Prune Trees in Winter to Prevent Oak Wilt

Prune Trees in Winter to Prevent Oak Wilt

Wintertime is a good time for tree pruning, when temperatures are cold, according to Wisconsin Department of Natural Resources tree health experts.

Trees affected by oak wilt
An oak tree dying from oak wilt (middle) and an oak killed by the disease (right). Photo: WDNR

Prune trees in winter

“Pruning is a good practice to help create or maintain good branch structure and minimize many tree pest and disease problems. However, if you are going to prune oak trees, it’s important to do it in winter to prevent introduction of oak wilt,” says Paul Cigan, a WDNR forest health specialist.

The best period to prune oaks is between the months of October and March. In spring, stop pruning when daytime temperatures reach 50 degrees.

This is because oak wilt — a fatal disease of oaks — is spread by beetles that feed on the sap of tree wounds. But these beetles are not active in winter.

Without leaves on trees, it’s also much easier to spot and prune broken, cracked, hanging, or competing limbs.

Because the beetles that spread oak wilt are attracted to tree wounds, be mindful during summer and fall of other ways that wounds are created: by accidentally puncturing the tree during mowing or weed-whacking, for example, or by breaking limbs as you drive vehicles along trails.

Do not move firewood

To further reduce the spread of oak wilt, avoid moving firewood from one location to another. “Many oak wilt infections in new areas are likely the result of infected firewood or other raw oak wood being moved from areas with oak wilt,” according to Cigan.

If you must move firewood, plan to leave oak firewood where it was cut for one year or until the bark is naturally loose. This will also help prevent the spread of oak wilt.

The disease is commonly found in the southern two-thirds of Wisconsin. It is creeping north, but is still uncommon in much of northern Wisconsin.

Taking precautions to prevent the spread of oak wilt will help keep it that way. “This is a current problem for which we can still make a difference,” Cigan says.

More advice on pruning

Trees should be pruned throughout their life to maintain strong structure and remove dead wood. “Pruning should not remove more than 25 percent of the live crown of a tree. The lower third of trunks of deciduous trees should be free of limbs,” says DNR urban forester, Don Kissinger.

For more advice on tree pruning and oak wilt prevention, visit dnr.wi.gov and search “tree pruning,” “oak wilt,” or “firewood.”

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This article was adapted from a Wisconsin DNR press release.