University of Illinois Extension

Plants, Bugs, Diseases and a Mild Winter

A mild winter can mean more insects and disease in the landscape next season, said Richard Hentschel, U of I Extension horticulture specialist.

“Rumor has it that our winter will be a mild one, not a wild one,” says Hentschel. “Mild winters can mean less snow or snows not lasting all winter. If it does not come down as snow, then it will be rain or sleet depending on what part of the state you live in. Open ground can have a strong impact on the depth of frost in our garden beds in the colder parts of Illinois.”

Mild winters, he noted, can create a lot of freezing and thawing of the ground, causing our perennials to heave up and out of the ground, exposing the plant crown to unnecessary cold temperatures.

“Warmer winter temperatures can also confuse our plants that are expecting to get cold and stay cold until spring,” he said. “We often see this in early spring bulbs and tender perennials that try to start to grow too early and damaged by frosts and drops in temperatures below freezing.”

Hentschel noted that gardeners can help avoid this by mulching those plants once the ground has gotten cold. Mulch will keep the ground cold and the plants dormant. This will keep them from heaving too. Mulching those late summer and fall transplants will further the plants’ ability to survive that first winter with a limited root system.

“There are some advantages to a mild winter,” he said. “Flower buds on fruits trees are less hardy than their vegetative buds, so a mild winter will allow more flowering and later more fruit but be sure to thin those fruits and not over burden the fruit tree. The same applies to our flowering landscape shrubs. Forsythia will bloom from the ground to the tips when winters are mild.”

Over 80 percent of our plant diseases are caused by fungi. A mild winter allows those fungal spores and structures to live on more, decaying moist plant parts.

“This past summer, apple scab became a serious situation for apple trees and flowering crabapples, even those varieties noted for disease resistance,” he said. “The overwintering fungal structures will be in the leaf litter waiting to strike again next spring during cool, wet weather. Raking and removal of as much of that leaf litter as possible will have an enormous impact on disease pressure this coming spring.

“This is the reason why gardeners clean up their beds, removing spent plant parts and weeds, to remove the overwintering fungal structures and lessen the disease pressure.”

A mild winter will not affect populations of insects routinely surviving much colder winter temperatures. It will have an impact on insects that typically move into northern parts of Illinois from southern or central Illinois or neighboring states that do not overwinter here. A mild winter will allow more of them to survive in their normal habit before migrating to other parts of Illinois.

For those insects that normally overwinter in northern Illinois, it would take a very cold winter over an extended period to lower those pest populations. Insects can overwinter as eggs, larvae, pupae or adults using a variety of tactics to survive.

“Leaf litter at the base of a shrub is a favorite of ladybug adults,”he said. “Scale insects overwinter as adults exposed on the branch and a more recent pest, the Emerald Ash Borer, as a larva just below the bark surface.”

No matter what kind of a winter we end up with, gardeners know that diseases and insects will develop right along with our landscape plants, so be on the lookout early and often for potential problems.