Kathy Zuzek, UMN Extension Educator
The effects of winter sun, wind, temperature fluctuations, snow, and ice can all combine to make winter a high-hazard time for tree and shrub health. Animal browsing is an additional challenge in our winter landscapes. Here's a checklist for gardeners who want to minimize injury to woody ornamentals in the coming winter.
Apply tree wraps and tree guards to prevent sunscald. Sunscald occurs when winter sun heats up bark on the south or west side of a tree enough to stimulate cambial activity. When shading or sunset causes the air temperature to drop quickly, the activated cambium is killed resulting in dead bark on the south or west side of a tree in the form of sunken, dried, or cracked areas. To prevent sunscald, tree wraps and guards should be applied in fall. They should be light-colored to reflect sun and to keep bark at a lower temperature on sunny winter days. Pay special attention to trees that have bark newly exposed to sun from recent pruning to raise tree canopies, trees that have been recently transplanted from shady sites to sunny sites, and young trees and other thin-barked trees such as cherries, crabapples, plums, maples, mountain ash, basswood, and honey locust. You will need to protect newly planted and thin-barked trees for several years but remember to remove tree wraps each spring and reapply them in autumn.
Avoid over watering and late season fertilizing that can encourage late season succulent growth. Late season growth is vulnerable to winter injury.
Avoid under watering of your trees and shrubs. Remember to water trees and shrubs in your landscape until the ground freezes. As temperatures drop in autumn and plants are acclimating for winter, it is easy to forget that they still need water. Inadequate watering stresses plants. Avoid this stress by watering until the ground freezes. The average freeze date of soils in Minnesota ranges from mid-November through mid-December; the average date for Minneapolis and St. Paul is December 6.
Roots are less hardy than stems of woody plants and can be injured when soil temperatures reach 10-15 ° F. Thankfully, soil temperatures are much higher than air temperatures in winter. A moist soil holds more heat than dry soil and helps to prevent deep frost penetration.
Evergreen foliage that turns brown during winter due to desiccation and injury is called winter burn (Photo 1). One way this damage occurs is when winter sun and wind trigger transpiration in evergreen foliage. During transpiration stomata (the openings on foliage that allow gas and water exchange with the atmosphere) open and water is lost from the foliage. Because the soil is frozen, the plant cannot replace the lost water and foliage desiccates and turns brown. Damage often occurs on the south, southwest, or windward side of evergreens but in severe cases, an entire plant can be affected. Winter burn can affect all evergreens but yew, arborvitae, and hemlocks are particularly susceptible. Minimize winter burn by watering evergreens until the ground freezes in late autumn or early winter.
Construct a barrier around plants susceptible to winter burn to protect them from winter sun and wind. A second way that winter burn occurs is similar to sunscald damage on tree trunks. Cellular activity in evergreen foliage can be stimulated by sunny winter days that increase tissue temperature in evergreen foliage. When shading or sunset occurs, foliage temperature drops, resulting in foliage injury or mortality. Whether it is caused by transpiration and water loss or by sun's activation of foliage tissue, winter burn is an indication that evergreens were not sited properly in a landscape. A short term and aesthetically unappealing solution to this problem is to construct a barrier of burlap or cut evergreen branches around plants to protect them from winter sun and wind (Photo 2).
Don't apply antitranspirant sprays to prevent winter burn. Most research shows that antitranspirant sprays do not protect evergreens from winter burn.
Apply several inches of mulch around your trees and shrubs to buffer soil temperatures. Mulch is especially important at preventing cold injury to roots during snowless winters. Snow is a great insulator and moderator of soil temperatures. In its absence, several inches of mulch will help to moderate soil temperature.
Protect your trees and shrubs from animal damage. Rodents, rabbits, and deer feed on twigs, bark, and foliage during winter. In severe cases, their feeding can girdle and kill stems or entire plants. Deer rubbing their antlers on trees can also cause damage. Protect tree trunks and shrubs from rodent and rabbit feeding damage by using tree guards or a hardware cloth wrap. Start your protection a few inches below the ground for mice and extend it 24 inches above the average snow line for rabbit protection. Or protect entire beds from rodents, rabbits and deer with wire fencing, repellent sprays, or by hanging repellant-drenched rags.
Prune to prevent snow and ice damage. Prune to eliminate multiple leaders and narrow branch angles (Photo 3) between branches or branches and trunks of trees. Removing these weak branch attachments makes trees less susceptible to snow and ice damage.