Protect oak trees by pruning after July, not before

By Don Kissinger, urban forester, 715-348-5746, Don.Kissinger@wisconsin.gov or Paul Cigan, plant pest and disease specialist, 715-416-4920, Paul.Cigan@wisconsin.gov

Anyone with oak trees in their yards or on forested lands should avoid pruning or cutting them from April through July to protect them from oak wilt.

Sap-feeding beetle on diseased oak tree in Sawyer County.

Sap-feeding beetle on diseased oak tree in Sawyer County.

The spring season often draws property owners outdoors to soak up rays of long-awaited sunlight, breathe in some fresh air, and to begin seasonal yard maintenance and cleanup projects. While spring is naturally a time to dust off yard tools like rakes, shovels, and weed clippers, when it comes to the health of oak trees, keeping those chainsaws and trimming tools a safe distance away will help ensure that your trees stay healthy for many spring seasons to come.

Pruning and cutting oaks in spring and early summer leaves them vulnerable to oak wilt, which rapidly kills trees in the red oak group and weakens those in the white oak group. Any damage during this time, including broken branches caused by storms, provides an opportunity for the oak wilt fungus to attack the tree by exposing living tree tissue beneath the bark.

Sap-feeding beetles introduce the disease by carrying oak wilt spores from infected trees, logs, or firewood to fresh wounds. Like most insects, these beetles have an incredible sense of smell that draws them to the surfaces of open wounds to feed on sugary sap in as little as 15 minutes after a wound is created. This is how most new oak wilt infections start.

The trees most likely to die from oak wilt infection are in the red oak group, including northern pin oak, northern red oak and black oak. The white oak group is more likely to survive infection and includes bur oak, swamp white oak, white oak and English oak.

Use of tree paint or wound dressing is normally discouraged on pruned or wounded surfaces, but if you must prune oaks for safety or other reasons from April through July, a light application of these products is recommended as soon as a cut is made to protect oak trees from becoming infected.

There are other important reasons to avoid pruning in spring. Deciduous trees that lose their leaves in the fall are just starting to grow new buds and leaves, so the trees’ food reserves are low. In general, the best time to prune is in winter when trees are dormant.

To date, oak wilt has been found in all Wisconsin counties except Ashland, Iron, Taylor, Door, Kewaunee, Calumet and Manitowoc counties. Several of these counties contain the highest abundance of healthy and productive oak forests in the state. Taking recommended precautions will help keep them that way for years to come.

Oak wilt and other diseases move easily on or in firewood logs year-round, so keeping firewood local, or purchasing Wisconsin-certified firewood, is another important component of protecting trees and keeping forests healthy. Learn more about responsible firewood practices on the DNR firewood webpage.

More information is available online at the DNR oak wilt webpage, including an educational video on oak wilt made by the DNR Forest Health team. Additional information about proper pruning techniques is available from community foresters or with this tree pruning poster from the DNR. 

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