University of Wisconsin–Madison

Wisconsin Geospatial News

Plant hardiness zones creep northward

Serious gardeners know to consider a perennial plant’s hardiness zone rating before expecting blooms year after year. Although a single brutal winter may kill a specimen that is marginally adapted to an area’s weather, climate averages are a good guide to plant selection.

The U.S. Dept. of Agriculture has recently revealed the second revision of its Plant Hardiness Zone Map which was published in 1965 and first revised in 1990. The latest map is the first to be designed as an electronic document.

The new map was produced in cooperation with the American Horticultural Society and draws on data from 7,000 weather stations from 1986 through 1990. The data used is the coldest temperature for each winter which is then averaged. The point values are then used to interpolate lines that mark the boundaries of hardiness zones.

A related Plant Heat-Zone Map shows zones of average high annual temperatures, another important factor relating to plant survival.

To see the maps, visit www.ahs.org. Look under "Publications." (source: The Capital Times, 4/5/2003)