“Girdling roots” are roots that grow around other roots or the trunk of the tree, eventually restricting their growth and choking off any nutrients they carry to the rest of the tree. The girdling can be on one side of the trunk, or in more sever cases, will encircle the entire trunk, causing the death of the tree. It is normally found on trees that are have been planted for several years, which gives the girdling roots time to enlarge and restrict the nutrient flow to the rest of the tree.
It is becoming a problem especially in urban locations, where trees are commonly planted in undersized holes in sidewalks along store fronts.
Research has shown that girdling almost always starts when the tree is young, even before it reaches the planting site. It turns out that a large percentage of the trees that experience root girdling were grown in containers to start. Because they are in a container, the roots grow in a circular fashion, and once planted, they continue to grow that way, eventually growing back on themselves or around the trunk.
Another cause of root girdling is planting trees too deep. Make sure you plant them at the “root flare” where the roots start to flare out from the trunk. Also, make sure that the planting hole is plenty large, because if the hole is too small, especially in a tight clay soil, it may have the same effect as planting it in a container.