Root pruning is the process of severing the roots of an established tree that is going to be dug and transplanted, to encourage the growth of new feeder roots along the root ball that will be transplanted along with the tree.
Established trees that have been growing in the ground have roots that reach out far beyond the branches or drip line. These long branches are used by the tree to anchor and support it. However most of the small feeder roots, which bring in food and nutrients to the tree, are likely to be found growing off the main roots at some distance from the tree itself.
When a tree is dug for moving and transplanting, generally the root ball taken is only the circumference of the drip line, sometimes less. To encourage the development of feeder roots closer to the drip line, root pruning is done.
Root pruning involves severing the roots of a tree, all the way around the tree's circumference at the drip line. This can be done by slicing down with a sharp spade. Ideally, root pruning should take place a year prior to digging and transplanting the tree. However in a pinch, 2-3 months should allow enough time for the tree to overcome the stress of root pruning and to start the process of developing new feeder roots.
Root pruning is also sometimes used to maintain a dwarfed size and to encourage flowering of a fruit tree or slow to bloom vine, such as wisteria.