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Glasgow, Portland and North Florida all hover around the border of 8/9, but does this actually mean that Glasgow and Portland are just as capable of growing subtropical flora as North Florida or that their climates are fundamentally similar in any way beyond the average annual low?
Hardiness zones are only useful in determining whether a plant will die during winter. It doesn't measure warmth, which is a requirement for subtropical flora. A combination of growing degree days and hardiness zone would be a much more accurate indicator of a climate's plant growing capability.
The more I learn about hardiness zones, the more useless I think they are. I actually think the USDA should abandon them and come up with another system which incorporates many more factors. The sunset climate zones are useful except that you can't extrapolate them to other parts of the world outside North America. Sunset Climate Zones - Sunset Mobile
the swedish vegetation zone map seems to incorporate both growing seasons and the minimum temperatures and some other stuff. couldn't find the full explanation, but here's a bit about that.
According to the National Association Swedish Garden zone map. Sweden is divided into eight growing zones. Zone I is the country's mildest parts and Zone VIII of the harshest regions. The higher zone number, given on a plant, the hardier it's considered to be.
It is not obvious that the "highest" zones always found in northern Sweden. Oceans and large lakes has a temperature-equalizing effect, and the climate tends to be milder in great waters vicinity. The height above sea level also shortens the growing season the higher up one goes. On the map above, you see, for example, that the area just south of Vättern zone V, which it is, among other things along the northern coast. Closest to Vättern is the zone II, which is visible in the more detailed county maps.
The more I learn about hardiness zones, the more useless I think they are. I actually think the USDA should abandon them and come up with another system which incorporates many more factors. The sunset climate zones are useful except that you can't extrapolate them to other parts of the world outside North America. Sunset Climate Zones - Sunset Mobile
The extreme winter min is an indicator, but not the be all and end all of what survives. I've learned from reading up a lot and listening to garden shows that the hours of the low temp combined with how warm the day gets have a lot to do with what can survive in winter.
One place that gets a low of 20F every night, and a high of 30F every day for a week would be devasting to subtropicals, while a place that goes down to 20F every night, and up to 45F every day would fare much better. Hardiness zone would be the same technically speaking. The South is a lot more like the latter rather than the former.
the swedish vegetation zone map seems to incorporate both growing seasons and the minimum temperatures and some other stuff. couldn't find the full explanation, but here's a bit about that.
The Finnish system is similar. It also takes the geography, quality of the soil and possible spring freezes into count. Unlike USDA the Finnish zones don't average the coldest annual temperature over 30 years, but only averages the tree coldest minimums in 30 years to play on the safe side.
The Finnish system translates in the Swedish as:
I = not in Finland
II = 1a
III = 1b
IV = 2 and 3
V = 4 and 4/5
VI = 5 and 5/6
VII = 6 adn 6/7
VIII = 7
Fjällregion = 8
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