When is it time to thin my tree?

Is it time to thin out my trees and cut off the lower limbs? No, and it never will be.

By Howard Garrett, The Dallas Morning News, October 14, 2019

This tree has been improperly ‘gutted,’ with inner branches and leaves being removed. Photo by Howard Garrett

It is a good time to prune, but pruning too much is the most common pruning mistake I see. Few landscape trees need major pruning or “thinning” every year. Trees can be severely damaged by over-pruning. Plus, it looks bad.

Landscape trees can be pruned any time of the year, but the best time is from fall to late winter. Pruning is part science and part art. Don’t try to change the character of a tree and don’t remove lower limbs to raise the canopy for more light to turf. Low-growing limbs exist for a reason.

It’s very unnatural to strip tree trunks bare. Remove dead, diseased or damaged limbs and the weakest of crossing limbs. Remove limbs growing toward the center of the tree and limbs that are dangerous or physically interfering with buildings or activities.

Thinning to eliminate a certain percentage of the foliage is usually a mistake. Heavy thinning of a tree’s canopy throws the plant out of balance, inviting wind and ice storm damage. Plus the resulting stress attracts diseases, parasites and insect pests. Gutting or “lion tailing” (heavy interior pruning) is never appropriate and can be damaging.

Removing one of the major co-dominate upward trunks or shoots to leave one strong vertical leader is a good thing. This eliminates a “V”-shaped crotch.

Flush cuts should never be made. Cuts leaving 1/16-inch stubs are also bad, unless the limb is tiny. Pruning cuts should be made at the point just outside the branch collar. The branch collar (actually part of the trunk) can protrude as little as 1/8-1/4 inch on small limbs but can be several inches to as much as a foot or more on large limbs. It’s also more pronounced at the bottom of the limb than at the top. Cuts made at the right place leave a round wound. Improper flush cuts leave oval wounds and cause cavities to form.

It’s a scientific fact that cutting into or removing branch collars causes problems. Flush cuts encourage decay at the top and bottom of cuts because the natural protective zone between the trunk and the branch is damaged. Resulting problems including decay, wet wood, resin pockets, cracks, sun injury, cankers and slowed growth of new tissue. Proper cuts are round, smaller and heal much faster. Long branch stubs can also be detrimental sometimes and should be avoided, but it is always better to err on the side of pruning stubs being left too long rather than too short.

As a final note, tree trimmings resulting from pruning should not be hauled away if possible. The large pieces should be used for firewood and the rest should be shredded and used as mulch around plants or mixed into the compost pile. This is excellent mulch for walkways in the landscape and vegetable garden.

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