There is an interesting article on p. 14 of the most recent Canadian Gardening Magazine which explains the differences between the Canadian and the US Zone maps.
Quoting:
IN THE ZONE?
Is it possible to convert USDA plant hardiness ratings to Canadian hardiness zones? Many of my gardening books are from the U.S. and only include USDA ratings.
SONIA SCOTT, EDMONTON
Anne Marie Van Nest, Canadian Gardening's horticultural editor, responds:
The United States Department of Agriculture hardiness map and the Canadian hardiness map (produced by Natural Resources Canada) shouldn't be used interchangeably. The two systems differ anywhere from half a zone (warmer or colder) to as much as two zones, preventing an exact exchange or even a standard means for conversion.
Additionally, the two maps are based on different criteria. For the NRC map, plant survival data and climatic information such as minimum winter temperatures, length of frost-free days and maximum wind speed are used to define its nine (0 through 8) zones, which are subdivided into colder (a) and warmer (b) regions. (The Canadian plant hardiness zone map can be found at www.canadiangardening.com/zones.shtml .)
The hardiness zones of the USDA map, on the other hand, are based only on average annual minimum temperatures. The map is divided into 11 zones (1 to 11), eight of which are subdivided into colder (a) and warmer (b) areas.
(The USDA hardiness zone map is found at www.usna.usda.gov/Hardzone/ushzmap.html .)
The best way to use USDA plant hardiness zones when NRC information is not available is to locate your garden on each map and note the corresponding zone. For example, Edmonton is indicated as 3a on the NRC map and 3b on the USDA map, so any plant listed as USDA Zone 3 or lower should survive. Also, always find out which hardiness system is being used by your local nursery or gardening reference; if it's USDA, ask if the NRC hardiness zone is available.
I note that for Ottawa where I live, I'm 5a on the Canadian zone map but 4b on the US.
Ann
