Submitted to: Landscape Plant News
Publication Type: Research Notes
Publication Acceptance Date: 8/1/1999
Publication Date: N/A
Technical Abstract: There are many climatic factors that limit the adaptation of woody landscape plants. In the North Central United States, one of the most widely studied limiting factors is winter injury caused by low temperatures. Mean annual minimum temperature is the basis for the USDA Plant Hardiness Zone Map, perhaps the best-known guide for defining woody plant adaptation in the region. Results from two, long-term evaluation projects to evaluate landscape trees and shrubs at multiple sites across the North Central U.S. indicated that winter low temperatures were not the only climatic determinant of plant establishment and overall survival. Those projects demonstrated that mean annual moisture balance was another important factor influencing plant establishment and survival. Mean annual moisture balance can be estimated by a moisture index proposed in 1955 by Thornthwaite and Mather. It is calculated on the basis of two variables, potential evapotranspiration and mean annual precipitation. This report presents a map of the moisture index for the North Central region, clearly demonstrating that mean annual moisture surpluses are found throughout the eastern half of the region, with the largest surpluses in Michigan's Upper Peninsula and the northeastern corner of Ohio. Much of the western half of the region experiences moisture deficits with the most serious deficits found in northwestern South Dakota and western North Dakota. This map can be used by horticulturists as a tool to improve the match between woody landscape plants and suitable climatic conditions.