Winter, when the branches of dormant trees are bare, is a good time to have them pruned. Oaks should only be pruned when they are dormant. (Beth Botts)
By Beth Botts
Morton Arboretum
The gray, still days of winter are the perfect time to prune trees. In fact, winter is the only recommended time to prune some species, such as oaks.
“If you prune oaks during the growing season, you risk spreading serious diseases,” said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle. One of the worst is a fungal disease called oak wilt.
The oak wilt fungus is spread by a beetle that is attracted to open pruning wounds in trees and carries the spores from tree to tree. Since the beetles aren’t active in winter, the safe period for pruning an oak is between Oct. 15 and April 15, Yiesla said.
Oak wilt is difficult or impossible to treat, she said, so the best way to fight it is to prevent it. The most important thing a homeowner can do to protect an oak is to prune it only in winter, unless it has been damaged by a storm.
Winter is also the best time to prune other trees, when they are dormant and not actively growing, Yiesla said. As with oak wilt, the cold will reduce the likelihood of spreading other pests and diseases.
The bare branches also make it easier for a trained arborist to see the tree’s structure and check its health. And if the ground is frozen when trees are pruned, surrounding perennial beds and other garden areas won’t be damaged by professionals’ equipment.
Large, mature trees should 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 website of the International Society of Arboriculture (treesaregood.org/findanarborist) or the Illinois Arborist Association (illinoisarborist.org).
Prune a tree yourself only if you can do it with your feet on the ground. Working above the ground to prune tree branches that may weigh hundreds of pounds requires the special safety training and equipment that professional firms have. “It’s easy to get seriously hurt pruning trees,” Yiesla said.
“Always err on the side of hiring a professional and keeping yourself safe.”
It’s a good idea to have large, mature, valuable trees inspected every few years, to catch any problems early. “That’s a good thing to do in winter too,” she said.
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.
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