Bob,
These maps are interesting somewhat. I live in NW FL zone 9 on Arbor Day map. Well, ironically, Tampa is also in zone 9 for the 2006 map! The climate is markedly different between here and there--at least a zone or more, I'd say.
All these maps are based on(to my understanding) is the actual low temperature for a given year, averaged over a 10 year(?) period. For example, the map of 1990 would be ab average of the low temps from 1980-1989. Or do they do a longer average?
In the 1990 map, I am in zone 8. I believe because our record lows in Dec 1983 and Jan 1985 skewed the results of the prior period because it was around 11 degress in 1983 and about 6 in 1985. (Last season's low was 18--which I consider about average.) The period from 1990-1999 didn't see any lows near 11 degrees--muchless 2 years of the 10 year period near single digits. This can materially affect the average. Remember also that it only takes a 1 degree average difference to change 1 zone(on paper) if your average low is near 20 degrees----the breakpoint. Zone 8 is defined as 10-20 degrees. So for many years, I have often said that I live in zone 8 1/2(which is borderline 8/9).
Another factor to consider when using the USDA temperatures is the fact that many of the reporting stations are now in urban areas that used to be rural. Urbanizing can increase the temperatures recorded by a degree or 2-----making it look like "GW", when it really isn't. It's a change in the surroundings of the measuring devices, instead. (instead of "GW", we have "overlooking something right under our noses"--which happens often these days, it seems.)
Regards,
Richard Harrison, NW FL
I'll get the email address of the researcher who spoke in LA last month who had the USDA zone charts and ask what year's chart he had to see if he supposedly had one since 1990. Personally, I believe I have seen 3 since I started growing fruit about 1977 or so. I COULD have been seeing an "old" USDA chart then, maybe they came out with a new one about 1980 and another in 1990. I'm not sure. But I'm pretty sure that I have seen at least 3---usually published in nursery catalogs.
--- On Mon, 8/3/09, Bob Randall <YearRoundGardening@comcast.net> wrote:
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