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Introduction
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 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.

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