Trees

An oak dying from oak wilt, left, and an oak killed by oak wilt.

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

“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 is important to do it in winter to prevent introduction of oak wilt,” said Paul Cigan, Wisconsin DNR forest-health specialist for northwest Wisconsin.

Oak wilt is a fatal disease of oaks. It’s spread by beetles that feed on the sap of tree wounds, but they are not active in winter.

Without leaves on the trees it is easier to see and prune broken, cracked, hanging or competing limbs.

To help reduce the spread of oak wilt another way, do not move firewood.

“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,” Cigan said.

Leave oak firewood where it is cut for one year, or until the bark is naturally loose, to prevent the spread of oak wilt. Oak wilt is commonly found in the southern two-thirds of the state. 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 said.

Trees should be pruned throughout their lives 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,” said Don Kissinger, DNR urban forester.

Visit dnr.wi.gov and search “tree pruning” or “oak wilt” or “firewood” for more information.

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