Scanned from the USDA Plant Hardiness Zone Map: issued January 1990: Miscellaneous Publication Number 1475. According to Henry M. Cathey, this map "shows in detail the lowest temperatures that can be expected each year in the United States, Canada, and Mexico. These temperatures are referred to as "average annual minimum temperatures" and 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."

Copies of this publication are available from the Superintendent of Documents, U.S. Government printing Office, Washington, D.C. 20402.