A Guide to Canadian Climate Zones
By Winston Gamache
What’s in a Zone?
The climate zones in Canada are drawn based on the climate of an area and how suitable it is for certain plants. The harshest zone is 0 up in the Arctic and the mildest zone in Canada is 8 along the West Coast.
These zones take into account many different factors that can affect how well a plant grows or survives the winter in an area. It is based on the temperatures of the coldest month, the frost free period, the amount of rainfall, the temperatures of the warmest month, maximum snow depth, and the strongest wind gusts. This differs from the USDA zone system, which is based mostly on minimum temperatures. Our zone 4 area is roughly equivalent to their zone 3 (minimum temperature -40 to -34°C or -40 to -30°F).
The map is made from the average of these conditions over many years and changes in weather from year to year can greatly affect the growing zone. Extra harsh winter conditions with extra cold or poor snow cover are known as test winters.
Edmonton’s Climate Zone
Based on the updated map, Edmonton and some of its surrounding area is considered Zone 4a. However, with the extreme and changing weather Alberta is known for, we can still get test winters that are a Zone 2b or 3. Much of the rest of Alberta is marked as a Zone 2 or 3. The map is a useful tool for understanding climate and choosing plants that will have the best chance of success, but should not be seen as an absolute.
Many gardeners and Edmonton Horticultural Society members grow plants rated for outside of our listed zone by changing the way they care for or place their plants. Seasoned gardeners also understand that at least once a decade we get a particularly nasty test winter and this takes out a few specimens and makes room for new plants. One of the exciting things about gardening is that we are at the whims of nature.
Stretching the Climate Zones
The growing zone system in Canada is meant as a guideline and is specifically designed for trees and shrubs that are subject to the elements. Perennials are more affected by the depth of snow received over winter than any other factor. Gardeners can easily stretch perennials a zone or three with protective measures. Areas, even in the same yard, can have different microclimates that would be considered a higher or lower zone depending on different factors. Factors such as sun exposure and snow drifting can greatly affect whether or not a plant survives. For more information on microclimates, see Using Microclimates to Stretch Growing Zones.
You can find an interactive map of these zones and a more in-depth explanation at Canada’s Plant Hardiness Site.
