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DNR recommends trimming oak trees before April 1

The Department of Natural Resources (DNR) reminds Minnesotans to prune oak trees before April 1 to prevent new oak wilt infestations.

"The best time to prune oak trees is during the winter months when the beetles that carry the oak wilt fungus are dormant," said Ryan Blaedow, DNR forest health specialist.

Beetle activity usually peaks between April and June; however, oak wilt can be transmitted any time during the growing season when it is warm enough for the beetles to feed on the sap of wounded oaks.

"The key to keep oak wilt from spreading is to remove diseased trees before spore mats form and by preventing the transfer of spores to healthy trees," Blaedow said. "If your oaks need to be pruned, get the work done before April 1, otherwise wait until fall. Unfortunately, people who prune their oaks in the spring find their tree is dead by the end of the summer."

To prevent the spread of oak wilt, follow these management guidelines:

If possible, trim oak trees during the winter.

Avoid wounding oak trees between April and July when sap beetles actively feed.

Immediately treat wounds with pruning paint during spring and summer months to stop flow.

Remove diseased trees before the following spring to prevent spore mat development.

It is best to remove trees in the winter to avoid wounding neighboring trees.

Prior to removing dead and diseased trees, sever root connections to healthy trees by trenching around diseased trees with a vibratory plow equipped with a 5-foot blade.

Destroy infested wood or treat wood on site by debarking, chipping or drying the wood.

To dry wood properly, cover split wood with plastic and bury the edges for at least six months to kill the oak wilt fungus and any insects.

Oak wilt is an aggressive disease that affects many species of oak, and kills thousands of oak trees each year in forests, wood lots and home landscapes in the eastern United States. Trees in the red oak group are more susceptible to oak wilt than trees in the white oak group. In Minnesota, black oak, northern pin oak and northern red oak belong to the red oak group, while bur oak, swamp white oak and white oak belong to the white oak group.

The disease is caused by a nonnative fungus that invades the water-conducting vessels of oak trees. Oak trees respond to the invasion by plugging their vessels, which causes leaves to wilt from the top of the crown downward. Infected trees in the red oak group often shed their leaves and start to die within a few weeks of symptom onset, while trees in the white oak group usually die slowly over one to many years.

Oak wilt can easily spread from infested trees to healthy trees through root connections.

Visit the DNR website at for more information on oak wilt and how to prevent its spread.