Pruning Mature Trees

Proper pruning of large shade trees is important to the health of a tree and will greatly extend its life. However, proper pruning is not a simple task and is often done incorrectly, leaving the tree stressed by the pruning rather than vitalized. The proper way to prune a mature tree is determined by the tree species, its size, and its current growth patterns. Every time a branch is removed, there should be a good reason for it based on an overall plan for the tree.

Studies done by the United States Forest Service show that trees increase their growth rate when properly pruned. Trees must grow to remain healthy. A growing tree will produce more leaves to manufacture food, seal over and compartmentalize decay, have a more efficient metabolism, develop a ready and vital defense system to ward off insect and pathogen attacks, and be much more tolerant of environmental stresses such as drought and cold.

A properly pruned tree is a safer tree. With correct thinning, wind will blow through the tree instead of up against it and help reduce limb breakage in storms. Also, removing dead, diseased, and weakly adhered branches will ensure that they don't fall on property or people.

To properly prune a large tree, a person should have training in tree physiology or be under the instruction of someone who does. Understanding how trees respond to pruning in general, as well as an individual species' response will ensure that the job is done the right way. Here are a few guidelines that should always be followed when pruning mature trees.

Never prune branches in the middle or leave stubbed ends. This is known as topping, heading back, or pollarding. This practice is very damaging to a tree and will greatly reduce its life. A branch pruned in the middle has no defense against decay organisms and often a decay pocket will form at the cut and spread down the inside of the branch. This sets up a situation where the branch that was cut in the middle will grow increasingly weaker. Often, a large tree that was previously topped will drop large branches for no apparent reason even on a calm day. Thus, people who think they are making a tree safer by topping to its height have actually made the tree more unsafe. If a branch needs to be cleared away from a house, power lines or other obstacles, it should be removed back to the trunk or to another main limb.

Topping trees has other negative side effects. It removes the terminal growth on the tree. This shocks the tree and induces it to grow large amounts of suckers. These suckers are weak and less metabolically efficient than the original terminal growth. Because of the large volume of suckers that are produced, the branches will become top heavy and very thick within a few years.