Timberlines

By: Shelly Van Landingham, Forester

November 28, 2003

Believe it or not, there is a science behind pruning! The difference between proper pruning techniques and incorrect pruning can result in a healthy tree, or a tree that has been seriously damaged. So let’s cut right to the chase.

Since each cut has the potential to change the growth of the tree, no branch should be removed without a reason. Proper pruning, with an understanding of tree biology, can maintain good tree health and structure while enhancing the aesthetic and economic value of landscapes.

The basic pruning cut is very simple, and will help the pruning wound "heal". A note on "healing": Trees do not "heal" like humans do. When a tree is wounded it must grow over and compartmentalize the wound, and the wound is contained within the tree forever. Cut as close as possible to the branch collar and do not remove the branch bark ridge. It is important that the branch collar is not injured or removed! There are actual chemicals in the branch collar that will compartmentalize the wound—removal of the branch collar destroys the tree’s major defense system, and can lead to excessive sprouting.

Sometimes branches do not develop a branch collar and/or a branch bark ridge due to the fact that they have grown too close to the trunk or other branches—they are "squeezed" together and are called codominant stems. These types of branch unions, referred to as "V" attachments, can lead to poor tree health, and possible dangerous situations as the tree continues to grow. The bark between these two branches is called included bark, but the branches DO NOT grow together—They actually push against one another, and are very WEAK attachments. These weak attachments are more likely to fail in high winds. It is best to correct this situation when trees are small, or when branches are small. Strong branch unions are attached at approximately a 45 degree angle. For pruning codominant stems, please see the diagram below.

It is important not to leave stubs either. Stub cuts are too far away from the branch collar, and the tree cannot compartmentalize the wound.

One of the main goals of pruning is to achieve a structurally strong tree that will have a much better chance to withstand damaging weather. A structurally strong tree has one main trunk with lateral branches attached at a about a 45 degree angle.

TOPPING hurts trees! Topping often removes 50-100% of the leaf-bearing crown of a tree. Since the leaves are the "food factories" of a tree, this can temporarily "starve" a tree. The severity of the pruning triggers a sort of survival mechanism. The tree activates latent buds, forcing the rapid growth of multiple shoots below each cut. If the tree is already stressed, say from drought, it may not be able to send out these shoots, and may die. I’ve seen a lot of this within the last two years of drought. The vigorous shoots that come out of topping are all WEAKLY attached! This means the tree will probably be more of a hazard a few years down the road than it was before it was topped. Topping also causes extensive decay within the entire tree because the cuts are not likely to be made at branch collars. Topping also completely opens up a tree to insect and disease problems. When trees need to be reduced in height and spread, try to remove branches at their point of origin. If a branch must be shortened, it should be cut back to a lateral branch that is large enough to assume the terminal role. A rule of thumb for this is to cut back to a lateral that is at least 1/3 the diameter of the limb being removed.

More pruning tips. Always use sharp pruning tools! Dull blades cause much injury to wood. We usually recommend spraying your pruning tools with Lysol (or other disinfectant spray) between cuts on a diseased tree, or disinfecting your tools when moving to a new tree. If a tree needs a lot of pruning, try to remove only 1/3 of the tree’s mass at one time (once a year). Light pruning and the removal of dead or diseased wood can be done at any time of the year. Pruning in the winter months is a good rule of thumb, as the tree is more dormant (and so are the bugs). Pruning should not be done during freezing weather. For summer or fall flowering trees that bloom on current year’s growth, prune in winter for best flower production. For trees that bloom in the spring from buds on one-year-old wood, prune when their flowers fade (this applies to most flowering fruit trees in our area).

Their have been thousands of pages written on pruning! It is indeed a science. But these few basic concepts will give you a head start in maintaining a healthy, strong, vigorous tree.

Sources for this article: An Illustrated Guide to Pruning Second Edition, Edward F. Gilman, 2002; International Society of Arboriculture brochures "Pruning Mature Trees", "Pruning Young Trees", "Why Topping Hurts", 1997; National Arbor Day Foundation Tree City USA Bulletin No.1, "How to Prune Young Shade Trees", Dr. James F. Fazio, 2000; Pruning Trees Near Electric Utility Lines Pocket Guide, Dr. Alex L. Shigo, 1990.