Wisconsin’s harsh winter climate is often responsible for damage to landscape plants; native and non-native plants alike can be killed or injured during the winter. Cold temperatures can harm plants if winter conditions are severe or if plants have been stressed by the environment. Injury is more prevalent and more severe when low temperatures occur in early fall or late spring, when there is little or no snow cover during the winter or when low temperatures persist. With proper care, we can minimize these injuries and prolong the lives of our trees and shrubs.
Browning or discoloration of evergreen needles during winter occurs for a couple of reasons. Winter winds cause excessive transpiration (water loss in the needles) while the roots are frozen in the soil and unable to replace lost water. This results in desiccation and browning of the plant tissue. Bright, sunny days can warm the tissue above normal temperature which in turn initiates cellular activity. Then, when the sun is quickly shaded, foliage temperature drops rapidly and the foliage is injured or killed.
There are several ways to minimize winter injury to evergreens.
- It is important to wait until mid-spring before pruning out damaged foliage. Brown foliage is most likely dead, but the buds, which are more cold hardy than foliage, will often grow and fill in areas where brown foliage was removed. If the buds have not survived, prune dead branches back to living tissue.Avoid planting evergreens on south or southwest sides of buildings or in highly exposed (windy, sunny) places.
- Prop pine boughs or Christmas tree greens against or over evergreens to protect them from wind and sun and to catch more snow for natural protection.
- Construct a barrier of burlap or similar material on the south, southwest, and windward sides of evergreens.
It is important to wait until mid-spring before pruning out damaged foliage. Brown foliage is most likely dead, but the buds, which are more cold hardy than foliage, will often grow and fill in areas where brown foliage was removed. If the buds have not survived, prune dead branches back to living tissue.
Heavy snow and ice storms cause damage by bending and breaking branches. Trees with multiple leaders, such as arborvitae, juniper, and birch, are most subject to snow and ice damage. Trees can be wrapped together or the leaders tied with strips of carpet, strong cloth or nylon stockings two-thirds of the way above the weak crotches (illustration to right). These wrappings must be removed in spring to prevent girdling. Proper pruning by a professional arborist, to eliminate multiple leaders and weak branch attachments, will reduce snow and ice damage.
Deicing salt can cause or aggravate winter injury and dieback too. Salt runoff can injure roots and be absorbed by the plant, ultimately damaging the foliage. Salt spray from passing vehicles can also cause severe injury. To prevent salt damage, do not plant trees and shrubs in highly salted areas. Avoid areas where salty runoff collects or where salt spray is prevalent. Burlap barriers may provide protection to some plants from salt spray.
Mice, rabbits, and deer can all cause severe damage to plants in the winter. These animals feed on the tender twigs, bark, and foliage of landscape plants during the winter. They can girdle trees and shrubs and eat shrubs to the ground line. Deer can cause significant injury and breakage by rubbing their antlers on trees during the fall.
Trees can be protected from rodent damage by placing a cylinder of ¼-inch mesh hardware cloth around the trunk. The cylinder should extend 2 to 3 inches below the ground line for mice and 18 to 24 inches above the anticipated snow line for rabbit protection. Hardware cloth can be left on year-round, but it must be larger than the trunk to allow for growth. For small trees, plastic tree guards are also effective.
Deer feed on and damage the branches of small trees and shrubs. Plants can be sprayed or painted with the repellent; however, the most effective procedure is to hang repellants around susceptible plants. Deer can also be successfully excluded with fencing. To be effective, fences must be high (8 feet or higher) and constructed properly.
Although winter injury is a common concern in Wisconsin, appropriate plant selection, selecting the proper site, good cultural practices, and preventive maintenance will significantly reduce or prevent damage or loss of landscape plants.
Tony Seidl, ISA Certified Arborist NY-5908A