In 1960, under the supervision of Henry T. Skinner, the second director of the U.S. National Arboretum, the first “Plant Hardiness Zone Map” was developed. In that map, the United States and southern Canada were divided into 10 zones based on a 10°F difference in average annual minimum temperature. The original map was revised in 1965 and again in 1990.
1990 USDA Zone Map
This map supersedes U.S. Department of Agriculture Miscellaneous Publication 814, “Plant Hardiness Zone Map,” which was revised in 1965. This 1990 version shows in detail the lowest temperatures that can be expected each year in the United States, Canada, and Mexico. These temperatures are based on the lowest temperatures recorded for each of the years 1974 to 1986 in the United States and Canada and 1971 to 1984 in Mexico. This is known as USDA Miscellaneous Publication No. 1475, and was issued in January 1990.
Link: USDA Website
Download: PDFs by state available at BHG.com
2006 Arbor Day Zone Map
Created by the Arbor Day Foundation in 2006. Using newer climate data from the 1980s, 1990s, and early 2000s, it takes into account warmer temperatures recently experienced throughout much of the United States.
Link: Arborday.org
Download: Zipped TIF version
2003 American Horicultural Society Zone Map (Draft)
Created by the American Horticultural Society in 2003. The AHS was commissioned by the USDA to create an updated version of their 1990 map. The updated USDA hardiness map shows in detail the lowest temperatures that can be expected each year in the United States. These temperatures are based on the lowest average temperatures recorded for each of the winter seasons from 1987 to 2001.
Link: AHS.org
Download: PDF version (warning: large file)
