Winter can be tough on Iowa's trees and shrubs. Low temperatures, rapid temperature changes, winter desiccation, and the weight of ice and snow can damage vulnerable trees and shrubs.
Iowa is located in USDA Hardiness Zones 4 and 5. The average annual minimum temperature in Zone 5 is -10 to -20 F. The average annual minimum temperature in Zone 4 is -20 to -30 F. The dividing line between Zones 4 and 5 lies roughly from Shenandoah to Ames to Dubuque.
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 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.
A rapid drop in temperature over a short period of time can also cause severe plant damage. In Iowa, severe damage to trees and shrubs often occurs when there is a sudden drop in temperature in the fall or early spring. The catastrophic effects of a sudden drop in temperature became painfully clear to apple growers in the spring of 1941. Prior to 1941, Iowa was one of the top apple producing states in the United States. However, on November 11, 1940, a blizzard accompanied by rapidly falling temperatures (temperatures dropped 40 F or more in just a few hours) destroyed approximately two-thirds of the apple trees in the western half of Iowa.
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 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 Iowa 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.
This article originally appeared in the November 7, 1997 issue, p. 148.