Canada’s plant hardiness zones map ready to bloom for 1st time in 10 years

A peace rose from Katharine Smyth’s garden.

Natural Resources Canada is working on the first update to its plant hardiness zones map since 2014, hoping to have the map – which gardeners rely on when determining what plants will thrive in their region – available sometime in 2024.

The update comes on the heels of the United States Department of Agriculture’s latest plant hardiness map, updated last month for the first time since 2012, which NPR reported saw roughly half the country shift into a new half zone as temperatures warm.

However, the USDA’s map relies solely on extreme temperatures, while Canada’s map takes into account seven factors, making predicting potential zone changes a little more complicated.

Still, early observations suggest change is coming more rapidly in Canada’s North and West compared with the East, where in some rare cases, researchers say there’s been a decrease in extreme minimum temperatures.

What is a plant hardiness 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.

“When you’re at the greenhouse and you’re purchasing plants, you’re always reading the tags to see what zone it belongs in,” says Sandra Mazur, second vice-president of the Ontario Horticultural Association and member of the Dryden Horticultural Society and the Thunder Bay Horticultural Society.

“That’s one of the questions that we get on all of our websites all the time from new gardeners, they’re always looking to see what zone they’re in.”

Much of the three territories falls under the 0 and 1 zones, while the northern middle of the provinces hovers around zones 2 and 3. Areas where much of Canada’s population is concentrated, farther south, tend to be in the 4, 5 and 6 zones. Victoria is the sole location to be listed as 9a in the 2014 map – a zone that didn’t exist in previous versions of the map.

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.

“Generally speaking, plants often have sort of a northern limit as to how far north you can plant them, how much cold tolerance they have. But a lot of times plants that are typically associated with more northern climates can grow in more southern warmer areas if you’re going to, like, bring them into a garden setting and … prevent the other competition that that sort of tends to keep them pushed off to the north,” John Pedler, one of the researchers behind the map, explains.

Canada’s map does not translate directly to the USDA’s map, but Canada does offer a second map that aligns with the USDA’s single-variable map.

“In some cases, plants are imported and used that are ascribed to the USDA zone and not the Canadian zone. So we’ve done both,” says Dan McKenney, who has played a key role in the maps for decades.

“It’s not the case that the zones are equally sort of transferrable. So, like, a 4a in the Canadian one is like a 5a U.S. one.”

What changes is Natural Resources Canada expecting?

McKenney, a senior scientist and director of the Integrated Ecology and Economics Division at the Great Lakes Forestry Centre, part of the Canadian Forest Service under Natural Resources Canada, says what’s known about climate change is that the West is changing more than the East, particularly when it comes to temperatures.

However, because the map is based on data spanning 30 years and on more than just extreme temperature, huge shifts aren’t expected.

Pedler, research scientist at Great Lakes Forestry Centre, says in the roughly 50 years between the initial plant hardiness map and the last update in 2014, parts of Western Canada saw a “drastic” three-zone increase, though that was uncommon.

“Overall, (there was) more like an increase of about a zone to a zone-and-a-half, whereas, in the eastern half of the country, it was more like half-zone increases,” he explained.

“Although there were even some places where there were no changes or even declines, not very often.”

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The very first map, which used data from 1930 to 1960, did not include projections for the Far North (due to a lack of weather stations in the Far North prior to 1950, McKenney explains) so researchers are unable to conclusively say how it changed in that 50-year span.

Pedler says next year’s update will allow them to compare the latest data with the map released in 2014 (which included 30-year data up to 2010).

“All of the climate change models are suggesting that the Far North is an area where we’re expecting to see the most kind of climate change as far as warming temperatures and such. So it could be the case that we see pretty big changes in plant hardiness zones up there as well.”

McKenney added that some of the changes in 30-year averages “could be ascribed to just normal variation” but “we’ve noticed that there have been changes that would be considered to be expected with climate (change).”

“For the most part, the extreme minimum temperatures have been going up,” he said

“But there are some parts of the country where we’ve actually had a decrease in extreme minimum temperatures through time, closer to the East Coast. And part of that is ascribed to things like icebergs spawning and going down the Eastern Seaboard.”

While the map is “quite popular,” McKenney says it’s actually “a little bit on the side of the desk” when it comes to his work, making it difficult to provide an exact timeline for the update.

“We’ve been working on it. We’re hoping that we’ll be able to publish something and get it peer-reviewed and published and sent out sometime in next year,” he said.

“People love it. You know, we enjoy it ourselves. We’ve done this work because we generate climate maps for lots of different reasons … to help people figure out the influence of weather and climate on growing plants, the distribution of birds, distribution of insects and forest diseases, even things like ticks. People use our data, our climate data, to do lots of different things.”

The plant hardiness zone map is also not the only map they produce: there are species maps, for example; there are maps showing where 62 different species of trees are growing in Canada; and they are in the process of creating maps for insects and diseases.

“We’re looking forward to providing an update. I don’t think there’s going to be a lot of changes. That’s my gut, but I guess we’ll see,” McKenney said.

What are gardeners noticing?

Penny Stewart, president of the Gananoque Horticultural Society and a member of the Ontario Horticultural Association, has been gardening for about 65 years, since she was five years old and her father gave her a vegetable garden.

