With spring on the way our thoughts may naturally be turning to outdoor garden chores like pruning. That's a good thing, because the best time to prune most trees, especially deciduous trees (trees that lose their leaves in the fall), is late in the winter before their leaves form. In other words, the month of March is a good time.
The main reasons for pruning ornamental and shade trees are safety, tree health and aesthetics. When done correctly, pruning can lengthen the life of your tree and increase its value to the landscape. If done poorly, pruning can cause a host of structural and biological problems, including pests, decay and a shorter lifespan for your tree.
To prune correctly you must understand how branch wood differs from trunk wood. Cutting a tree's branches flush with the trunk robs the tree of natural chemicals it needs to close the wound, leading to decay in the tree. Cut only branch tissues when pruning so the stem tissues of the tree will be less likely to decay and the wound will seal more effectively. The guiding principle of good pruning is to cut the branch, not what's called the "branch collar."
What's a branch collar? Take a close look at the branch: the slight swelling on the underside of the base of the branch is the branch collar; and on the upper surface branch — where it's connected to the trunk — is the "branch bark ridge." When pruning, avoid cutting into either the branch bark ridge or the branch collar.
For young trees that have been in the ground at least three years, a very light pruning is usually all that's needed. Arborists call this type of pruning "structural pruning," because the goal is to give your tree the kind of structure that will ensure a long, healthy life. Trees with a main stem or trunk that branches into a narrow fork often form a "v-crotch" with "included" (embedded) bark in it, a structurally weak part of the tree. Remove one of the branches or stems to create a strong control leader. You can do this by retaining the stronger, more vigorous, larger crowned side, and removing the less desirable limb.
With mature trees one of the primary objectives of pruning is reducing potential hazards by removing deadwood, weakly attached limbs and broken branches. Take care not to remove too much foliage; a good rule of thumb is less than 25 percent of the crown — often far less is needed. Also, refrain from removing any live foliage from a tree stressed by nearby construction; the food produced by their leaves is particularly critical to them for healthy tissue, roots and replacement branches.
Don't "top" your tree. Tree topping is the indiscriminate cutting back of a tree's branches to numerous "stubs." It often results in the development of weakly attached and unsightly epicormic sprouts. Many people mistakenly "top" trees because they interfere with views or sunlight, or they simply grow so large that they cause worry for the property owner. However, without its protective crown of leaves and branches, a tree cannot feed itself or protect its sensitive bark from damaging sun and heat. Topping weakens trees, leaves them vulnerable to insects and disease, and shortens their life span.
Proper tools are essential for quality pruning. Hand pruners are used to prune small branches, and many different kinds are available. Slightly larger branches that can't be cut with a hand pruner may be cut with small pruning saws or lopping shears. Keep tools clean and sharp; hand pruners, lopping shears and pole pruners can be sharpened with a sharpening stone.
Cynthia Orlando is a certified arborist with the Oregon Department of Forestry