Why Limbs Fall in your Yard
March 23, 2006
Travel around a leafy 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 Peter Gerstenberger, senior advisor for safety, standards
and compliance with the Tree Care Industry Association. "Homeowners can educate themselves about
the 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 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 tree. The term for bark growing inside the tree is "included
bark." As more and more bark is included inside the tree, the weak union is formed that is more 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: European mountain ash, green ash, hackberry, boxelder,
willow, red maple, silver maple, Amur maple, cherry and littleleaf linden.
Epicormic branches (also called water sprouts) are formed as a response to bad pruning, injury or environmental
stress. Epicormic 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.
Epicormic branches, by their very nature, form weak unions because they are shallowly attached 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 and may fall to the ground
because the underlying wood cannot support their 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. 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.
Incompetent Tree Trimming
Falling limbs are also caused by poor pruning practices by licensed and unlicensed contractors. Lions
tailing is one of the major causes of limb failure. After this type of pruning the ends of the limbs grow, some
times 4 times faster than usual, causing the tip weight to increase at a faster rate than the limb can support.
Cabling or actually the lack of cabling on old growth, very large trees allows the weight of the entire limb
foliage and wood weight to increase beyond the capability of the union at the trunk. Most companies choose
to remove large limbs like these or cut them in half damaging the tree to a degree it will not recover from.
When simple cabling will solve the problem with NO damage to the tree.
Partially Reprinted from Tree Care Industry Magazine
For more information please contact your local tree care specialist.
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