By Amy Chapman, University of Minnesota Extension Gardener
The benefits of trees are many. Not only are they a pleasure to view; they offer social, environmental and economic benefits. We often don’t realize what we have in a tree until it is gone.
Since we value our trees and want to preserve them, it’s important to prevent problems. Extensive (and expensive) repair work in older trees can be prevented by early corrective pruning.
To start, planting time is a good time to correct minor structural problems. Remove branches that are diseased, dead, broken, rubbing or crossing each other.
One or two years after planting start pruning the tree to improve its form. Select the main (scaffold) branches–these should be evenly spaced along the main stem. They are smaller in diameter than the main stem, and distributed around the tree so that a full crown can develop. Choose branches with wide angled crotches as narrow crotches are generally weak and could split in high winds.
The position of a limb on a trunk essentially remains the same for the life of a tree. Decide at what height you’d like branches when the tree is mature. However since these branches are important to the development of the trunk, allow lower branches to remain for a year or two to develop a stronger tree.
Prune the young tree ‘up’ gradually so the unbranched trunk is never more than one-third of the total height of the tree.
Pruning young, established trees. Develop a regular pruning schedule while the tree is young and prune out the following problems: Dead, dying, or diseased branches; crossing branches and branches that grow back towards the center of the tree; crossing branches will rub on each other, which damage the bark and attract diseases and insects. Water sprouts (young, weak growth that grows straight up off horizontal branches); sprouts at or near the base of the tree trunk; multiple leaders (one main upright trunk) where a single leader is desirable; don’t cut back the leader; and branches with narrow angled crotches.
- Use a sharp pruning shears to prevent tears. For branches more than one inch in diameter, use a hand saw.
- Always prune back to the main trunk or side branch, don’t leave a stub.
- Take care to not remove any part of the branch collar. The branch collar is a thickened area of wrinkled bark from which a branch emerges. It contains hormones that help form a callus that in turn seals the pruning wound and protects the tree. The final cut should be a ‘collar cut’ made at about a 45-degree angle along but not into the swollen area (the branch collar). (See diagram)
- Wound dressing or ‘painting’ is not normally needed on pruning cuts and can promote disease. However, if wounds need to be covered to prevent insect transmission of certain diseases such as oak wilt, use latex rather than oil-based paint.
- To remove large branches, use three cuts to avoid tearing the bark. Make the first cut on the underside of the branch about 18 inches from the trunk. Undercut one-third to one-half way through the branch. Make the second cut an inch further out on the branch; cut until the branch breaks free, then make the final collar cut. (See diagram.)
The late dormant season is best for most pruning. Pruning in late winter, just before spring growth starts, leaves fresh wounds exposed for only a short time before new growth begins the wound-sealing process. It’s also easier to make pruning decisions when leaves are not obscuring the branch structure.
Pruning at the proper time helps to avoid certain disease and physiological problems:
To avoid oak wilt disease do not prune oaks from April to October. If oaks are wounded or must be pruned during these months, apply wound dressing or latex paint to mask the odor of freshly cut wood so the beetles that spread oak wilt will not be attracted to the trees.
Prune apple trees, including flowering crabapples, mountain ash, and hawthorns in late winter (February-early April). Spring or summer pruning increases chances for infection and spread of the bacterial disease fireblight.
Some trees like maple and birch have sap that ‘bleeds’ after late winter or early spring pruning. This bleeding causes little harm.
If you have a large established tree that needs attention, realize most pruning is beyond what a homeowner should attempt. Contact a tree-care company with arborists who are certified by or are members in a professional organization.
Before you make any cuts, use research-based information for the type of tree you are going to prune. The Minnesota Landscape Arboretum Yard and Garden Line 612-301-7590 can answer questions and provide direction.