Pruning and Pinching
by Brent Walston
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 inexcruciating 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. This is usually done to a shoot with several
internodes to be removed or with a larger lignified (woody) existing
branch. It doesn't matter if you use concave cutters, your fingers,
pruners, or chain saw, as long as you do a neat job.
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
internodes 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.
Armed with this knowledge, you will be able figure out how to
achieve your goal. But of course you have to know what your goal is,
and that is always the most difficult part, especially for beginners,
who have not seen enough trees to know what they want to do.
Alternate versus OppositeIn
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, weak
secondary pattern usually desired for bonsai. If one bud is nipped out
as they expand, one can mimic an alternate species. The same thing is
achieved in the training of Black Pine, where great care is taken to
make sure that all the branches fork, whereas in nature they would form
whorls. Only after many years of wear and tear would they be reduced to
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
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
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.
And finallyIn 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.
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