Remove Circling Roots When Planting
Circling roots can cause a plant to decline. Circling/girdling root systems begin in the nursery with plants that are too large for the container size or kept too long in the pot. Some roots hit the edge of the container and circle; other hit the edge and dive down. Both can cause the root mass to be so dense that it prevents roots from penetrating into the landscape soil after planting. They can weaken a root system and predispose a plant to root disease. Circling roots can also girdle the plant as it grows and eventually kill it. The plant decline from circling/girdling may take months to cause observable damage and sometimes takes years before plant death occurs.
Circling roots should be evaluated for removal at the time of planting, but root removal can stress the tree. Stress from root removal will only last for a relatively short period and will decrease with time as the tree recovers from the loss of the root. Leaving the root defect in place can increase stress with time. UF/IFAS research has showen that 25% of the roots can be removed without impacting the appearance of the tree.
Circling roots can be found on container-grown trees, field-grown (balled-in-burlap, or B&B) trees, or bare-root trees. Eliminate this root defect by cutting roots at planting. First, carefully remove the soil from around the base of the trunk so you can see the root flare. Then, make clean pruning cuts back to a lateral root if practical. Cut roots at the point where they begin to circle so new roots that grow from the cut will point away from the trunk. New roots will grow more-or-less parallel to the direction of the root section just behind the cut, but some could grow in as much as a 90 degree angle to the cut. Certain species such as magnolia, maple, holly, and others appear especially sensitive to this root defect; however, any tree can be affected by circling or stem girdling roots. Removing or cutting roots that circle, touch the trunk or cross over main roots can improve tree health in most circumstances.
More information here: http://hort.ufl.edu/woody/remove-circling.shtml.