Why Limbs Fall in Your Yard.

• Position: TREES

Travel around a neighborhood after a storm and you will see tree limbs, large and small, scattered about the ground. Why do some limbs fall in high winds or after ice storms while others merely bend? Should you worry about that large limb overhanging your driveway?

"One reason trees fail is weak branch unions," says Tchukki Andersen, staff arborist with the Tree Care Industry Association, Londonderry, N.H. "Homeowners can educate themselves about tree limbs, but they should call a professional arborist if they are worried about an overhanging branch."

Trees may suffer from naturally formed imperfections that can lead to branch failure at the union of the branch and main stem. There are two types of imperfections that create weak unions: a branch union with included bark and an epicormic branch.

Branch unions can be characterized as strong or weak. Strong branch unions have upturned branch bark ridges at branch junctions. Annual rings of wood from the branch grow together with annual rings of wood from the stem, creating a sound, strong union all the way into the center of the tree.

A weak branch union occurs when a branch and stem (or two or more co-dominant stems) grow so closely together that bark grows between them, inside the trec called "included bark," As more and more bark is included inside the tree, the greater the likelihood that this weak union will fail.

In storm-damage surveys conducted by the Forest Resources Department at the University of Minnesota, Minneapolis, 21% of all landscape trees that failed in windstorms did so at weak branch unions of co-dominant stems. Some species are notorious...