My area does not even fit into one of those zones unless I am reading the Centigrade incorrectly??? I don't think most of this continent gets below -5 unless its up in the alps (our small mountains) or maybe in the desert. How many areas in the world would this zoning apply to. Is it a US, Canda only zoning? Or is there one for Australia. Found this for Oz is that the same thing? http://www.anbg.gov.au/hort.research/zones.html Liz
Well, the USDA zoning numbers are in use worldwide, but Liz your continent goes above their regularly used numbers. So does mine. Your Aussie scale there in that link shows the US zones parallel to the OZ zones; looks like your country doesn't go below USDA Zone 7, and goes right up past 11 to 13 or 14. Here in Ecuador, the habitable areas start at USDA Zone 10 and go up from there (Paramo, which can be as cool as Zone 6, is considered to be too high-altitude to grow much of anything). The heart of our Amazon is USDA 14. Your scale is calibrated the same way, but in such a manner as to be useful to Australians - you never see USDA Zone 1 conditions (-50 C) so why use a scale created for an arctic and semi-arctic pair of countries? For what it's worth, I used to grow Zone 7b plants outdoors in Zone 3a. The USDA system is a bit misleading the way it calcuates its average lowest temperatures.... This link has USDA-zone numbered maps of the world, if you want to compare OZ zoning to USDA zoning.
Although, Liz, this might be more useful for you; it's the Australian Horticultural Society Heat Zones.
Yes the 1990 US map is just average annual minimum temperatures for a period of time, 13-15 years (I don't remember). And the way they stretched the zones like melted cheese it doesn't line up with actual conditions here in the mountainous west. I've read an Oregon company is putting together a new one that will be accurate, using state of the art climate analysis based on multiple data sources. Plants have a mininum temperature below which they are damaged. The 1990 USDA zones are the range within which the average minimum temperature for the time period falls. So the 0F-10F for Zone 7 for instance isn't the bottom floor (absolute mininum) for that zone, that's why it's suggested that plants hardy to temperatures below the average range given for each zone are selected.
The zones are average temps; always buy plants hardy to at least 1 zone north is a good rule here in New Jersey, USA...when the exceptional (or 10th year storm) hits, the plants can weather it.
