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Root Pruning and Root Restriction
Removing or pruning a portion of a tree’s root system will temporarily
reduce the top growth of the tree. The ancient art of bonsai is
dependent on this technique of root pruning. Time of root pruning
and distance from the tree trunk have an effect on the degree and
length of growth control achieved. Root pruning in the spring near
bloom has a greater dwarfing effect than root pruning in mid- or late
summer. When a tree’s roots are severed, growth of new roots is
stimulated in the area of the cut. The tree’s natural root:shoot equilibrium
is upset, and, in response, assimilates are directed away from
top growth to the roots. The result is a decrease in shoot growth. As
new roots are produced, the root:shoot equilibrium is reestablished
and the dwarfing effect is lost. When properly applied, the effect of
root pruning may last an entire growing season. Other complex and
interactive physiological mechanisms also contribute to the change
in top growth when trees are root pruned. Absorption of water and
nutrients is reduced, and hormone synthesis, particularly cytokinins,
is decreased. A reduction in water absorption results in water stress,
which reduces transpiration, causing stomatal closure and a reduc-
70 CONCISE ENCYCLOPEDIA OF TEMPERATE TREE FRUIT
tion in photosynthesis. In combination, these effects contribute to reduced
shoot growth. Because many of these physiological responses
to root pruning are short-lived, root pruning may need to be repeated
several times during a growing season to achieve effective growth
control.
Dwarfing can also be achieved by restricting roots to a small area.
This effect is readily observed in potted plants or in the culture of
bonsai trees. In the field, hardpans restrict root growth to a shallow
soil, resulting in dwarfed growth. In Australia, soil hardpans restrict
root growth and allow apple trees to be grown at higher densities on
vigorous rootstocks with reduced shoot growth. Planting trees at high
density results in root competition and restricted horizontal spread of
roots, a form of root restriction. Under these conditions, roots grow to
greater depths in the soil. More recently, in-ground fabric containers
have been used to restrict root systems, thereby reducing shoot growth.
The physiological effects of root restriction have not been well studied
in fruit trees. Unlike root pruning where root growth is stimulated,
root restriction does not result in new root growth. When roots
are restricted, root density increases, leaf transpiration and photosynthesis
decrease, and foliar nutrient levels decline. These changes are
probably responsible for reduced growth.