I'm afraid you have misunderstood the USDA standards for the
climate zones. The
climate zone records the AVERAGE winter low temperature. If the
lowest
temperature one year is -20 and the lowest temperature the next
year is zero,
you have an average minimum temperature of -10.
And, of course, there are quite a few more than 2 years from
which to work out the average, so it is fairly accurate.
We who live south of the Mason-Dixon line (USA), of course, wish
plant people would adopt the American Horticultural Society's
Heat Zone index. It is MUCH more important to us. I've seen it
used in a couple of (non-bonsai) books, but the nurseries have
yet to adopt it.
A good alternative would be the Sunset zones, which are based on
a number of factors. I do question, however, how valid those
zones are outside of the 10 western states that are Sunset's home
turf.
I was doing a Google for climate zones the other day and found
references to climate zone schemes in North America, Canada,
Europe, Great Britain, and Australia before I found what I was
looking for and quit the search. There MAY have been one for
Africa, too?????
Jim Lewis -
- Tallahassee, FL - Nature
encourages no looseness, pardons no errors. Ralph Waldo Emerson
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