Most of Canada sees changes under newly updated plant hardiness zones map

About 80 per cent of Canada's land mass saw an increase in its plant hardiness zone, mostly of about half a zone to one full zone.

Gardeners will have an updated roadmap to help them plan next year as Natural Resources Canada has released an update to its plant hardiness zones map.

The last map came out in 2014 and, since then, researchers say about 80 per cent of land in Canada has shown an increase in zones, typically between a half and a full zone.

In the simplest terms, the plant hardiness zone map shows what can grow where. The zones go from 0 to 9, and each zone is divided into two: a and b.

The maps are created based on data from seven criteria averaged out over a 30-year span: monthly mean of the daily minimum temperature of the coldest month, mean frost-free period above 0 C in days, amount of rainfall from June to November, monthly mean of the daily maximum temperatures of the warmest month, rainfall in January (important because freezing temperatures following rainfall can be bad for roots), mean maximum snow depth and maximum wind gust in 30 years.

The new map, which came out in July, is based on data from 1991 to 2020.

In southern Ontario, specifically, most locations have increased by half a zone from the previous map.

“One notable exception is the GTA, which is a big area, so it actually covers a few different zones, but on average it’s increased from a 6A to a 7A,” said John Pedler, research scientist at Great Lakes Forestry Centre, part of Canadian Forest Service — Natural Resources Canada

McKenney says some of the change in Toronto could be due to the “heat island effect.”