The weather has turned chilly and trees are losing their leaves and entering dormancy. Soon it will be winter, the safe time to prune oak and elm trees.
“These trees should only be pruned during the cold months to protect them from serious disease threats,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic of The Morton Arboretum in Lisle. That’s because the insects that spread these diseases are not active in winter.
Oak trees are vulnerable to a serious fungal disease called oak wilt. It can kill some species within just a few months by interfering with the transport of water from the roots to the branches and leaves. Branch by branch, the tree dries out and dies.
The fungus is spread by sap-feeding insects that are attracted to open pruning wounds and carry the spores from tree to tree. Having trees pruned when the insects are inactive reduces the risk that oaks will become infected through those wounds.
The period from Oct. 15 to April 1 used to be considered safe, but those dates are less certain now that weather has become increasingly variable in both autumn and spring. “The best bet is to have oaks and elms pruned during the coldest part of the winter, in December, January and February,” Yiesla said.
The tree species in the red oak group — red oak, black oak, Hill’s oak and pin oak — are most susceptible to oak wilt and will die most quickly once infected.
Species in the white oak group, such as white oak, bur oak, English oak and swamp white oak, may survive with the disease for several years while they slowly decline. Meanwhile, the fungus will have formed mats under the bark, full of spores that insects can carry to other oaks. The disease can also spread when roots of neighboring trees become grafted together underground.
The first sign of oak wilt is usually a single branch where all the leaves die back during the growing season. A laboratory analysis is needed to confirm the disease and can be arranged by a trained, certified arborist. “If you see a dying branch on an oak tree, call in an arborist right away to check for oak wilt,” Yiesla said. “Especially for red oaks, time is of the essence.”
Hire an arborist certified by the International Society of Arboriculture, who will have the expertise to spot signs of oak wilt or other diseases.
An infected oak can sometimes be saved if the disease is contained in one branch that can be removed. Once the fungus has spread throughout the tree, there is no cure.
That means prevention is key. “If you need to have an oak pruned to remove damaged branches or improve its structure, have it done in winter,” Yiesla said.
Elms, too, should be pruned only in winter, because the deadly Dutch elm disease is spread by bark beetles that are inactive in cold weather.
If any kind of tree needs to be pruned, winter is generally the best time to do it, she said, because the tree will be dormant and not actively growing. With the branches bare of leaves, a trained arborist will be able to see the tree’s structure and check its health.
Mature trees should only be pruned by certified professionals. “A certified arborist has the training and equipment to do it safely,” Yiesla said. You can find a certified arborist through the websites of the International Society of Arboriculture (treesaregood.org/findanarborist) or the Illinois Arborist Association (illinoisarborist.org).
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
