Introduction
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 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.  The map shows 10 different zones, each of which represents an area of winter hardiness for the plants of agriculture and our natural landscape.  It also introduces zone 11 to represent areas that have average annual minimum temperatures above 40 F (4.4 C) and that are therefore essentially frost free.

How to Use the New Map
Zones 2-10 in the map have been subdivided into light- and dark-colored sections (a and b) that represent 5 F (2.8 C) differences within the 10 F (5.6 C) zone.  The light color of each zone represents the colder section; the dark color, the warmer section.  Zone 11 represents any area where the average annual minimum temperature is above 40 F (4.4 C). The map shows 20 latitude and longitude lines.  Areas above an arbitrary elevation are traditionally considered unsuitable for plant cropping and do not bear appropriate zone designations.  There are also island zones that, because of elevation differences, are warmer or cooler than the surrounding areas and are given a different zone designation.  Note that many large urban areas carry a warmer zone designation than the surrounding countryside.  The map-contains as much detail as possible, considering the vast amount of data on which it is based and its size.