Hi Master Gardeners, as the Winter Solstice approaches, I look forward to settling down with all my delicious plant catalogues and planning my garden in Toronto. I get my catalogues from Canada (Ontario), the US, and from the UK. The plant descriptions usually include the plant’s hardiness zone. Could you please explain hardiness zones to me and tell me what Toronto’s zone would be under the three systems? Many thanks.
Dear Gardener, the question about hardiness zones has come up a number of times. Gardeners write in and wonder why their plant did not survive the winter. Canada, the US and the UK have different ways of calculating hardiness zones. Toronto will have a different hardiness-zone number depending upon which country the plants comes from.
Toronto has a hardiness zone of 6a or 6b using the Canadian system, and 5a by the USDA system. It is a bit difficult to find a map that includes Canada that uses the UK system, but I think that plants would need to be at least H7 by the UK system to thrive in Toronto. (That is colder than the UK itself which varies from H1-H6).
Please keep in mind that the hardiness zones do not take into account small microclimates. Your garden probably has little pockets of warmth protected from the wind that you can taken advantage of. A garden that is further from Lake Ontario or on a south-facing slope will also be warmer.
You can get more information on plant hardiness zones and how they are calculated from these interesting articles:
Happy catalogue shopping.