Source: Penn State University Extension

Pruning the roots of field-grown trees is a practice nurseries use to slow growth, develop a high concentration of roots in a smaller root ball, or prepare a tree for off-season digging. To minimize transplant shock, Penn State University Extension educator Emelie Swackhamer recommends root pruning several months to one year in advance of a move, depending on the size and type of the plant.

Pruning the roots will encourage the plant to produce a flush of new feeder roots, which are responsible for absorbing most essential nutrients and water. The goal is to allow the plant to develop new feeder roots within the zone of the future root ball that will be moved. This will reduce transplant shock.

Before root pruning you should consider the size of the root ball that will be moved. The greater the root ball diameter, the more roots will be included in the move. Also remember that bigger root balls weigh more. Consider how the plant will be lifted and moved. Ball carts, wagons, tarps, or thick folded cardboard can be helpful in transporting the dug plant to its new location.

When to root prune depends on when you wish to move the plant. For most plants, root pruning is recommended in the fall, followed by transplanting in the spring. This allows the plant to grow new feeder roots in the pruned zone over the winter without the burden of supporting new growth. For larger plants, you may want to root prune one year or more before transplanting. Keep in mind larger plants will need more time to become established after transplanting. Alternatively, root pruning in the spring for a fall move is possible; however, the root pruned plant will need to be watered during summer dry spells. Some plants do not respond well to being moved in the fall, especially those with thick and fleshy roots (e.g., magnolia, tulip poplar, oaks, birch, rhododendrons, hemlocks, and flowering dogwood).

Methods for root pruning vary. One method called spading involves cutting through the existing roots with a spade, making a circular cut all the way around the plant. The edge of this cut should be just inside the edge of the future root ball. Spading works best for small plants. Once the roots are pruned, special care should be taken to assure the root ball receives sufficient moisture, especially in the event of a dry fall or winter season. Check for soil moisture levels by feeling the soil. If the soil is dry two to three inches below the surface, give the tree a good soaking, assuring that the trench area is well watered. A 2- to 3-inch layer of mulch over the root ball but not in contact with the trunk or stems of the plant can help hold moisture in the soil and also protect the roots from cold temperatures during the winter.