Published: January 22, 2008 by the Central Office
MADISON - Snow shoveling isn't the only winter yard work home and landowners may want to undertake. Winter is a good time for tree pruning especially on oak trees. Winter pruning greatly reduces the likelihood of spreading oak wilt and other tree diseases and minimizing pruning stress on trees, according to tree health experts.
"I tell people the best time to prune trees in Wisconsin is during the winter, when the tree is dormant, which is before April, for a number of reasons," said Don Kissinger, an urban forester with the Department of Natural Resources in Wausau. "Insects and diseases that could attack the open wound aren't present 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 Wisconsin for probably a century or more, according to forest health specialists. The 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 in Fitchburg. "Once a tree is infected, water and stored nutrients can't move upward from the root system, causing 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 this disease. 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 and in the extreme northeast corner of Wisconsin. Prevention is the best defense against this disease say tree experts as the only other treatment options are costly fungicide applications or trenching between healthy and infected trees to sever connected roots
DNR foresters recommend people stop pruning, wounding, or cutting oak trees in the urban setting from April through July (April, May, June, July). A more cautious approach limits pruning until Oct. 1.
"The most critical time for oak wilt infection through 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 oak 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.
Before planning any tree pruning, tree owners should consider some rules designed to help further 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:
FOR MORE INFORMATION CONTACT: Don Kissinger: (715) 359-5793 or Kyoko Scanlon: (608) 275-3275