My trees have a few low branches hanging over my patio and I would like to prune them to avoid bumping my head. Is this an acceptable time to do this work?
— Jody Stein, Wheeling
I’m glad you noticed this during dormant season. The recent warm weather is about to end the dormant pruning season, so prune away while you still can! Take the time to make proper pruning cuts. Pruning creates injuries to the tree, causing it to respond by isolating the damage instead of growing new tissue. Prune less aggressively or wait another year or two if the tree is under stress due to drought, disease or insect damage or has been aggressively pruned in the last couple of years. A tree that has low energy reserves due to stress may be unable to respond to pruning. I prefer to remove low-hanging branches entirely rather than pruning the ends to raise the branches.
You will get the best results by pruning just outside the branch collar so that it stays intact. Trees form a protective barrier inside the branch collar, so leaving this zone intact after pruning allows the tree to heal. A proper pruning cut starts at the top of the branch collar and ends at the bottom. Look for a distinctive bulge (ridge of bark) at the base of the branch where it connects to the trunk. Branch collars can be thick and prominent on some species or flat and close to the trunk on others. This makes them hard to see on some trees. Your pruning cuts may vary from a half inch or less from the trunk and vertical to 1 inch or more at an angle away from the trunk.
The position of the branch collar should guide the placement of your cut. The general rule is to prune just outside of it. A thick branch collar may protrude from the trunk when you have finished pruning, but you should avoid cutting too far away from the collar and leaving a stub. A branch that has been dead for years may have a more developed branch collar that is a few inches from the trunk of the tree, and you should prune to leave it intact. When you make the proper cut, the new growth to cover the wound will develop evenly. If this does not happen, you may cut through the branch collar. There is no need to treat the wounds after pruning, as pathogens seldom spread into a tree when branches are pruned properly.
Before you make the final pruning cut at the branch collar, shorten the branches to about a 4-inch stub. Otherwise, the branch can tear away and damage the trunk as it falls. Make the initial cut about a third of the way through the branch on the underside and 2 to 3 inches away from the trunk. Then make the next cut to remove the limb on the top of the branch within 1 inch outside of your first cut. The branch may break away quickly as it begins falling and hits the undercut, which will prevent the bark from tearing away from the trunk. Then you can carefully make your final cut at the branch collar. I hold the stub as I make the final cut, so it does not fall and tear bark off of the trunk.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.
