The first such map for North America, released by the United States Department of Agriculture in 1960, was based only on minimum winter temperatures. In 1967, Agriculture Canada scientists created a plant hardiness map using Canadian plant survival data and a wider range of climatic variables, including minimum winter temperatures, length of the frost-free period, summer rainfall, maximum temperatures, snow cover, January rainfall and maximum wind speed.
Most plant catalogues highlight the hardiness zones that the plant will thrive. Most cold hardy plants will survive in warmer zones but not the other way around.
Information from http://sis.agr.gc.ca/cansis/nsdb/climate/hardiness/intro.html