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The size of leaves, the length of internodes, and the thickness of twigs are related to the:
Of these five factors the first and
second have the greatest importance for bonsai, but the other three
also play a role.
To manipulate growth in bonsai it is
essential to be thoroughly familiar with this cycle and all of its
ramifications.
If you top prune a tree while it is dormant, you
remove outlets,
that is buds, for receiving food in spring, the result is the over
stimulation of the existing buds, that are released from apical
dominance (no more 'stop' hormones from the now cut off terminal buds).
This results in long internodes and huge leaves, gigantic whips on some
plants like apples and plums.
If you root prune a dormant
plant without top
pruning, you remove part of the food supply for stimulating buds and
new growth. The result is that released buds will form smaller leaves
and drastically shorter internodes. Plants seem to know how many buds
to stimulate and how much new growth can be supported by the roots. I
don't know the physiology, but I have seen it enough times to know that
it works. Root pruning succeeds in reducing top growth up to the point
that so many roots are removed that water transport becomes critical.
At this point, the plant cannot support any new growth and dies. This
is called overdoing it.
If you top prune a tree that has just leafed out,
you remove
the food factory that has just been created with most of the stored
energy of the roots. The roots will have to use whatever reserve is
left to issue a new set of leaves. This severely taxes the roots, and
the new growth will have shorter internodes and smaller leaves. We use
this principle in defoliation, although some time is usually allowed to
restock food in the roots first. Multiple defoliation will result in
ever decreasing leaf size. Liquidambar can be
defoliated three times a season if they are in good condition, and are
in a region with a long enough growing season.
The manipulations based on this principle allow one
to do all sorts of
things to control the speed and character of growth. This is the basis
for most of the pruning practices used in bonsai.
Roots confined in a space tend to get woody and begin to lose their
ability to store food readily. One of the first symptoms of a root
bound plant is chlorosis resulting from the inability of the aged root
system to take up essential nutrients. As the plant stores less and
less food relative to the amount of top growth that accumulates, the
leaves get smaller and the internodes shorter. This is one reason why
these trees are in tiny pots, aside from the aesthetic value.
Keeping a seedling a little on the hungry side and a little root bound
can dramatically shorten the first internodes. This becomes valuable
later on in the training process when the plant is trunk chopped to
introduce a low curve on the trunk and attain taper. The places where
the internodes formed will be dense with adventitious buds which will
break easily on most deciduous plants. This is particularly important
for the Maple genus, Acer. Most strongly affected
by this phenomenon is Japanese Maple, Acer palmatum.
Of all the nutrients, the one affecting leaf and
internode size the
most dramatically is nitrogen, so it is best to keep the level of
nitrogen balanced in the feeding program.
For more information about manipulation of growth in bonsai see the
companion article "Using Principles of Growth to Manipulate Plants for
Bonsai".
copyright 1996, all rights reserved
The
Balance of the Canopy to the Root Mass
The roots and top growth are in
constant cyclical
motion to stay balanced. In spring, the roots pump most of their stored
energy to produce a full canopy of new leaves, and then shoots for more
leaves. In summer the process reverses and the leaves resupply the
roots with food, and provide energy for increased top growth. In fall
the leaves stop producing food, but food continues to move down the
stem from tissues. The roots
continue to grow until the soil temperature falls below 60 degrees,
using some of their stored food to increase their mass, and yet retain
enough reserves to start the process again in spring.
The Age and Density of the Roots
New roots growing in fresh
medium easily absorb
water and nutrients and increase their mass very quickly, upsetting the
balance in favor of larger leaves and longer internodes due to the
excess food stored. The Level of Fertility
The
level of fertility is somewhat obvious. Plants with good nutrition have
normal size leaves and internodes. Virtually no one recommends not
feeding bonsai to reduce the size of the leaves and the internodes.
However, it is sometimes done to plants in training, particularly
seedlings in the first year or two to get a series of close internodes
low on the trunk. Presence of Absence of hormones
Some
hormones can affect leaf size and, particularly, internode length. The
most important of these hormones is probably gibberellic acid, which
can dramatically increase internode length. These affects have little
importance for bonsai.
Light
Levels
Plants
with good nutrition growing in the maximum of light that they can
easily tolerate will have the smallest leaves and the shortest
internodes.
And
finally
Spend
some time reviewing the first two growth principles, and thinking about
their ramifications. The answers to most questions about the timing of
pruning, and the correct procedure to achieve design objectives can be
deduced from analysis of these two principles.