Pruning and Pinching
by Brent Walston, Evergreen Gardenworks
Editor's Note: This article appeared in our May 2005 newsletter.
Pruning and pinching deciduous plants is an integral part of bonsai. It is how we create and maintain the fine branch structure and the pleasing outline that is essential to the beauty of the plant. Most people learn how to do this by rote, not really understanding the growth principles behind the methods they practice. This article explains in excruciating detail the why of pinching and pruning.
Pinching versus Pruning
First, let’s discuss the role of ‘pinching’ as opposed to ‘pruning.’ Pruning is used (with reference to branches) to shape the branch, change the direction of the branch, and create taper within the branch. This is all done by pruning back to a bud aimed in the direction that you want it to go.
Pinching can achieve the above for very small branches on some species, but for the most part, when we speak of pinching, we are talking about achieving ramification, or twigginess. When pinching the scaffolding of the branch is complete, it only needs detail work or ramification. This is done by removing the tip of the growing branch, usually removing two nodes of a three-node shoot (a node is where buds appear or leaves grow). This does two things: 1) it shortens the branch, 2) it releases the buds behind it, interrupting apical dominance. The buds are held from opening by the release of a hormone (auxin) manufactured by the last (terminal) bud. Now the remaining bud is free to open and the buds behind this one.
The result is that by pinching out the terminal bud, or removing a two- or three-bud shoot, several of the buds remaining on this stem are free to open. Usually one or two will open before the new terminal bud starts forming the hormone and stops the process. Thus, where you once had a soft straight shoot with increasing internodes (usually), you now have two buds opening forming a forked branch with shorter internodes and achieving ramification.
Achieving Shorter Internodes
I emphasized increasing above, because what most people have never bothered to observe (at least I never see it mentioned) is that the first two or three internodes of a released bud (achieved by pruning) are shorter than the internodes of the rest of the expanding shoot. By pinching back to one or two buds, we take advantage of this phenomenon, keeping the short internode sections and discarding the strong shoot with the longer internodes. This is most important as we reach the outer portions of the branch which, like the branches on the trunk itself, should have leaves and nodes closer together as we approach the extremities.
Internode length is also influenced by the season and growth rate. Recently repotted plants will form longer internodes. Plants heavily fertilized in the spring, or pruned in the winter will also form longer internodes. Plants fertilized and/or pruned in early summer (but not spring), will form shorter internodes.
Alternate Versus Opposite
In addition to this, some knowledge of the species is important, because not all woody species react in the same way to pruning (although in general they do). There is also the major distinction that divides the woody species, those that have alternate leaves (buds), and those that have opposite. Alternate species are in general easier to work with, since the last bud will open before the one before it. This creates a strong main branch extension from the last bud and a weaker, smaller branch fork (secondary) from the next to last bud. Just keep this in mind and nature will do the rest.
In opposite species, the two buds are directly opposite each other and will often open simultaneously with equal strength. If not corrected this will give a fan pattern rather than a strong branch.
You must form a branch before you can ramify it. It doesn't matter how many internodes are in the first section of the branch, the only thing that matters is whether the branch will sprout from where you cut it to form a bend or create a secondary (side branch) and a new main extension. In general, if you remove the end of a branch, or otherwise prune it, you will change its direction, since a bud on the side of the branch will break and start the new extension in a different direction. The same is true for chopping the trunk. If you want a straight branch, don't prune it. Pinching buds at the end of the branch, means just that. Remove the one terminal (and its leaf) and leave the other buds and leaves alone. The bud at base of the next leaf will be the first to open and form a new shoot in a new direction. The rest of the branch may or may not have leaves, it is of no concern to us here. A branch for pinching purposes constitutes one straight section, or twig, with buds and possibly leaves but with no side branches. Each branch, twig, or shoot if you prefer has only one terminal bud, at the very end of the shoot.
Consider the Species
The other thing that you need to know, is how each species grow. For example, how Ulmus parvifolia grows in response to pruning. This species is dense with dormant buds embedded in the bark as well as the single bud contained in the leaf axil. When you prune this species, you get branches all over the place. This is the beauty of this species for bonsai, and also the reason I tell folks not to worry about branches until the trunk is formed, since you can easily get branches wherever you want them by simply pruning it hard.
Pruning this species hard will give you a little fuzz ball of branches. These are very soft and succulent, and there are very many more than you need. If you begin to prune them off as they emerge you weaken the plant and many of them will die back. Let them grow out about six to eight inches, or more, so they can harden and have a better chance of surviving. Then you can remove the ones you don't want and start thinking about what to do with the ones remaining. If you want one to fork at a certain location, prune it there. You can see that you must have an image of the tree that you want to obtain before you can proceed.
One the other end of the spectrum you have species like beech, Fagus, that have strongly determinate buds. Beech will usually only form one set of buds and leaves a year, although an occasional hard pruning will force dormant buds to break. Since you can't rely upon buds breaking wherever you want them, you must plan ahead and rely upon pinching as your main tool of branch formation as well as for ramification.
In a beginning bonsai book, you might get the advice, “Let a shoot grow three sets of leaves and pinch it back to one or two.” While this, in fact, may be good advice, you can see from the above that it’s not quite that simple. Knowledge is power.