Take advantage of winter to have trees pruned

The bare branches of trees are starkly beautiful against the winter sky. They also make it easier to have the trees pruned.

Without leaves, it is easier for a trained arborist to see a tree’s structure and check its health, said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum.

Winter pruning is safer for the tree because cutting into the wood now is less likely to spread diseases or insect problems. “When a tree is actively growing in spring or summer, pruning wounds are open doors to disease,” Yiesla said.

That’s why winter is the only recommended season to prune some species, such as oaks, which are especially vulnerable to several easily spread diseases.

Oak wilt, for example, is a potentially deadly fungal disease spread by a species of beetle that is attracted to open pruning wounds and carries the spores from tree to tree. “These beetles are not active in winter,” she said. “That’s why you should only prune oaks between Oct. 15 and April 15.”

The risk of spreading problems is also lower in winter for other kinds of trees. “The dormant season is generally the safest time to prune any species,” Yiesla said. The Arboretum does much of its tree pruning in winter.

If you delay until late winter, after the end of February, some tree species may leak sap in their pruning wounds because they are already preparing for spring. These species include maples, walnuts, birches, beeches, hornbeams and yellowwood. “The sap can be unsightly, but the leaks don’t harm the tree,” she said.

To avoid the unsightliness, you may want to prune these species in the deep cold of winter or wait until very early summer, when the tree has leafed out completely.

Of course, circumstances may make it necessary to prune trees at any time of the year. “If a storm breaks the trunk or branches, or a branch poses a danger, it needs to be pruned promptly,” Yiesla said. “But if you have a choice, or for routine tree inspections, it’s a good idea to have the work done in winter.”

Prune a tree yourself only if it is small enough so that you can do the job while keeping your feet on the ground. “If you would need to climb a ladder to prune a tree, call a professional,” she said. “Pruning large trees is a dangerous business.”

The tools are sharp. It’s easy to fall. And large branches that can weigh hundreds of pounds may drop or swing in unexpected ways that can cause serious injury.

A certified arborist has the training and equipment to prune a big tree safely and also has the expert knowledge to spot problems such as cracks, rot or disease. 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 more guidance about having trees pruned, see mortonarb.org/pruning-trees.

Mature trees are valuable assets that increase property values. “It’s worth the investment to have them pruned when they need it and inspected every few years to catch small problems before they become bigger, much more expensive problems,” Yiesla 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.