Spring Pruning of Trees

Leif’s article in this week’s Northfield News discusses thoughts on spring pruning of trees.

Property owners who are eager to be working on their yards and gardens need to be careful this time of year to choose tasks that help prepare for another successful gardening season, but won’t damage soils, plants or property. We often hear that pruning trees is best done in the winter time, but most of us lack enthusiasm for pruning during cold weather.

Now that temperatures have warmed to comfortable levels, and we are in the transition from winter to spring, there is a short window of opportunity to do pruning of trees and shrubs before the early season growth flush is fully under way, and before oak wilt, Dutch elm disease and canker are active. Try to complete your pruning projects on oaks and elms and honeylocust in the next ten days and on the other plants before they leaf out.

Pruning before woody plants leaf out is easier because leaves do not obscure your view of the structure of the tree or shrub that is being shaped. This allows for better decisions on which branches to remove, and results in a better plant architecture that stands up to storms, ice, snow and is more visually appealing.

A common pruning mistake on shade trees is waiting too long to remove lower level branches. The longer you wait, the thicker their lower branches become. It is not unusual to see shade trees in people’s yards with branches 3″ to 6″ thick just 3 to 6 feet above the ground. When finally removed, the pruning cuts leave wounds 3″ to 6″ wide or larger. Wounds of this size usually take from 4 to 10 years to have bark grow over and seal the wound. This is not only unsightly for a long period of time, but also a long time during which decay may start.

It is far better to prune shade and ornamental trees early and often. During the first 10-15 years in the landscape, these valuable parts of your landscape should be pruned every 1-2 years, and after that every 3-5 years. My rule of thumb is that any branch which needs to be removed should be trimmed out before it is any thicker than your thumb. This means that pruning wounds are generally ½” to 1″ in diameter and heal over in just one to two growing seasons, greatly reducing the risks of decay, and also giving a much more pleasing look.

Another way to prevent decay when pruning trees is to make cuts just outside the branch collar, rather than cutting flush with the trunk. The branch collar is the slightly wider area of the branch right where it joins the main stem. On branches the size of human fingers, I usually leave about 1/8″ to ¼” inch of the branch collar in place. This makes for a slightly smaller diameter wound, but avoids the appearance of leaving a protruding stub.

(This picture shows the protruding stub which needs to be taken back a little further, but not flush with the trunk)

The branch collar tissue which remains after the cut secretes phenols which wash over the cut. These phenols help prevent decay from beginning, but are only produced by the collar tissue, and not the regular bark. If you cut away all the collar tissue by cutting perfectly flush with the trunk, you remove part of the trees natural decay prevention systems.

If you fail to prune your tree for many years, the results can be far worse than the limited perils of going ahead with pruning. Branches that are too low on a large tree will block out sunlight penetration to the soil surface, causing grass to die out and creating an unsightly yard where little else will grow well.

Failure to prune also increases the risks of catastrophic damage during wind, ice and snow storms. This typically happens when weakly attached branches grow to large size, and then catch lots of wind or ice. The very weak attachment then splits out under the heavy load, leaving a huge gap in the canopy of the tree.

By constantly removing very steeply angled branches and leaving branches that are from horizontal to about 45 to 55 degree angles to the trunk, strong attachments are likely to form, giving decades of resistance to storm damage.

Over a period of 10 to 20 years, you can gradually remove lower branches until the canopy of the tree begins at 10 to 20 feet above ground. Careful thinning of the crown will let a little filtered light through to keep grass growing, and the high canopy also allows early and late day sunlight to reach grass and other landscape plants from the sides. This will create both a beautiful tree and vibrant grass and gardens.

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