After a certain length
of time and growth, whether in ground beds or the field, or in containers
of any size, close spacing of tree seedlings has some very detrimental
- restricts horizontal
- may reduce light to
inner lower leaves
- reduces secondary
branch development and
- reduces the stem flexing
by air movement.
Each of these factors
can stimulate vertical growth, which is not desirable.
Seedlings of nearly
all tree species begin life by forming a single vertical stem
on which there are leaves positioned from the tip down almost
to the soil line. Each leaf contributes energy for growth,
with that from the younger leaves assisting terminal growth,
while that from the older leaves goes mostly to stem diameter
increase and root growth.
Since stem diameter
increase and root development are primarily served by energy
produced in the older leaves and those on the lower branches,
anything which stresses these or affects their production
will directly affect growth.
Many cultural factors
can affect these leaves, including nutrient deficiencies (nitrogen,
phosphorus, potassium or magnesium), toxicity of excess micronutrients
such as manganese, copper or boron, or damage from sodium, bicarbonates
or chlorides. Anything interfering with normal root activity such
as root rot or other diseases,insect or nematode damage, being root
bound, persistent overwatering or with a poor drainage and low oxygen
situation in the soil. Getting these conditions right is the challenge
of growing a good seedling. What isn't generally realised, however,
is that they can also be seriously affected by overcrowding which
prevents light from reaching them to allow photosynthesis to occur.
How much space is enough
to prevent this? The smaller the pot and the closer together they
are, the sooner the plants will need to be moved on and spaced to
avoid the loss of light to the lower leaves. Sooner or later, plants
in any size pot will need to be given additional space between them.
This cannot be done on a predetermined calendar date, since it depends
on the growth rate, the species, the time of year, the fertilizer
levels and the other factors that control growth.
With additional space,
most tree species develop more side branches; spacing is critical
for this, and leaves on these branches are the primary contributors
of energy for stem and root growth. These branches should not be
removed prematurely, and the criterion for the timing of this is
that sufficient stem diameter and taper must have developed to give
support to the top of the tree before any are taken off.
When the lower branches
are prevented from forming, or are densely shaded, or are taken
off too soon, the stem development is typically like a piece of
pipe: that is, all sides of the trunk are parallel. Once this situation
develops, it is likely to persist even if the spacing is improved.
The foliage is mostly at the top of the tree, and the energy which
it produces is used first there. Any excess travels down the trunk
in the phloem tissue, but is tapped by what branches remain so that
little reaches the trunk at the base of the tree to cause the growth
that gives the desirable taper. In addition to this, the more you
prune off the lower branches and reduce leaf surface, the more the
top extends vertically trying to produce more leaf surface to replace
what has been lost.
On the other hand, once
the tree has produced a good taper, the lower branches can gradually
be removed without losing the taper. These trees usually have a
superior root system to match their strong trunk, which supplies
more water and nutrients to the leaves and in turn increases the
amount of energy available for all parts of the tree to draw on
Trees with a good taper
should not need staking. Their trunk allows them to respond to wind
by bending along their whole length like a good fishing rod. Trees
with no taper that have to be staked, flex just above the soil line
in a wind, and are easily damaged.
Dr Carl Whitcomb
is president of Lacebark, Inc. Publications and Research, Stillwater,
OK. Tel 405 377 3539
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