Time to stop pruning oaks

The safe period to prune oaks is over, according to the Department of Natural Resources. With recent warm temperatures, oaks in most of Minnesota are now at risk of becoming infected with oak wilt.

Oak wilt is a fungal disease that kills oaks. It is spread in part by a beetle that is attracted to sap seeping from a fresh wound on an oak. Oaks can become infected with oak wilt if they are trimmed or damaged when the fungus is present and the beetle is active. This high risk period usually runs from April through July.

“A majestic red oak that took more than a hundred years to grow can be killed within weeks after becoming infected with oak wilt,” said Brian Schwingle, DNR forest health specialist. “So don’t prune your oaks from April through July.”

According to Schwingle, oak wilt spreads from infested trees to healthy trees in two ways—by beetles attracted to fresh wounds and the fungus moving between connected roots of nearby oaks.

To prevent the spread of oak wilt, follow these guidelines:
Trim oaks during the winter.

  • Avoid wounding or cutting oaks when sap beetles are active in spring and summer.
  • Immediately treat wounds with pruning paint during spring and summer months.
  • Prior to removing dead and diseased oaks, sever root connections to healthy trees by trenching around diseased trees with a vibratory plow.
  • Remove diseased oaks in the winter and destroy or treat wood before spring by debarking, chipping or drying the wood onsite.
  • To keep the wood for use, cover it with plastic and bury the edges from the following April through July to prevent insects from spreading the oak wilt fungus.

Oak wilt symptoms start in the tree’s crown and move downward. Trees in the red oak group can die within weeks of becoming infected while trees in the white oak group can take a year to many years to die. 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.

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