’s words, “When you see a big leaf, take it off.”

Another partial defoliation possibility, sometimes applied on maples and other opposite leaved trees, is to remove one leaf from each pair in an alternating pattern leaving a leaf on one side of the shoot at one node and on the other side at the next node.

Leaf Blade Shortening:
One more foliage reduction practice, perhaps the least weakening of all, is to remove just part of each large leaf blade. Incidentally, Japanese beech (Fagus crenata) is one tree species on which there seems to be almost universal agreement that it is best not to defoliate completely. Yet, it is often recommended that leaf surface of overly large Japanese beech leaves be reduced by clipping off half or two thirds of each leaf blade. This technique, said to not only limit growth but to activate buds that would otherwise remain dormant, can also be used on other species.

Other Notes:
Reading bonsai literature, it soon becomes clear that some species respond better to leaf pruning than others. Defoliation of Maples (Acer species) is often recommended. Other kinds also mentioned with regularity as being tolerant of leaf pruning are Hornbeams (Carpinus), figs (Ficus), ginkgo (Ginkgo), elms (Ulmus) and zelkova (Zelkova).

Experienced growers often recommend limiting complete defoliation to two out of three years or to every other year to keep one’s bonsai healthy.

A little personal experimentation can be very helpful in determining which approach will work best for you.

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