CATHEY, H.M. AND J. HERITEAU. 1990. Mapping it out. Am. Nurseryman 171(5): 55-63.

The long-awaited revision of the US Department of Agriculture's hardiness zone map is complete _ and it reveals some significant changes in North American weather patterns. The biggest alteration is a general southward drift of zone boundary lines, particularly in the Southeast and the Midwest. During the past 15 years or so, observations of local weather patterns made it clear to nurserymen and gardeners that the map's zone boundaries did not match reality. Thus many plants' hardiness zone classifications are no longer considered valid. A map revision was clearly needed. The new map is based on massive amounts of validated meteorological data from almost 8,000 government stations in the US, Canada and Mexico. The staff of Meteorological Evaluation Services was responsible for sorting through millions and millions of data pieces. The new map retains the familiar 10 zones, each of which represents an area of winter hardiness. Zones are separated by 10° increments. Each zone's temperature parameters are unchanged. In addition, the map introduces Zone 11, representing areas with average annual minimum temperatures above 40°.

• © 1991, International Society of Arboriculture. All rights reserved.