Contact(s): Paul Cigan, DNR forest health specialist, 715-416-4920, Paul.Cigan@wisconsin.gov and Jodie Ellis, DNR forest health program, communications specialist, 608-266-2172, Jodie.Ellis@wisconsin.gov
MADISON - State forestry specialists say trees should be pruned throughout their entire lives to maintain strong structure and remove dead wood. Young trees should be pruned to establish good branch structure and shape, while older trees are pruned to remove dead and/or hazardous limbs.
"The best time to prune hardwood trees in Wisconsin is during winter when a tree is not actively growing," said Paul Cigan, forest health specialist with the Wisconsin Department of Natural Resources. "Pruning is easier in winter when the leaves are gone, which makes it is easier to see damage on tree branches and limbs."
"Pruning should not remove more than 25 percent of the live top of a tree. The lower third of trunks of hardwood trees, such as oak and maple trees, should be free of limbs," Don Kissinger, a DNR urban forestry specialist, said. The DNR offers a pruning brochure that has more detailed, step-by-step tips for tree pruning. Find it by searching the DNR website, dnr.wi.gov, for keywords "tree pruning [PDF]."
Another advantage is that many tree pests "hibernate" and cannot spread tree diseases during winter. The fungus that causes oak wilt, a fatal disease of oak trees, is spread in warmer months by tiny sap-feeding beetles, which carry the pathogen between oak trees as they feed where sap is leaking. Since the act of pruning causes some seepage, it is best to prune in winter when it is too cold for the beetles to be out.
"The DNR recommends pruning oak trees between October and March when the risk of spreading oak wilt is the lowest," Cigan said.
Oak wilt is also spread in firewood. Several recent finds of infected trees in northern Wisconsin were likely the result of infected firewood brought from other areas where oak wilt is established. A good practice to prevent the spread of the disease is to keep oak firewood where it is cut for one year or until the bark is naturally loose. Besides oak wilt, Kissinger said firewood can carry many kinds of diseases and pathogens, so it is important people who move firewood in Wisconsin be aware of the risks, obey the law and take recommended precautions.
A list of certified arborists who offer pruning and other tree care services is available on the Wisconsin Arborist Association website at: www.waa-isa.org/arborist-for-hire/ (exit DNR).