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Growing Green: Protect Your Trees and Shrubs from Winter Injury

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In April and May, I get many calls from distressed homeowners concerned about trees and shrubs that did not fare well during the treacherous winter months. Many of these plants are suffering from cold damage such as sun scald, dieback, root injury, and frost heaving. A few cultural practices now can help prevent such often irrecoverable damage from occurring.

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Sun scald is caused by the bark heating up on cold winter days, then chilling rapidly when the sun goes down or is blocked such as from trees and clouds. The heating causes the growing tissue in the bark, near the surface, to become active. The sudden drop in temperature then kills this tissue. This usually occurs on the south side of trees, and results in sunken, elongated and darkened areas of bark. Such damage is most common in young trees and thin-barked species, including: cherry, crabapple, basswood, maple and mountain ash.

To prevent sun scald, wrap the bark of susceptible trees in late fall with either tree wrap tape, plastic tree guards, or a light material such as burlap. Remove the wrap in the spring after the last frost, to avoid warm season damage caused by insects and disease. Wrap newly planted trees or young trees for at least two winters after planting. Wrap thin-bark trees for at least five winters.

Cold can injure evergreens through discoloration and browning, particularly in late winter. Such browning is often referred to as "desiccation", or a drying out of needles. Sunny days in winter, or wind, can cause evergreen needles to lose water or "transpire". Since the ground is frozen, roots can't take up replacement water, so the result is the needles drying out and browning.

The best solution to avoid winter browning is to make sure evergreens go into the winter well-watered. Trees require 10 gallons of water/inch diameter of trunk/week. Make sure to water in the fall until the ground is totally frozen and will no longer allow water to penetrate. Another solution is to erect a protective screen around evergreens. Such screens should be on windward sides of the plants, and on the south sides to lessen sun heating.

For an online publication on how to prevent other winter damage to trees and shrubs from snow and ice, road salt, and animals cold damage, visit http://www.extension.umn.edu/distribution/horticulture/dg1411.html

Robin Trott is a Horticulture Educator with University of Minnesota Extension.

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