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Over the Garden Fence

Plants, diseases & bugs over winter

Our weather can influence how well our landscape plants over-winter. Boxwood, rhododendron, azaleas and evergreen groundcovers get through the winter without all the desiccation associated with cold winters and come out in the spring looking a lot better. Limited or no snow can drive the frost deeper into the soil profile. A winter with temperatures that allow cycles of freezing and thawing soils will heave shallow rooted perennials and groundcovers out of the ground by spring.

There are some good things that happen if our weather is mild for most of the winter. Flower buds that are more sensitive than vegetative buds to cold weather survive in much higher numbers. A good example is forsythia. In a very cold winter, flower buds are killed down to the snow line, if we have snow (we call that a snow skirt of bloom), otherwise clear down to the ground. In a mild winter, we get that great bloom show from the ground to the tip of every branch. Another sensitive plant is the peach tree. You can count on foliage every year, yet flower buds are killed and why a peach may only bear fruit every few years (a mild winter).

However, a downside is that a larger percentage of overwintering adult insects can survive during a mild winter since they are set up to withstand much colder temperatures without dying. An example of an overwintering adult are scale insects on just about any kind of shrub or tree. Recently magnolia scale has been on the increase. Scale insects affect the long-term health of landscape plants and the home orchard. Insects can overwinter as eggs, larvae, pupae too. Eggs are often found in the cracks and crevices of the bark and buildings, larvae and pupae in the soil just below the surface, or in the case of some moths, bagworms and butterflies, as a cocoon attached to a branch. Overwintering adults will collect in strong numbers in cracks and crevices. Lady bugs often will be found in the leaf litter at the base of shrubs.

Plant diseases are going to overwinter in a stage that are likely to survive no matter what kind of weather we have. What will make a difference is extended cool or cold, wet weather in the spring where leaf litter containing the fungal spores has a longer infection period. This is why those pesticide labels address the length of the spray schedule based on local weather patterns.

No matter what kind of a winter weather 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.

About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.