Source: Natural Resources Canada

The Plant Hardiness Zone map is based on seven variables that contribute to growing conditions:

1. The monthly mean of the daily minimum temperatures of the coldest month
2. The monthly mean of the daily maximum temperatures of the warmest month
3. The mean frost-free period (above 0°C) in days 
4. The amount of rainfall from June to November
5. A winter harshness index related to rainfall in January
6. The mean maximum snow depth
7. The maximum wind gust in a 30 year period 

The zone at the coldest end of the range is 0a, which covers much of northern Canada. Nunavut is exclusively Zone 0a, while the Yukon and Northwest Territories consist of Zones 0b, 1a, 1b, and 2a. You'll find the hardiest plants in these regions, which can survive at temperatures as low as -46°C. Warm southern regions typically fall under higher zones, like Zone 9a in Victoria where temperatures rarely drop below -7°C. Environmental factors such as elevation can influence growing conditions and create climatic differences and one province may cover a huge ranges of zones. British Columbia includes regions of Zone 0a, Zone 9a, and everything in between! Most of Atlantic Canada falls within Zones 3b to 6b.

How to Use Plant Hardiness Zone Information

We recommend looking up the Hardiness Zone of your region before selecting plants for your garden. Most perennial plants sold at garden centres will come with a tag that specifies growing information including the hardiness zone for that species. Growing zones are usually specified in a range, but some plants may be quite limited in where they can thrive. Not all plants in the same species will be hardy in the same zones, so it's important to check before you buy. Selective breeding has produced many varieties that grow outside the typical range of a species, enabling northern gardeners to grow select types of perennial flowers and fruit trees previously limited to southern zones. 

Note: Some Canadian garden centres may carry plants produced in the US, where hardiness zones are categorized differently. Always check with a staff member if you're not sure a plant will work in your area.

It's important to note that just as tender, tropical plants can't survive freezing winters, plants that grow well in colder zones may not thrive in warmer areas. Varieties planted in too cold a zone will typically be killed by hard winter frosts or the freeze and thaw of unpredictable spring weather. Varieties planted in too warm a zone may fail to spread and flower, and will lose vitality over time.