Today, the USDA map, which was last updated and released in 1990
(based on weather records from 1974-1986), is generally considered the standard
measure of plant hardiness throughout much of the United States. Hence we
have the USDA Plant Hardiness Zones.
The USDA plant hardiness map divides North America into 11 hardiness zones.
Zone 1 is the coldest; zone 11 is the warmest, a tropical area found only
in Hawaii and southernmost Florida (and maybe the very southwest corner of
San Diego County, California). In between, the zones follow a fairly predictable
pattern across the continent, though a closer look will reveal scattered
patterns of variations. Generally, the colder zones are found at higher latitudes
and higher elevations.
Plant encyclopedias may refer simply, for example, to "Zone 6," which generally
means that the plant is hardy to that zone (and will endure winters there),
and generally can withstand all the warmer zones below. More detailed information
may indicate a range of zones (i.e., "Zones 4-9"), which means the plant
will only grow in those zones, and will not tolerate the colder and warmer
extremes outside them. But remember, zones are only a guide. You may find
microclimates that allow you to grow more than the books say you can; by
the same token, you may find to your dismay that some precious plant -- one
that's "supposed" to be hardy in your zone -- finds its way to plant heaven
instead.
You can roughly translate the USDA hardiness zones by finding out how low
your area's temperatures can reach, and then use the chart below to find
your corresponding zone.
Zone 1: below -46 C (below -50 F)
Zone 2: -46 to -40 C (-50 to -40 F)
Zone 3: -40 to -34 C (-40 to -30 F)
Zone 4: -34 to -29 C (-30 to -20 F)
Zone 5: -29 to -23 C (-20 to -10 F)
Zone 6: -23 to -18 C (-10 to 0 F)
Zone 7: -18 to -12 C (0 to 10 F)
Zone 8: -12 to -7 C (10 to 20 F)
Zone 9: -7 to -1 C (20 to 30 F)
Zone 10: -1 to 4 C (30 to 40 F)
Zone 11: above 4 C (above 40 F)
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