Root pruning is the process of slicing through the roots at the drip line of an established tree that is going to be dug and transplanted. This is done to encourage the growth of new feeder roots along the root ball that will be transplanted along with the tree. A smaller root ball, with many feeder roots, will help the tree acclimate faster to its new spot in the ground.
Established trees that have been growing in the ground have roots that reach out far beyond the branches or drip line—the distance the branches reach out above the ground. 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 portion of the roots taken, the root ball, is only the circumference of the drip line, sometimes even less.
Since the tree or shrub will be dependent on this root ball for most of its nutrients and water, it will need plenty of feeder roots to continue to sustain itself during the shock of being transplanted.
To encourage the development of feeder roots closer to the drip line, root pruning, cutting off the long anchor roots, is done.