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I voted for 8a thru 10a. In my opinion an average annual min temp of 10-15F (hz=8a) isn't too cold to be considered subtropical. Places like Nashville, Atlanta, Fl panhandle, Huntsville, etc hit this mark somewhat often, but they are certainly not continental climates - they're way too warm to be. Average numbers are what really matter, so a few sparse winter cold spells don't change the entire climate classification of the area. However, I draw the line between 7b and 8a, because once you get single-digit temps almost every year, it's a bit overambitious to call it humid-subtropical imo.
8b-9b sounds about right. I'm in 8b, and some flora like cabbage palms thrive here. 8b starts right around the fall line in Georgia and points south. Granted, we see freezing weather a lot in winter (it's currently 32 F out right now), but this only happens either late at night or early in the morning under clear skies. Winter storms are just too rare down here and don't happen as often as Atlanta/the piedmont. 9b seems to include all of central Florida and the far inland areas of south Florida, so that seems about right in regards to the southern limit of this "climate". Areas in 10a might see a freeze only about once in a decade, allowing many tropical flora to grow there.
This photo pretty much is my point in excluding 10b from the poll. Fruiting coconut palms in a 10b zone? Not very subtropical IMO . Photo is from Sanibel Island near Cape Coral, FL. So, to me, subtropical is perfect for subtropical flora, not for barely growing tropical flora. So, no 10b in the poll
I think I agree with this assessment. 8b even in the Cfa US south is too cold in winter to be truly subtropical IMO as the number and severity of freezes each winter is too great. With this distinction, most of the inland south and even a small part of the Florida panhandle aren't subtropical IMO.
So a place in China like Nanchang that is zone 9a, but has an avg Jan high/low of 46/37 is "subtropical", while a place in Georgia zone 8b with Jan avg high/low of 63/37 is not? You can't use USDA zones to define a climate. Therefore London is subtropical and Vancouver. USDA zones use the years 1976 to 2005. Some of these 8b places in the US, when you look at their temps since 1990 are zone 9a. The 80's were a crazy cold decade in the eastern US.
When you can grow Queen Palms and Bougainvillea don't you think that is subtropical.
Last edited by tom77falcons; 11-28-2014 at 11:04 PM..
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. Photo is from Sanibel Island near Cape Coral, FL. So, to me, subtropical is perfect for subtropical flora, not for barely growing tropical flora. So, no 10b in the poll 