While she was born and raised in Woodstock, Ont., she’s been in the Kingston area for about 40 years. In that time, she’s noticed that the region is seeing more frost-free days, “meaning it’s a little milder earlier in the spring and definitely the falls seem to be warmer and longer.”

“When I first started gardening here on this location, I would consider anything after about, you know, the 15th or 20th of September, you could get a frost. Now we’re getting an extra month,” she said.

“It’s made a difference in some of the types of annuals and vegetables I can plant.”

She added that other gardeners are also having luck growing kale year-round, provided they protect the bed with a covering, something she doesn’t believe people were attempting 30 or 40 years ago.

In Ontario’s southernmost county, Katharine Smyth says there hasn’t been much of a change in the length of the seasons, but they seem to be shifting, with frost still a potential threat even past May 2-4 but warmer temperatures lasting much longer into the fall.

Smyth lives in Tilbury, not far from Windsor, and is the past president of the Ontario Horticultural Association and current secretary of District 11, which includes Chatham-Kent, Essex and Sarnia-Lambton.

“The last, I would say three, four years, it’s been a cold, wet May or a cold dry May, depending on what year it was and you just didn’t want to go and buy your flowers because you were afraid if you planted them, you’re going to have a frost and they would be dead or else they would die because of the cold weather,” she said, adding that she now waits until almost the first week of June before planting flowers outside.

“I had begonias, I had echinacea, I had rudbeckia … but I had them blooming right up until the 20th of November, which never used to happen. We always got a frost in October. Always. And even the tomatoes — all the tomato factories here were right almost to the end of October.”

Smyth also notes that conditions can change quickly throughout the region. Just 20 minutes down Highway 401 in Windsor, tulips are often blooming almost a full week earlier than where she is in Tilbury.

More on Canada

Sandra Mazur recently moved to Atikokan after spending three years in Dryden, though she has spent much of her life in Thunder Bay.

She says where she is now has gone from a 2b to a 3a.

“We were a little bit warmer in Dryden than we were in Thunder Bay area. I had a little bit more luck with my garden,” she said.

“Sometimes we get a later fall, but quite often by October we’re getting snow already. So our season is quite short. But it does seem to be that things have warmed up a little bit and I have brought different plants home from greenhouses in southern Ontario that they say wouldn’t survive here and, if I protect them, they did survive.”

For example, she had sweet woodruff growing for years before a particularly brutal winter killed it.

“I know that down there it multiplies and just spreads rapidly. And I had to baby it, but it survived for me for over five years.”

As for concerns about introducing non-native species, she notes that oftentimes plants that are perennial in warmer areas, meaning they keep growing year after year, are treated as annuals farther north.

“There are a lot of things that are invasive in southern Ontario that we buy at our greenhouses and they’re just an annual. They die in the winter because they don’t survive the cold.”

Those on the Pacific coast are experiencing significant changes, as McKenney and Pedler noted.

Sarah Beck, executive director of the non-profit education-focused organization Pacific Horticulture – which covers California, Oregon, Washington, British Columbia, Alaska and Hawaii – says that while the region is vast, it has “some elements in common” with the Mediterranean climate. For example, much of the West Coast experiences a dry summer and wet winter as well as “big extremes” in elevation.

“I’m not a climate scientist … but I will say that we have different conditions than the whole rest of North America because of the influence of the Pacific Ocean,” she said.

“Our ability to grow plants is much more determined by our ability to have moisture during the wet season and our best planting time is the fall.

“In Ontario, you’re probably … living for that last frost and then hoping to wait on the first frost. And for those of us in the Pacific region, we just can’t wait for that first rain to start.”

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Beck says the extremes of climate change are greatly impacting the region, as evidenced by heat domes, drought, extreme fires and flooding. Gardeners, horticulturists and researchers are making plans for the future.

“When we think about adapting plants to these situations, it’s not enough to just say, ‘Oh, well, it’s dry conditions, right? Well, let’s get some desert plants or let’s get some arid plants.’ No, that does not work because what else are we getting? We’re getting monsoonal rains. We’re getting completely massive water events.”

She says gardeners are able to “push the zones,” with many in Oregon and Washington states planting “some really interesting plants that are from Southern California.”

While climate change is of particular concern for the coast, Beck is practising optimism, noting that while there are challenges and changes, there are also “so many potential solutions.”

Want to get involved?

Stewart encourages those interested in gardening to get involved in their local horticultural society. In Ontario, for example, there are 270 across the province.

“Horticultural societies are a wealth of information,” she says.

Beck simply encourages everyone to become familiar with the environment they live in.

“Just try to figure out what plants are likely to be the happiest in the conditions you have,” she said.

“There is so much benefit and joy that comes from getting to know your place and to get to know the plants that are from that particular place and to garden and to interact with your own ecosystem. To be part of the collective habitat-building in a place is just such a satisfying experience.”

As for the map itself, McKenney says there is a bit of “citizen science” involved, with the team accepting submissions from the public for the longitude and latitude of what grows in their gardens. Master Gardeners in Ontario can get credit for doing so, which goes towards their accreditation to be certified as a Master Gardener and many students have also provided contributions, he says.

“It’s a combination of, I would say, professional and amateur contributions, which is a nice thing and it gives people a way to contribute to some useful science that basically can help determine what can grow where in the country.”