Trees are best pruned for structure when young, as early as a season after planting. By the time I am normally called to a property to assess the trees for health and safety, the trees are either mature, or very nearly so. As a result, drastic steps to improve the structure of the tree are no longer feasible, and I am forced to manage the tree’s shape and form as best I can. In climates that receive a much longer growing season, like Victoria, BC, hard pruning to improve structure can be done later in a tree’s life. On the Prairies, we are at the mercy of a very short growing season, and recovery from hard pruning may take a very long time, making large structural pruning cuts problematic.
Perhaps the best description of structural pruning is that given by Dr. Ed Gilman, a professor at the University of Florida, who is arguably considered North America’s pruning “guru.”
”Many shade trees in the forest grow straight, tall trunks as they compete with neighboring trees for sunlight. In the landscape, however, the abundance of sunlight encourages trees to develop multiple, competing trunks or leaders. This type of structure is susceptible to mechanical breakage and can reduce tree longevity. But trees with one dominant leader and small well-spaced branches, like trees in the forest, are less likely to suffer this type of mechanical failure. The dominant leader structure also makes trees better able to retard the spread of decay within the tree.
“Structural pruning in the landscape aims to develop the strong tree structure we see in the forest. Structural pruning selectively favors a single, dominant leader by suppressing competing leaders using reduction cuts. Reduction cuts shorten stems back to lateral branches at least one-third the diameter of the cut stems. Structural pruning on shade trees should occur regularly when the tree is less than about 20 inches trunk diameter to establish good form early. It is normally performed every few years to gradually encourage more growth in the selected leader. Proper structural pruning should be performed on most tree species that become large at maturity to promote longevity, decrease future maintenance costs, and reduce conditions in the tree that could place people or property at risk.
“There are six main strategies in executing a structural pruning program. These include the following:
- Develop or maintain a dominant leader
- Identify lowest branch in the permanent canopy
- Prevent branches below the permanent canopy from growing upright or too large
- Space main branches along a dominant trunk
- Keep all branches less than one-half the trunk diameter
- Suppress growth on branches with bark inclusions.”
Dr. Edward Gilman, Professor, Environmental Horticulture, University of Florida
I practice structural pruning techniques on virtually everything that I prune, to some extent, even on the multi-stemmed trees like crabapples and chokecherries. It is a powerful pruning technique, especially here on the Prairies, where properly structured trees can resist or defy the worst spring snow storms.