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USDA
Plant Hardiness Zone Map—Map Details |
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How the New Map Was Created
The average annual minimum temperature data were analyzed for Mexico,
the United States, and Canada. Of 14,500 stations that measured
temperature during the period of interest, almost 8,000 could be identified
by latitude and longitude and by a valid average annual minimum temperature
(i.e., an average based on at least 10 years of data). Data from
only the latter stations were used in the map. The data were archived
by Servicio Meteorologico Nacional (Tucubaya, D.F., Mexico), the National
Climatic Data Center (Asheville, NC), and Environment Canada - Canadian
Climate Centre (Downsville, Ontario). Temperature data were compiled
and maps prepared under contract with the Meteorological Evaluation Services
Co., Inc., 165 Broadway, Amityville, New York 11701. The
map is an Albers Equal Area Projection. Standard parallels of 29.5
, and 45.5 , were used to generate the map of the three countries. The
map was computer generated by latitude and longitude. Because of
the large area involved, it is not possible to draw one map that is accurate
for all of North America. The part representing the United States
has the least distortion. The Agricultural Research Service proposes
to periodically evaluate weather data and issue updated maps as necessary
and appropriate.
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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