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USDA
Plant Hardiness Zone Map—Map Details |
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Why the New Map was Created in 1990
The "Plant Hardiness Zone Map" was published in 1960 and revised in
1965. Since then, many changes, new interests, and new responsibilities
have emerged in North American landscaping:
Changes in Weather. We have been losing
from our landscapes plants that apparently survived the 1940's to the
1960's. Many of the hardiness zone classifications of plants are
no longer considered valid. In North America, the ranges of temperature
and moisture for the past decade were wider than those recorded for the
1940's through the 1960's.
Introduction of New Elite Forms. Our
landscape industry has worked with plant explorers and breeders to introduce
many new forms of traditional plants that are adapted to a wider range
of environments than the older forms.
Scope. The continental United States
is contiguous with Canada to the north and Mexico to the south. We
share more than a common border-we share indigenous plants and many introduced
plants that can be grown successfully. We needed to expand the
scope to include the whole of North America.
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