The USDA (US Min of Ag) has developed a map of "plant hardiness" zones
based on average minimum temperature range. Sometimes these are also
associated with first and last frost dates. It's used as a rough
(very) guide to what sort of plants will successfully survive winter
temperatures in a given area. There are more detailed maps taking
micro-climate conditions into account published by a company that used
to specialize in western (US) gardens, but has now ventured into other
areas. These are the "Sunset (magazine) zones."
USDA zones run from 1 (-50F) for Fairbanks, Alaska to 11 (greater than
40F) for Honolulu, Hawaii. They hedge their bets a bit with a-b
sub-designations. I.e., the SE corner of Virginia is either 7b or 8a
(minimums between 5 and 15F.
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