Winter is a great time to prune trees, and with pruning often comes the question of how to treat pruning wounds. The simple answer regarding tree wound treatment is the opposite of what you might think: Doing nothing is better than doing something.
Doing nothing to a pruning wound means not using pruning paint, wound dressings, bandages, wax, hydrogel, spray foam, concrete, concrete sealer, bricks and mortar or any other products touted as being good tree wound treatments. (For the record, I have seen all of these items in use.)
The “news” about not treating tree wounds is really not news, but I am surprised at how few people know the truth about tree wound treatments. Alex Shigo, a plant pathologist with the United States Forest Service, published research in the early 1990s proving that pruning paint and other wound treatments inhibited the tree’s natural process of compartmentalization.
Compartmentalization is the process by which trees isolate damage physically and chemically. Trees wounds do not heal like animal wounds. Instead, trees compartmentalize and form callus tissue around the edge of the wound that will eventually enclose the damaged areas. Some trees are more efficient at compartmentalizing damage than others.
Shigo’s work also proved that use of pruning paint and other chemicals on pruning wounds sometimes sealed in moisture that made a better environment for disease-causing organisms and could serve as a food source for some pathogens.
Instead of using pruning paint, concentrate on making good pruning cuts. Always prune branches to another branch or to the main trunk instead of leaving long stubs, and make cuts just outside the branch collar. The branch collar is the ridge where the branch attaches to another branch or to the trunk of the tree. Cuts made flush with a connecting branch or trunk should also be avoided as they create a much larger wound than necessary. Large wounds take longer to compartmentalize and leave the tree more susceptible to pathogens.
This is the point when someone usually asks why we even need to prune trees. The answer to this is simple too — to make trees healthier and safer. Trees grow differently in a forest where they compete for light and live in an ecosystem conducive to tree health. Residential lawns and urban landscapes are often built on poor soils. Trees compete with grass or other plants that have higher water and nutrient needs, and they are subject to abuse from passing lawnmowers, weed trimmers, bicycle locks, and cars that leave the roadway.
Many species of trees self-prune although some species are better at it than others, and tree health can be promoted by doing some of the work for them.
Pruning for tree health usually includes removal of dead, diseased and insect-infested wood; removal of broken and damaged branches; thinning to increase air movement through the tree; removal of crossing or rubbing branches that wear away bark and create weak points; and structure improvement.
Pruning for tree safety includes removing branches that interfere with utility lines and lines of sight at intersections and by removing branches with potential to break or fall and cause injury or property damage.
Good mulching and watering practices will also help overall tree health. Ensure a two- to three-inch layer of mulch in a large-diameter circle around the base of the tree, but keep mulch pulled away from the trunk. Water deeply and infrequently over extended dry periods. As a general rule, trees and other landscape plants need about one inch of water per week.
Someday researchers might present a better option for tree wounds than doing nothing, but for now, stay away from snake oils.