Pruning tree roots is drastic, but can restrain growth or bring young trees into flower when other, less severe, measures such as summer pruning and spring bark girdling are ineffective. Pruning tree roots reduces vigour and promotes the formation of flowers instead of shoots. Mature trees should not be root pruned as they have less resilience than young trees.
Root removal is thought to lead to more carbohydrates in the shoots, which induces flower bud formation.
Root
pruning of trees and shrubs can be carried out over two years,
in winter, removing half the roots at a time, by digging a
trench around the tree, 30-40cm (12-16in) deep and wide, about
120cm (4ft) from the trunk. The trench is refilled at once
and the tree staked and watered in subsequent summers. A milder
effect can be achieved more easily by inserting a spade to
its full depth around the tree to cut the surface roots. Most
tree roots are near the soil surface, rather than deep in
the ground. If these treatments fail to have sufficient effect,
root pruning can be repeated working more closely, 50cm (20in),
from the trunk.
Shrubs and trees growing in pots are more easily root pruned. Here up to 20 percent of roots may be shortened by about a quarter, and the other roots trimmed to ensure the plant will fit back into the pot, and the plant replaced in its pot using fresh compost, with a controlled-release fertiliser such as Osmocote.