Prevent winter damage to trees and shrubs

By Gary Johnson, University of Minnesota Extension

Don’t automatically blame cold weather for killing your plants over the winter. Most plants that suffer chronic winter damage and die are stressed plants that would have died from something else.

Keep the plants healthy enough to survive the stresses of winter and prevent damage when possible.

Winter damage usually falls into one of four major categories: mechanical, desiccation, cold temperatures or chemical injury. Mechanical damage is the most common and includes animal-feeding damage, heavy snow and ice loads that break branches, and snow blower or plow damage.

Desiccation or drying of living tissues damage occurs commonly on plants that have evergreen foliage or large buds. The drying winter winds on fully exposed landscape sites constantly dry out evergreen foliage tissues and large flower buds, especially on more sensitive evergreens, such as yews, arborvitae, and hemlock. Desiccation may also happen to plants exposed to the bright southern sun of winter that warms up buds and foliage, causing the tissues to lose too much moisture as they transpire.

Cold temperatures do sometimes cause damage. True frost cracks (cracks through the wood of a stem) are partially due to cold temperatures. Unusually early autumn frosts and unusually late spring frosts can kill plant tissues either before or after they are safely dormant. Plants that are hardy to 30 degrees below zero when they are fully dormant can be damaged by temperatures as “warm” as 25 degrees when they are not dormant.

Finally, chemicals from de-icing salt spray drift or runoff may cause the worst injury to plants. Plants within 60 feet of a busy highway or street are most at-risk for this type of damage. Most of the time, salt spray damages foliage and buds, disfiguring the plant but not killing it. Salt runoff that accumulates in the soil can become toxic and eventually kill plants.

The following four easy steps will help you minimize winter damage:
< Keep plants healthy. It is especially important to keep plants watered until the ground freezes. Moist soil freezes slower and stays warmer than dry soil.
< Mechanical injury requires physical protection. Some examples of protection are hardware cloth, snow fencing and deer netting.
< Rely on mulch rather than snow. Four inches of organic mulch is as effective as a good snow cover for keeping soil warmer and roots healthy, and it’s more reliable.
< Don’t plant close to roads. Most plants are best situated at least 60 feet from highways and 30 feet from streets that use a lot of de-icing salt.

(Gary Johnson is an urban and community forestry professor with University of Minnesota Extension.)