Using an underground blade in a manner similar to that of the cutting bar just described, nursery growers prune back the root systems of shrubs and young trees. Done in the fall after the plants have ceased their active growth for the year, the pruning technique promotes a compact root system. Such a root system gives the plants a better chance of surviving their eventual harvesting, handling, and transplanting. Without root pruning, most of the fibrous nutrient – and water-absorbing roots would be lost at the time of harvest, and nursery plants would have little more chance of survival than plants collected from the woods.
For larger trees and shrubs, the sickle bar technique is inadequate. Instead, several months to a year in advance of the harvest, a trench must be dug around each plant just outside of what will be the soil ball at the time of harvest. The trench should be at least the width of a spade and as deep as the soil ball will be. The trench is then backfilled with sand. This process cuts the lateral roots of the plant and encourages new ones to develop within the soil ball. The sand discourages root growth out of the ball.
Containerized nursery plants do not require root pruning since the entire system is intact within the container. If kept too long in the container, the plants may send roots out through the drainage holes and into the soil of their holding area, making it hard to lift them. This can be discouraged by spreading a layer of heavy black plastic beneath the containers. The plastic also discourages weeds from growing.