When is the best time to prune trees and shrubs?

by Steve Turner

I am often asked when is the best time and what should be pruned. The ideal time is when the tree is dormant, which is late fall to early spring. Since the tree is not active, the removal of limbs will be less stressful to the plant. It is also easier to see the structure of the tree without its leaves to better find problem areas like rubbing branches, weak crotches, cracked or broken limbs and competing leader branches. The tree will have more time to seal the wounds before the growing season starts and there will be less sap running from the wounds. Also, insect and disease organisms are not active at this time, so it reduces the risk of infection or pest infestation, which is why birches and American elms should only be pruned in the winter. The worst time to prune trees is late spring and early summer when their leaves are expanding and the tree is putting its energy toward growth.

A common myth is that large, mature trees can tolerate more pruning than small trees. In fact, the opposite is true. Only the outside inch or two of a tree is alive; the rest is made up of dead cells whose main function is to support the tree. That’s why a tree can be hollow but still be perfectly healthy. If you compare the percentage of live cells to dead cells between a small tree and mature tree, the difference is huge. A young tree might be made up of 90 to 100 percent live cells, while an old, mature tree might only have 10 percent or less. As you can imagine, the younger tree has a better chance to adjust to change.

A good rule for mature trees is that less is better. Try to avoid removing large limbs over 6 inches in diameter because it will be difficult for the tree to close the wound before decay sets in. Other options are cabling or thinning these large limbs as opposed to removing them. Good maintenance items for mature trees are removing deadwood and hazard limbs and thinning for weight reduction when necessary.

Young trees up to 8 to 12 inches in diameter need to be pruned properly for structure while they are still young to avoid the need to remove large limbs in the future. When pruning trees, keep in mind that the leaves produce the energy for the plant and that removing too much leaf surface from a tree or limb will starve the plant, causing die back or heavy sucker growth from that part of the tree.

“Topping” trees destroys them. They will never grow back to their natural shape and every branch that grows out of that wound area will be an accident waiting to happen. As the new limbs grow bigger and the rot in the wound increases, the branch will eventually break. A topped tree soon becomes a maintenance headache, with constant storm damage due to the weak branch attachment. If a tree is too large for its location, consider having it removed and replanted with a smaller species rather than topping it.

It is important to know what a tree’s natural shape will be when it matures before you prune it. For example, I see many weeping cherries that are pruned into a ball shape because all of the upright limbs were removed as they grew out of the top. If allowed to continue growing, these limbs would have eventually bent back down, creating the weeping effect that gives the tree its unique look. So it pays to do a little homework and find out what the tree’s natural shape is and help it reach its full potential. Not all ornamental trees need to be pruned into the traditional lollipop shape!

When it comes to shrubs, prune flowering shrubs soon after they are done blooming. Non-flowering shrubs and evergreens can be pruned as necessary to achieve a desired shape. Keep in mind that wider at the bottom is better than narrow — if you allow more sun to reach the bottom of the shrub, the plant won’t thin out as much. Avoid heavy, late summer pruning; too much pruning at this time will encourage new growth that may not harden off in time for winter. For those plants that don’t tolerate shearing well, like red or green twig dogwoods, it is best to prune them back to the ground when they become too large or sparse, and let them start over again by growing new limbs from the base.

Steve Turner, Certified Arborist, is from Arboricultural Services in Fenton, MI.