Pruning trees in winter can help reduce the spread of oak wilt - WQOW TV: Eau Claire, WI NEWS18 News, Weather, and Sports

Pruning trees in winter can help reduce the spread of oak wilt


Madison (News Release) – Winter is a good time for tree pruning, according state forestry specialists, who say winter pruning greatly reduces the likelihood of spreading oak wilt and other tree diseases, and it also minimizes pruning stress on trees.

"The best time to prune trees in Wisconsin is during winter when a tree is dormant," according Don Kissinger, an urban forester with the Department of Natural Resources. "Insects and diseases that could attack an open wound on pruned tree aren't active in winter. And without leaves, broken, cracked or hanging limbs and branch structure are easy to see and prune."

Timing is especially critical for pruning oak trees in order to limit the spread of oak wilt, a devastating fungal disease of oaks that has been present in the state for probably a century or longer. Oak wilt fungus spreads from tree to tree by "hitchhiking" on sap-feeding beetles that are attracted to freshly pruned or injured trees and root grafts between neighboring trees.

"Oak wilt causes the water- and nutrient-conducting channels in the tree to plug up and fail," explains Kyoko Scanlon, DNR forest health specialist. "Once a tree is infected, water and stored nutrients can't move upward from the root system, and that causes the tree's leaves to wilt and fall. The tree dies shortly afterward in some species of oak.

"Red oaks, which include red, pin, and black oak, are particularly vulnerable to oak wilt. Once wilting symptoms appear, trees in the red oak group die very quickly, often within a month."

Oak wilt is found mainly in the southern two-thirds of Wisconsin. It has also been found in Barron, Burnett, Florence, Langlade, Marinette, and Polk Counties.

Both Kissinger and Scanlon said prevention is the best defense against this disease. The only other treatment options are costly fungicide applications or trenching between healthy and infected trees in order to sever connected roots.

DNR foresters recommend people stop pruning, wounding, or cutting oak trees in urban settings from April through July. A more cautious approach limits pruning in urban areas until October 1.

"The most critical time for oak wilt infection being spread by insects is the spring and early summer," Scanlon said. "In some years, spring comes much earlier than we expect. If daytime temperatures begin to reach the 50-degree Fahrenheit mark, stop pruning oaks at that time, even if it's still the middle of March."

Communities where oak wilt disease is a problem include Adams, Baraboo, Black River Falls, Durand, Eau Claire, Fort McCoy, Green Bay, La Crosse, Madison, Menomonie, Mosinee, Onalaska, Richland Center, Shawano, Stevens Point, and Waupaca.

Additional information on oak wilt can be found on the forestry pages of the DNR Web site.

Pruning can be beneficial for trees

Before planning any tree pruning, owners should consider some guidelines that will support the health of their trees. Trees should be pruned throughout their entire life, with more attention paid during the first 10 years (every other or every third year) to foster strong structural or "scaffold" limbs. Once proper structure is established, pruning can occur less often (about every five years) to maintain the structure and remove larger pieces of dead wood.

"Pruning should not take more than 25 percent of the live crown of a tree while the lower third of established trunks of deciduous trees should be free of limbs," Kissinger said.

Kissinger offered these tips for pruning shade or deciduous trees and encourages folks to review the DNR pruning brochure [PDF].

  • Remove limbs growing toward the ground.
  • Remove limbs that are crossing, rubbing, or growing parallel to one another, competing for the same space in the tree crown.
  • Remove limbs growing vertically or toward the interior of the tree.
  • Remove broken, cracked, diseased, or dead limbs.
  • Maintain one central trunk or "leader" for as long as possible.
  • Never remove so many interior branches that leaves are only present at the outside edge of the tree.
  • Never prune a branch flush to the trunk as the large wound reduces the tree's natural decay barrier. The cut should begin just outside the branch bark ridge and continue at a slight outward angle.
  • Never "top" trees. This makes the tree vulnerable to decay, it sucks energy from the tree, and it could lead to an early tree death.
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