Winter-Damaged Trees May Need Pruning

By Laurie Casey

The Morton Arboretum

This time of year wind and ice storms may threaten to break weakened tree branches and send them hurtling at homes, cars or people. It's wise for you or a certified professional arborist to inspect your trees every winter for structural problems. Corrective pruning this month, while trees are dormant and leaves aren't obscuring their structure, can repair problems, according to Donna Smith, display horticulture supervisor at the Morton Arboretum.

Start by removing any dead, diseased, damaged, crossing or rubbing branches. Then attend to the tree's form. Trees should develop a straight central leader, along with well-spaced branches and a generally symmetrical crown, depending on the species. Here, prevention is the best medicine.

"Young trees need pruning to help give them a good start. In many new city and suburban subdivisions, people want to green their neighborhoods with new trees," said Smith. "If young trees aren't pruned well, structural problems become more pronounced and difficult to correct as trees mature."

For example, if the central leader is cut or broken off, two or more leaders may form. Instead of an "I" shape, the tree creates less stable "V" shaped unions. When the tree is young, it is possible to train a side branch to replace the broken leader. But if a mature tree loses its leader, it's almost impossible to restore good architecture. For this reason, never "top" a tree by lopping off the central leader (often done under power lines).

Another common structural problem to look for is a weak branch union, where two main limbs join the trunk. "The two branches begin growing so closely that bark grows between the branches, inside the union," said Smith. The weak bark can't hold the limbs together and the union may split or crack down the middle.

Trees that have an upright habit, with branches that connect to the central leader at less than a 45 degree angle, may develop weak branch unions. Bradford callery pear is especially susceptible to this problem. To remedy, an arborist might remove or shorten one of the limbs, or possibly insert a bolt to hold the union together.

Very old trees may begin to decay, enabling large branches or even the whole tree to get blown over in a storm. Internal decay may not be apparent, so an arborist should inspect the tree periodically. In some cases, a tree may be so hazardous it must be taken down. In an otherwise healthy tree, an arborist can run cables between heavy limbs in danger of breaking off, but the expense is often only justified if it's a special or valuable landscape tree. Otherwise, Smith doesn't recommend cabling: "Trees should stand on their own!" she said.

Laurie Casey is a staff writer at the Morton Arboretum in Lisle.