Pruning is not a task that many homeowners enjoy or look forward to. However, from now until the end of winter is a good time to evaluate the structure of your younger or more recently established ornamental trees and shrubs, and to prune them as needed.
As I mentioned in the opening, it is easier to see the branch structure without leaves present. This makes it easier to see those problem areas, such as crossing or rubbing branches, dead or damaged limbs, narrow crotch angles of limbs in relation to the trunk (those that form a tight “V”), multiple central leaders, and other factors. Most trees can be pruned anytime during the winter without adverse consequences, but we generally recommend the late winter period before spring growth to avoid periods of severe cold temperatures and to minimize the time wounds are exposed before the sealing process begins in earnest in spring. Plus, it’s more comfortable to work in milder temperatures as we approach spring.
Another advantage to winter pruning may be more of a regional decision to avoid certain pest problems. American elms should be pruned in winter to avoid having a fresh wound when the beetle vector that spreads Dutch elm disease is active. An insect vector is an insect that spreads the fungus that causes the disease. The same goes for oaks and the threat of oak wilt. Oaks in the red and black oak group (bristle-tipped leaves) are more susceptible than those in the white oak group. If you prune any of these trees in the spring, this is the rare instance where tree wound dressing may help prevent infection by the vectoring beetles. Normally, tree wound dressing on other species is not recommended or required.
Some trees with a heavy sap flow in late winter, such as maple, birch, yellowwood, dogwood, beech, and hornbeam will leak sap through newly pruned cuts. These trees will ooze sap onto anything below the tree and appear unsightly, but it has little negative effect on tree health. Choosing a time for pruning after this heavy sap flow may be a better choice for these species.
There is one distinct disadvantage of winter pruning on early-flowering trees and shrubs, such as magnolia, lilac, and azalea – you prune off flower buds. While this should not harm the health of the tree or shrub, it will decrease the spring show of flowers. For these species, pruning should be delayed until after flowers fade in the spring.
Finally, homeowners should practice best practices when it comes to the types of pruning cuts, the location of those cuts, using the proper tools, and how much branch material is removed. These and other important factors are discussed and illustrated in the Purdue Extension publications, Tree Pruning Essentials, and Pruning Ornamental Trees and Shrubs. Each is available at Purdue Extension’s Education Store, www.edustore.purdue.edu.