Flowers, Fruits, and Frass

URBANA, Ill. - Many gardeners have experienced that awful moment when the lawn mower or weed whacker accidentally comes in contact with the bark of a valued tree in the landscape and a wound is created, said a University of Illinois Extension educator.

"No matter how careful you are, accidents happen, and then you're left wondering what you can do to help the tree repair this wound," said Candice Miller. "In most cases the answer is to really let the tree repair the wound on its own."

Upon being wounded, trees begin a natural process of callusing over the wounded area with new bark and wood. In the spring, when trees are growing vigorously, this process will naturally occur quickly. During other times of the year when growth is not as vigorous, try to keep wounded trees growing as vigorously as possible, she said. For example, trees should be fertilized properly and watered during droughts for example.

"Keeping the tree as healthy and happy as possible is really your best bet," she said. "However, in the case of some trees, a wound made during the active growing season may mean that insects or diseases could be more attracted to the wound and potentially pass on pathogens."

Miller said that pruning wounds on oaks and elms can attract borers and beetles that are carriers of diseases such as Dutch elm disease and oak wilt.

"In this case, some may recommend that a tree wound dressing be put on that wound. The effectiveness of this is debated though. A tree wound dressing is a petroleum-based product used to cover freshly cut wood to inhibit decay or insect infestation," she said.

According to research done by Washington State University, wound dressings do not prevent entrance of decay organisms or stop rot from occurring. They do, on the other hand, seal in moisture and decay, sometimes serve as a food source for pathogens, prevent wound wood from forming, inhibit compartmentalization, and eventually crack, exposing the tree to pathogens. "All of which are problems," Miller said.

She added that if you must prune a disease-prone species when insects or fungi are active (that is, during the warmer times of the year), a light coating of an insecticide or fungicide may be warranted.

"Other than that, avoiding wounds is the best practice. Mulch around trees and shrubs to avoid mowers and weed whackers getting close to trees and prune during the dormant season when insects and pathogens are not active," she said.