Travel around a neighborhood after a storm and you will see tree limbs, large and small, scattered about the ground. Why do branches fall in your yard from 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, CTSP, BCMA* and staff arborist with the Tree Care Industry Association. “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. This 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 with included bark
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. This can create 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 tree. The term for bark growing inside the tree is “included bark”. As more and more bark is included inside the tree, the greater the likelihood that this weak union is likely to fail.
In storm-damage surveys conducted by the University of Minnesota’s Forest Resources Department, 21 percent of all landscape trees that failed in windstorms failed at weak branch unions of co-dominant stems. Some species are notorious for having included bark. These species include European mountain ash, green ash, hackberry, boxelder, willow, red maple, silver maple, Amur maple, cherry, Bartlett pear, and little leaf linden.
Epicormic branches (also called water sprouts) are formed as a response to poor pruning practices, injury, or environmental stress. These types of branches are new branches that replaced injured, pruned, or declining branches. Commonly, epicormic branches form on the stems and branches of topped trees. When old, large epicormic branches are growing on decaying stems or branches, the epicormics are very likely to fail.
An epicormic branch, by its very nature, forms a weak union. They have a shallow attachment instead of being attached all the way to the center of the stem. Epicormic branches grow very quickly so they become heavy very quickly. After a time they lose their connection to the main branch. This may cause the branch to fall to the ground because the underlying wood cannot support its weight.
“If a weak union is also cracked, cankered, or decayed, the union is likely to fail, causing the branch to fall off the tree,” says Andersen. “Sometimes, ridges of bark and wood will form on one or both sides of a weakened branch union in order to stabilize the union. The branch is very likely to fail when a crack forms between the ridges.”