Trees which will be professionally planted on
new sites, are usually purchased from tree
nurseries where, they have been grown, they
have been re-planted several times and their tap
root removed. This gives a more compact root
ball which is close to the trunk compared to a
tree which has grown from a seed in one place.
When commercially grown trees are sold, they
are removed from ground by a three bladed
shovel or with what is more commonly used
now, a circular knife. The root balls are covered
by a bale cloth so that the roots are not exposed and the bale is then
enclosed in wire netting. The root system on a commercially grown tree
is considerably smaller than the bale on a naturally grown tree, hence the
reason for some type of securing system when planting.
The most commonly used method for stabilizing newly planted trees, is
a wooden tripod or its derivatives. This is not an optimal solution, neither
esthetically, nor with concern to the physiology of the tree. Consequently
more and more underground anchorage systems are being used.
Underground systems are superior to tripod systems allowing the tree to
be more esthetically displayed, and allowing the tree to develop correctly
and establish a healthy root system. This type of anchorage also removes
the problems associated with above ground securing, such as vandalism,
Test results
From the results, he was able to determine that trees
with foliage, which were planted in sandy humus
soil at Bad Zwischenahn in Germany, didn’t need
any additional anchorage up to storm force 10. Of
the trees planted, 74% were able to take up to storm
force 11 and 48% still didn’t need support up to storm
force 12. This determines that only a relatively small
support is actually necessary. In addition to the newly
planted trees, further trees with a standing time of
one to three were tested to determine how long a tree
should be supported until it can be left on its own.
Oaks, chestnuts, horse chestnuts, maples, sycamore
and linden trees were used, groups all of the typical
planting size of 25cm – 30cm diameter trunks as well
as solitary trees with 50cm – 70cm diameter trunks
were used. The results show that additional anchorage
should be used for dynamic and biological reasons.
Biodynamical background
A young tree strives to grow as tall as possible.
Without any stimulus by wind, most of the trees
resources go into vertical growth. During the
vegetation period, there is a constant flow of auxin
from the tree tops to the roots. This boosts the vertical
growth and therefore inhibits the budding of side
shoots, the growth of a wide crown and the main
root system. In contrast, in a tree that sways naturally
in the wind, phytohormone ethylene is produced.
This hormone inhibits the transportation of auxin
and creates a growth
of cells in a radial
direction. This results in
a redistribution of the
trees growth resources
with a reduction of
vertical growth and
the reinforcement of
stem growth and of
the main root system.
Therefore, trees should
be grown without
additional support or
anchorage.
60 Landscape & Urban Design
continual maintenance, the chance of litigation due to obstruction and
additional difficulties in maintaining grass around the planting.
The general thoughts and guidelines on underground securing is that
it is acceptable as long as the system does not damage the stem, crown
or roots and that the bale diameter is large enough and the ground is
appropriate. The bale should not be compressed or disturbed in a way that
would damage the roots.
There are several above ground and underground securing systems
available and it is important to make direct comparisons before choosing
what you consider to be the best for you. One essential question has
been answered by field trials. How much anchorage does a tree need as a
minimum? Extensive research by Dr Lothar
Wessolly of Germany who is recognized
as the worlds leading expert on tree
stabilization, and using his “inclomethod”
which uses the angle of inclination under
a so called “wind supplement load”
in connection with an analysis of the
wind pressure on a crown, you can get
a generalized inclination graph and can
determine the maximum wind force a tree
can safely take. Dr Wessolly conducted these
tests on more than 7000 trees.
Possible securing methods
There is a simple principle in engineering, that stiff or rigid systems are not able
to absorb and dissipate imposed loads. Dynamic systems on the other hand are
able to do just that. This means that a if a tree is supported or anchored by steel
cables or stiff tripods, all of the winds force goes into that construction, the tree
does not participate in the absorption and the support system is unnecessarily
strained. This results in the necessary over-dimensioning of these systems or,
if that is not done, the support mechanism will fail. The tree also allows for this
support in its system and remains too thin and weak for its height.
Test results also showed that rigid systems with steel cables that are kept under
tension, lead to a tightening of the cables and a constriction of the stem which,
results in a predetermined breaking point.
If the principle that load distribution, is supposed to function between the
tree and its securing system, the possible movements and the maximum bend
of the tree have to be known. This was also examined at a tree nursery at Bad
Zwischenahn, and it was found that it is