Pruning Dormant Trees
by Michael Stein

Proper tree pruning is one of the most important things you can do to take care of your trees. Well-pruned trees are less likely to suffer from insect and disease problems, and will better withstand the damaging forces of wind or snow. There are three basic reasons for pruning a tree: safety, health, and aesthetics. Timing of tree pruning is also an important factor in maintaining tree health.

Pruning to increase safety is generally a top priority for most tree owners. Safety pruning entails removing dead and dying limbs, either cabling or removing limbs with weak points of attachment, and selectively removing live limbs to decrease wind resistance of the tree. Sometimes lower limbs may need to be removed to prevent damage by a car or truck, injury to a person or to improve visibility. Another important consideration is pruning to maintain a proper clearance from utility lines.

Pruning can substantially increase the health of large or overly mature trees. Dead branches left on a tree for too long can become avenues of decay that inhibit the tree's natural healing process. Selective removal of live growth to increase sunlight and air penetration into the canopy is important in preventing infestation by insects or disease. Branches that cross and rub each other or the trunk can create wounds and should also be removed.

Trees may be pruned for aesthetic reasons as well, simply to improve their appearance. With precise pruning, even large trees can be made more pleasing to the eye. In a formal landscape setting, however, there are many specific methods of pruning that are intended to control the shape and size of plants. Examples of this include the art of bonsai that forces trees into miniaturization, as well as pollarding, a popular method of keeping trees small in Europe. When large but desirable trees obstruct a view, careful pruning can create lines of sight through the canopy without harming or needing to remove the tree.

The best season to prune a tree varies according to the species of tree and the desired outcome. When pruning to remove dead and dying limbs, any time of the year is fine. However, when removing live branches, discretion should be used to ensure that no more than ten to 15 percent of the canopy is removed. For more extreme pruning, and for most fruit tree pruning, late winter is often considered the optimum time of year. Winter pruning is advantageous for a few reasons. In the absence of leaves, a trained arborist can better determine which limbs and branches need to be removed. Also, environmental conditions in late winter greatly reduce the chance of insect attraction to freshly pruned branches, as well as disease infection of the tree.

Dormant trees are sustained by utilizing reserves of sugars and carbohydrates produced through photosynthesis in the previous season; these reserves are stored in the trees' roots. When trees are pruned while dormant, new growth is delayed until spring. On the other hand, a tree that is heavily pruned in late summer or fall will instantly respond with new growth. But this new growth can drain the tree's energy and diminish its ability to build reserves for winter. Another reason to be cautious when considering late summer or fall pruning is that the new growth created by pruning may not have enough time to mature and may not survive the winter.

Always use care in pruning of trees. However important proper pruning is to tree health and safety, improper pruning can be more damaging than not pruning at all.

Michael Stein is an arborist at Integral Tree and Landscape, Stanfordville.