I just discovered Canada has a different zone designation system than the US. (or vice versa? ). Canadian zone numbers are generally one higher than for the same temp range in the 'States. Crossing the border from 'Pacific Northwest' to 'Lower Mainland,' zone 7 American for example, jumps to Canadian zone 8. The Canadian numbers claim to take account of more than just min. temperature, but the temps look as if they line up anyway, apart from that +1 zone jump.
Then there's some outright misinformation, even on what look like authoritative websites. One I came across declares incredibly that 10 Celsius degrees equal 12.2 Fahrenheit degrees!!