Growing Black Pine for Bonsai

by Brent Walston

Introduction

I am always giving this subject a lot of thought. I consider the training of black pines, Pinus thunbergii, to be one of the most difficult aspects of bonsai, as well as one of the slowest goals to achieve. After over fifteen years of playing with them, I am only now beginning to get to the ramification stage, and I emphasize 'beginning'. Of course I am not talking about one inch trunk Shohin here, but full blown three inch trunk monsters like you see in the books. Most of the finest ones I have had the privilege to see have been in training for approximately fifty years or more. This seems to be the general minimum age for really fine trees. I think it can be done faster than that, but it is still a long term struggle, not the least of which is the learning curve.

I have now classified the techniques into three basic categories. These are based on the physiology of the plant's growth response to pruning. Understanding the response to pruning gives you the tools to design and train your tree.

Removing Candles in their Entirety

This is the pruning technique that John Naka suggests and the one I have mostly relied upon to shape my trees. It is slow but gives the most refined growth. Removing a candle at its base will stimulate the dormant buds in that node (whorl) primarily. These buds will produce shorter secondary candles, the length depending mostly on the timing of the pruning. Prune early and they will be long, prune late and they will be short.

You can manipulate this principle to get just the right size candles for the next branch extension. Timing can vary even within the same tree. Prune the candles of the lower, weaker branches early for longer secondary candles, prune the upper, stronger candles later to slow down the growth of its secondary candles. In this fashion you can balance the growth of the tree and make the new growth even or shorter in the top of the tree for a finer apex.

Pruning candles back to the node will always produce mature, typical wood. That is what I like about this technique. You can rely upon the response to produce fairly short needles with strong nodal response, that is, there will a strong node at the base of the candle and at its tip (the following season). This is in contrast to juvenile growth discussed below.

Pruning Below or Between Nodes

Pruning between nodes gives a different response, and in general produces atypical, juvenile growth. Juvenile growth arises not from a dormant bud at the node, but from an adventitious bud along the internode at the base of a needle. The color is typically bluer, the needles finer, stem slightly wavy and thin. After a year of two (after the needles are gone) this juvenile growth will be indistinguishable from stems of mature growth.

This kind of pruning can be thought of as 'forcing growth', or forcing the stem to back bud. It is pretty reliable as long as the needles are still present on the remaining stem section. Removing the node and its terminal buds removes the flow of auxin that keeps the adventitious buds dormant. This releases the buds at the base of the needles, and often it releases almost ALL of them, so a typical response is produce a 'broom like' effect with very many shoots just below the cut, one for each needle.

This kind of pruning can be used when you have branch or branch section in just the right spot, but the internodal distance is too great. To save and use the branch you have to make a bud break somewhere in the middle so you can prune it back. The object in training any of the branches of a black pine is to produce a series of forked branches with the length of each fork DIMINISHING as they proceed toward the end of the branch. This is usually accomplished by timing the secondary candles as explained above. However, sometimes you blow it, or you acquire a pine with branches in the right position, but with internodes are too long, particularly the first internode.

If there are still needles on the section you want to stimulate, simply cut the branch off just before the node (whorl) at the end of this section. I usually do this in late winter, but I don't think the timing is all that important. This is another fuzzy area for me, and I will probably know the answer in another five years. I did learn one new trick though. A colleague of mine prunes in late winter as I do, but doesn't prune the needles until later, usually in July. Removing the tips of the needles further reduces the auxin flow and will help in releasing the adventitious buds. It certainly works for him, he creates branch stubs with little fuzz balls at the end of them after one year. So cut the needles in half on the remaining branch stub in summer. New buds will be evident by fall.

If the section you want to stimulate doesn't not have needles (older wood) you cannot use the above procedure. Conifers are very sensitive to the hormone feedback system between the roots and the branches. I think this partly due to the long lag time (much longer than deciduous plants) for bud break. Strongly growing branches send the hormone auxin to the roots (actually specific roots) which stimulates those roots to increase their growth to support the new foliage. Roots thus stimulated send more of the hormone cytokinin to the branch to stimulate stronger branch and foliage growth. If the roots fail to receive the auxin signal which is produced by the terminal buds and foliage, they will cut off the cytokinins and eventually wall off the area causing the branch to die.

This is what we call 'keeping a sap line' in conifers. It is true of some other plants as well, but is critical for conifers. That is why you hear the warning about always leaving some foliage at the end of each branch when pruning pines and junipers.

So what do you do to stimulate buds in older wood where the needles have long since fallen? You can start 'forcing' the growth back by starting farther up the tree, or farther out on the limb. Prune just ahead of a node that DOES have needles left on it. This will stimulate bud breaks before it. In addition it will often produce a very few adventitious breaks on the older wood in the internodes (or nodes) in the needleless sections. Once these buds break, be very careful with them, they will be extremely fragile, and it probably won't happen twice. Let them grow undisturbed for two years, then begin forcing growth on them. After a few more years, you may be able to force growth back even farther by hard pruning of these stems.

The key to a strong bud breaking response is to have a very vigorously growing plant. There must be a strong root system and strong upper growth as well. Before doing any of this work, invigorate the plant by repotting it, or planting it in the ground, giving it optimum water and sunlight, and fertilize it adequately. Alternate heavy pruning years with light pruning years to allow the plant to 'pump up'.

Pruning Between Candles

I haven't done much of this, since I was so strongly influenced by Naka, but apparently Ernie Kuo has done a lot of it and has published an article on it. Many people remove only part of the candle instead of the candle at its base to achieve ramification. They might even remove various fractions of the candles, one half, one third, two thirds, etc. The object here is not necessarily to stimulate new shorter secondary candles from the node, but to use existing candles that would otherwise be too long. By cutting the candle you stimulate the adventitious buds at the base of the new needles near the end of the cut and you will get two or more bud breaks in the same season. Ernie uses this technique to get more than one set of forked branches (ramification) in a single season. I think this technique is probably best suited for warm, long growing season areas.

And finally

There are also some general growth issues for training black pine and design considerations that must be tailored to these pruning techniques. These are addressed in the companion article to this piece: Training Black Pine for Bonsai