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.

Growing in Ontario

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.”

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“As populations grow, there’s more of an influence of concrete and buildings in the actual heat that’s experienced by people in large centres. It’s something that’s studied by climate scientists, not us per se, but we see it represented in the maps that we make.”

The other area in Ontario to see a large jump is the Windsor region, which moved from a 7A to a 7B.

“That’s the first time we’ve seen 7B in Ontario.”

The change in zones means gardeners in Zone 7 can grow canna lilies or even dahlias as perennials rather than annuals. Pedler says gardeners could even try their hand at peaches, nectarines and even figs and pomegranates.

However, Dan McKenney, a research scientist and director of the Integrated Ecology and Economics Division at the Great Lakes Forestry Centre, noted that the maps are a guide based on 30-year averages, and one particularly cold winter could be too harsh for those plants.

“Plants experience things on a day-to-day basis. Every year is different.”

McKenney pointed to Natural Resources Canada’s species models, which allow people to look up the hardiness of individual plant species based on location.

“Some of these other approaches might be best for people who are thinking about commercial growing, depending on your risk tolerances.”

The big picture

Some areas saw changes up to two full zones — mostly in Western Canada, southern and northwestern British Columbia, and the foothills region of Alberta — though none in urban centres.

“As far as some of the big urban centres go across the country, Victoria was a big winner with an increase of 1.5 zones, and it went from a 7B to a 9A; 9A is the highest hardiness zone in all of Canada, so that’s limited to pretty much the area immediately surrounding Victoria and Vancouver,” Pedler said.

“Coming across the country, Vancouver jumped from an 8A to a 9A, Calgary jumped from a 3A to 4A, Winnipeg jumped from 2B to a 3B, Toronto jump from a 6a to a 7a.”

Notably, the eastern portion of Newfoundland actually dropped by half a zone.

“The going theory there is that climate change is actually driving more spawning of icebergs in the North Atlantic, and so you’ve got more icebergs moving down through Iceberg Alley and actually almost counterintuitively having a cooling effect on the eastern part of Newfoundland,” Pedler said, adding that climate change is likely driving the major changes in zones across Canada.

“While it’s fun to think about the novel planting opportunities that come with increasing plant hardiness zones across the country, the larger context is this very concerning phenomenon that is projected to have significant negative impacts on natural and human environments – some of which we’re already seeing, such as the longer and more intense fire seasons in Northern Canada.”

Canada’s map does not translate directly to the United States Department of Agriculture’s map, which is based solely on extreme temperatures.

“(That approach) works well for them because they don’t get a lot of snow in all parts of their country but snow cover can make a big difference to plants,” McKenney said.

Canada’s interactive plant hardiness zones maps, including previous maps covering 1961-1990 and 1981-2010, can be found on the Natural Resources Canada website.

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