Root pruning a horticultural technique in which the roots of a plant are trimmed. While this might sound like an alarming proposition, root pruning can actually be good for plants when it is done properly, and it can resolve a number of issues associated with trees and shrubs. For root pruning, well sharpened, clean tools are needed, and some gardeners recommend keeping a separate set of tools just for work on roots, since grit from the soil around the roots tends to grind up the tools quickly.
Roots serve a number of functions for a healthy plant. They help the plant take up nutrients from the ground, and they also work to stabilize the plant. Some plants may utilize their roots to force out competition, as well, taking over the soil so that other plants cannot settle in it. When people engage in root pruning, they remove nonessential roots, leaving behind the small, delicate roots which the tree uses to access nutrients.
One reason for root pruning is transplantation. When trees and shrubs are transplanted, people often note that their roots are quite sprawling. Instead of trying to get all the roots, people root prune down to the root ball, the cluster of roots in the dripline which actually gathers nutrients, and then transplant the tree. The tree will put out new roots for stability later.
Root pruning can also be used to manage size, and to control plants being grown in containers. For bonsai in particular, root pruning is critical, as it will keep the roots from filling the entire container. When plants become rootbound, they have trouble intaking nutrients, and the volume of the roots can actually force the plant up out of the container, with the plant literally uprooting itself.
Another reason for this practice is to control buckling of sidewalks and pathways. Roots have a way of disturbing these features in the landscape, and while barriers can be put in place to encourage them to grow elsewhere, occasionally a determined root will snake its way into an inappropriate place. This root can be removed without harming the tree to prevent further damage to the walkway.
It is important to time this pruning activity carefully. Pruning tree roots does damage and shock the plant, although it will not kill it. It is best to prune roots in late fall or early spring, when the plant is fairly dormant. During peak growing periods, like in the late spring, root pruning can damage a plant.
