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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