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 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.

Growing Bonsai

Growing Bonsai

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