Pruning for tree health

Joanne Skelly

Special to the Nevada Appeal

Proper pruning can be good for trees, but people should not think that trees can be pruned or treated in any old way and stay healthy.

For centuries, man has damaged trees in the name of pruning. Alex Shigo, a well-respected tree researcher, presents a number of valuable tips in his book "Tree Pruning." He asserts that when pruning trees, people often only think about their needs, and pay little attention to the fact that pruning affects tree health. He points out that proper pruning preserves a tree's beauty and its defense systems. Proper pruning starts with understanding how trees grow.

Trees can live longer than any other organism. A tree is made up of wood - living, dying and dead cells in an ordered arrangement. Sapwood is alive, and is in trunks, branches and roots. These living cells store energy reserves used in many tree processes, including defense. Pruning can damage sapwood, which can inhibit growth and defense mechanisms if improperly done.

Avoid flush cuts or stub cuts, which injure trees and destroy their defense mechanisms. A flush cut damages the collar of a branch. The collar is that part of the tree that compartmentalizes the injury, which is the pruning cut in this case, from the rest of the tree. It is the "doughnut" shape that develops after a limb has been appropriately pruned. Stubs are branches that aren't cut close enough to the branch collar. There is no way the protective doughnut can develop with a stub in the way.

Topping causes serious injury to trees and leads to hazardous conditions. Over-pruning destroys a tree's framework, weakens roots, and initiates excessive weak sprouts that become hazardous. Wound dressings do not stop rot, and may actually stimulate rot in some cases.

Proper pruning begins with selecting the right tree for the right place. Know the soil type and drainage. Do not plant trees that grow large, such as a sycamore, in small places, under power lines or close to buildings. Buy healthy trees that are free of wounds or injuries. Start pruning three years after planting to establish the framework shape of the tree. Prune only up to one-quarter of the entire tree each year. Prune regularly to maintain the structure of the tree.

Shigo also has a few pointers about safety. Use a protective hat, gloves and glasses, and wear a long-sleeved shirt. Use sharp tools. Do not work near power lines. You should probably hire a professional arborist if the pruning job requires a ladder or a chain saw, if the tree has been storm-damaged, or if the limbs are very heavy (greater than 2 inches in diameter).

For more information, e-mail skellyj@unce.unr.edu or call me at 887-2252. You can "Ask a Master Gardener" by e-mailing mastergardeners@unce.unr.edu or call your local University of Nevada Cooperative Extension office. Check out many useful horticulture publications at www.unce.unr.edu.

n JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension.