Root pruning is practiced to encourage the development of fibrous roots which are the plants’ suppliers of food and water. Plants that are being readied for transplanting or that need invigorating, and trees whose roots are over-taking gardens, lawns and paths are often root pruned.
When fruit trees consistently fail to set fruit, though all other conditions are favorable, the grower may resort to root pruning. In the fall a trench about two feet deep and six feet from the trunk is dug around the tree. The trench exposes the big anchor roots for cutting. If no big roots are found, there is very likely a wild taproot that must be located and cut. Any ornamental tree that has spread its roots out into areas where they are not wanted can be treated in the same way. A metal or cement barrier set in the trench will prevent subsequent spread.
When planning to move a deciduous shrub, it’s a good idea to prune its roots by forcing a sharp spade into the soil during the summer. In response to the pruning, the plant will develop more roots and so become easier to take up fall. Sometimes judicious root pruning force a recalcitrant flowering shrub into a system of root pruning and sometimes used to keep tub plants small.
In the nursery, trees and shrubs are lifted several times or planted wide apart, roots pruned regularly until they are sold. These methods force trees and shrubs to mass of fibrous roots rather than a few wide-spreading ones that would make it difficult to move and establish successful nurseries, special machines are used to roots under as well as around the plant.