This problem is a serious cause of many trees in a landscape. Circling roots, rope, string or wires that are left behind after transplant are usually the cause of girdling. It generally takes many years after transplanting to discover this condition and usually causes the death of the tree or shrub.
Girdling is caused by some sort of restriction of the base or stem of the plant. Generally these restrictions are twine, roots growing in a circle close to the trunk, tags which were not removed or wire left on the tree after staking. As time goes on the tree grows and gets bigger, any of these restrictions will impede the transport of materials necessary for the growth of the plant and cause the plant to decline. Usually the damage is not seen for quite some time and once seen it is too late to save the plant.
Generally the plant with this type of problem will have reduced growth and less vigor than what is expected. Branch growth may be slowed, leaf colour may be lighter and or scorched and the plant will not respond to good cultural practices such as watering fertilization and pruning. Another good indicator is the lack of a buttressing root on one side of the stem. By digging up some soil on that side you will usually find a circling root. By checking for swelling of the stems you may find girdling material such as twine wires or rope. Often these wires are left behind when the tree was staked early in its life for stability. Often the culprit will be a nursery tag used for identification left on too long.
If a girdling root causes the problem you can remove it IF it is not too large. If the offending root is too large you can cause more damage to the tree by removing it. These girdling roots are best prevented at the time of the original transplant. By making 4-6 vertical cuts through the outer edge of the root ball should be enough to change the pattern of the root mass. When transplanting remove all tags, wire, or twine. Try to remove as much of the burlap as possible. By making your hole at least 2 times larger than the root ball you will encourage outward versus circular growth of the roots. If you need to stake the plant ensure you remove them in the next growing season. By following these simple steps you will be well on your way to preventing a problem with your trees and shrubs in the years to come.