When you are moving an established tree or shrub, you will increase your chance of a successful transplant if you root prune in the spring or fall prior to digging.
Root pruning should only be done when the tree or shrub is dormant; after the leaves of deciduous trees have dropped in the fall or before bud break in the spring.
There are two methods of root pruning – spading and trenching.
Spading is the simplest method and is sufficient for small/young trees and shrubs. It involves using a sharp spade to cut straight down through the soil about 12″ deep around the entire tree or shrub. This should be done a few inches closer to the trunk than the size of the root ball you will eventually dig. This slices through the roots and stimulates the growth of new feeder roots. Be sure that the spade you use is sharp so it makes a clean cut.
Trenching involves more work but is the better method to use when you plan to move larger, more established trees and shrubs. With this method, you basically dig a 12″ – 14″ deep, 6″ – 8″ wide trench around the tree of shrub. The outer edge of the trench will define the outer edge of the root ball. After the trench is completed, backfill the trench with a mixture of the native soil and some good quality compost. Feeder roots will develop and grow into this loose soil. It may take a while to get good root production throughout the trench so you may want to do this a year in advance of moving the tree.
After root pruning (regardless of which method you use), water the tree or shrub well, apply a good organic fertilizer like Espoma Plant-tone or Holly-tone, and mulch with 3″ of a good quality organic mulch. Be sure to provide water as needed until the plant is dug.