Oak trees are special for many reasons: they’re Iowa’s state tree, they’re important to our ecosystem and — they shouldn’t be pruned during the spring growing season. If you’re looking for something to do in the garden this winter, get out and see if your oak tree needs a trim.
Pruning oaks in the spring leaves them susceptible to infection with oak wilt, which can kill the tree in a matter of weeks. Sap beetles, the vectors that carry the disease from tree to tree, are attracted to the sap that seeps out of pruning cuts when the tree is active.
What is oak wilt?
Oak wilt is a disease only found in North America, most commonly in the upper Midwest of the United States. It’s caused by a fungus, Bretziella fagacearum, which is transmitted by sap beetles and through tree to tree root grafts. The disease results in wilting symptoms and, most often, death of the tree.
When a tree is infected, the length of time it takes to die and its possibility of recovery depends on the type of oak.
Red oaks are extremely sensitive to oak wilt and can die within a few weeks or a few months. If infected, the leaves in one area of the canopy will turn brown at the edges. The browning will progress through the rest of the canopy, and eventually the leaves will fall off. It's not uncommon to also see fungal mats on the bark.
White oaks are slightly more hardy and are able to form structures within the tree to slow the infection. They tend to die over the course of several years, but in some cases may survive. Symptoms tend to be more variable in white oaks, but leaf discoloration is still common, though often slower to progress.
How to prune your oak trees (and avoid oak wilt)
The golden rule is to prune your oak trees during the dormant season. Depending on the year, it can be safe to prune from late November to early March, but December through February is the safest time to prune. Sap beetles become active in the spring, and they could infect your tree if its pruning cuts are recent.
Trees heal faster from pruning during the growing season, so February toward the end of the dormant period, but before sap beetles appear, may be an ideal time.
If you’re worried you may be cutting it close, the Iowa DNR often sends out news releases about the best times to start and stop pruning oaks. You can also try their predictive model for determining the risk of oak wilt transmission on any given day.
What to prune:
Young oak trees of five to ten years need pruning to help shape the tree. Most mature oak trees need very little pruning.
Look for and remove branches that are brushing each other, which creates open wounds. Also prioritize broken branches, because pruning makes the wound smaller and therefore less susceptible to infection.
Using hand pruners, loppers or a folding pruning saw, clip the branches close to the tree, leaving the swollen area at the base of the branch (known as the branch bark collar). You should always consult an arborist before cutting branches that are too large for hand tools.
If for any reason you have to prune your oak tree outside of the dormant season, protect the cut by brushing on latex paint.
Preventive care for your oaks
As oaks grow older they become more susceptible to disease. Taking care of them by providing water and nutrition as needed will increase their chances of survival, especially for white oaks.
If you have multiple oak trees and one becomes infected with oak wilt, it’s important to remove the tree carefully and sever any root grafts it may have with other oaks to prevent the spread.
What to do if you suspect your tree has oak wilt:
To confirm oak wilt, you may need to talk to an arborist or send pictures and samples to the Plant and Insect Diagnostic Clinic at Iowa State University. Especially if you have a white oak, experts may be able to suggest management options, such as fungicides, to try to save the infected tree.
