Winter is Tough on Trees and Shrubs
by Don Janssen, Extension Educator
Winter can be tough on trees and shrubs. Low temperatures, rapid temperature changes, winter desiccation, and the weight of ice and snow can damage vulnerable trees and shrubs.
Nebraska is located in USDA Hardiness Zones 4 and 5. The average annual minimum temperature in Zone 5 is -10 to -20 degrees F. The average annual minimum temperature in Zone 4 is -20 to -30 degrees F. The dividing line between Zones 4 and 5 lies roughly along Interstate 80.
Woody plants gradually acclimate to cold temperatures. Cold hardiness is initiated by decreasing daylength and temperature. Trees and shrubs gradually become more cold hardy during the fall and early winter season and possess maximum cold hardiness in mid-winter. Cold hardiness then decreases. As a result, a temperature of -5 to -10 degrees F in January is generally not a problem for hardy plants. However, a temperature near zero in early November or late March may cause considerable damage to poorly adapted trees and shrubs.
The best way to prevent damage caused by low temperatures or rapid temperature changes is to select trees and shrubs that are hardy in your area. Marginally hardy plants should be planted in protected sites, such as courtyards or eastern exposures. Avoid late summer pruning and fertilization of trees and shrubs. Late summer pruning and fertilization stimulate late season growth and delay the hardening process, making the plants more susceptible to winter injury.
Narrow and broadleaf evergreens lose considerable amounts of moisture through their leaves or needles, buds, and stems during the winter months. The cold, dry winds and sun are mainly responsible for the water loss. Once the ground freezes, however, plant roots are no longer able to absorb water. Plant foliage that loses a large amount of moisture may dry and suffer desiccation injury.
Plants susceptible to desiccation injury should be planted in protected areas. A shield or screen can be erected to deflect drying winds or shade exposed plants. A simple screen can be constructed with wooden posts and burlap. Anti-desiccants can also be used to prevent desiccation injury. When sprayed on plant foliage, these materials form a protective film that slows water loss. In dry years, water evergreens susceptible to desiccation injury in the fall.
Major damage to trees and shrubs can also be caused by the weight of ice or heavy, wet snow. Multi-stemmed evergreens, such as arborvitae, and weak-wooded deciduous trees, such as Siberian elm, green ash, and silver maple are most susceptible to branch breakage. High winds during an ice or snow storm can greatly increase tree and shrub damage. Oak, crabapple, pine, spruce, and fir are less susceptible to winter storm damage.
When heavy, wet snow accumulates on shrubs and small trees, home gardeners can gently shake the snow from their branches or carefully brush off the snow with a broom. Sharply bent, ice-covered branches on small trees and shrubs can be propped up slightly to prevent breakage. Don't attempt to remove the ice by beating the tree or shrub with a broom or rake. This may only cause greater damage.
Individuals should stay away from large, ice-covered trees. Nothing can be done to prevent damage to large trees. Individuals, however, can be severely injured or killed if a large, ice-laden branch or tree were to suddenly crash to the ground while underneath it.
Trees and shrubs in Nebraska often have to endure a long and harsh winter. Proper plant selection, placement in the landscape, and pruning can reduce winter injury to woody ornamentals.
University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office