The following is information on dealing with circling roots in trees and shrubs in the nursery and prior to planting in the landscape.
Dr. Ed Gilman, tree researcher and professor at Univ. of Florida, suggests we need to take drastic steps to correct tree root defects in container grown trees and shrubs. Rather than just slicing, he advises removing the outer inch of the rootball from container-grown trees. To see photos on how to do this, go to http://hort.ifas.ufl.edu/woody/circleremoving.shtml. Any large diameter surface roots should also be cut with pruners just before the points where they kink or bend. New roots should branch from those cuts and grow outwards as nature intended (unless planted too deep, but that is another topic), establishing a good root system for the long run.
These practices can certainly add stress to the tree during establishment, making frequent watering during the first season more critical than ever. Root defects are most common in container production because hitting the container wall disrupts the natural outward direction of root growth, instead causing the roots to circle and/or dive downwards. However, even B&B trees may have hidden root defects from container growing early in the production cycle, or from roots bent when planting bare-root liners into the production field. Once the roots become woody, their kinked shapes are permanent defects.
Good nursery practice includes root pruning each time the plant is stepped up, but that is not always done. If not corrected when planted into the landscape, the circling roots near the trunk continue to enlarge and may end up girdling the stem. Even if stem girdling does not result, few branch roots develop to the outside of circling roots, which can cause the tree to be unstable and have a reduced feeder root system. Most people don’t realize that tree decline and death in 10-20 year old trees may have been predetermined by the root architecture present when planted.
Root defects are less severe on shrubs because they typically have more fibrous and branched root systems than trees. Most shrub root problems can be prevented by purchasing plants that are not root bound and properly planting them in decent soil. Slicing exterior circling roots and removing any matted root layer at the bottom of the pot should give a shrub a good chance of living a long and productive life.
So, to slice or not to slice? I say “yes”, do slice but don’t stop there. The green industry as a whole needs to address root defect problems from propagation to landscape establishment, to help stem the epidemic of tree failures in the landscape. As a landscaper, retailer or producer, you can help by inspecting roots and refusing to buy (or sell) trees and shrubs which are rootbound or have visible root defects. Growers should consider using containers which have been designed for air-root pruning and step up unsold trees and shrubs into larger containers rather than holding them in the same pots once roots begin to circle. Teach your workers how to prune roots when transplanting, whether the plant is going into a larger container or into the landscape.
Information from Cathy Neal, Ph.D., Specialist in Ornamentals, University of New Hampshire Cooperative Extension Adapted from her article in News & Views for New Hampshire’s Green Industry, University of New Hampshire Cooperative Extension, July 2009.