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 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. 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.