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Voilà! A New Canadian Plant Hardiness Zone Map
by Art Drysdale
by Art Drysdale


Art Drysdale, a life-long resident of Toronto and a horticulturist well known all across Canada, is now a resident of Parksville, British Columbia on Vancouver Island, just north of Nanaimo. He has reno-vated an old home and has a new garden there. His radio gardening vignettes are heard in south-western Ontario over radio station Easy 101 FM out of Tillsonburg at 2 PM weekdays.

Art also has his own website at

May 13, 2001

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"Joan O'Neill, president of the Canadian Tulip Festival, speaks during the unveiling of the new Canadian Plant Hardiness Zone Map in Ottawa, May 11, 2001." Author photo.

It was a way back in 1966 when the late Art Buckley (curator of the Dominion Arboretum, and horticulturist with the Plant Research Institute, Ottawa), Larry Sherk (research officer, also at the Plant Research Institute, now at Sheridan Nurseries in Toronto) and myself met in a small room at the Central Experimental Farm in Ottawa, to review a huge list of woody plants and assign each of them a zone, based on the zones of the then new Canadian Plant Hardiness Zone Map. It was solely an exercise based on each our respective experiences in knowing what woody plants grew where. At that time we only discussed woody plants. No consideration was given to herbaceous perennials, as Agriculture Canada officials involved with the map did not consider it to be suited for use with herbaceous plants. Actually, we didn’t do a bad job--most of the assignments we made remained for the next 35 years!
The first actual hardiness map for North America was developed by Arnold Arboretum in Massachusetts in 1927, and underwent a couple of modifications up to the 1940s. It was followed in 1960 with a major project by the United States of Department Agriculture (USDA). Both of these maps (which included the southern portions of Canada) were done based solely on minimum winter temperatures.
In stark contrast, Canada's original map was created using plant survival data and a wide range of climatic variables, including minimum winter temperatures, length of the frost-free period, summer rainfall, maximum temperatures, snow cover, January rainfall (to bring the freeze-thaw aspect into play) and maximum wind speed. In other words, our Canadian map has always been superior to even the newest version of the USDA map.
Now, I know the Canadian map (which its producers stated applied to over 99 percent of the country's population) has received considerable criticism from folks in the North. Donna Dawson, web mistress of the website reminds me that in her five years of working with visitors to her site, there has been constant criticism that the map only pertained to about two-thirds of the country. From a land mass point of view that is likely true. And, even though the North may have a small population, obviously they like to garden too. On my recent trip to Victoria, I met a Weather Network fan of mine who lived, and gardens, in Yellowknife. In Vancouver, we had a discussion about cold-climate gardening as he awaited his flight to Edmonton (and a further transfer to Yellowknife), and I prepared to depart the airport for VanDusen Gardens.
As this is written I'm on my way back to Toronto from Ottawa where, yesterday (May 11), I took part in the announcement and unveiling of the revised Canadian Plant Hardiness Zone Map. To quote the authors of a major article in the January/February issue of the Canadian Journal of Plant Science, "we updated the plant hardiness zones derived by [the authors of the original map,] C.E. Ouellet and Larry Sherk, [in 1967] using relatively new climate interpolation methods, a new digital elevation model of Canada and more recent climate data from across the country. There are at least two compelling reasons for the plant hardiness zones to be re-examined. The first is that the methods presented here provide a more repeatable, objective approach to mapping climate, and hence plant hardiness. Secondly, Canada's climate is changing and therefore the plant hardiness zones are likely to have changed."
In the same paper, the authors state in their 'Concluding comments', "We have provided a revised and updated map of Canada's plant hardiness zones that appears to be consistent with the original work. Our approach is repeatable, incorporates elevation effects that were not previously possible and includes an assessment of overall quality through the statistical diagnostics. Canada's plant hardiness zones appear to have changed in many parts of the country. The changes are most pronounced in western Canada, and this is consistent with what is known about climate change. Perhaps surprisingly, the index value seems to be decreasing in parts of eastern Canada [this means that overall the various factors being inputted are decreasing]. It is important to be explicit about the exact nature of the hardiness index. The zones are associated with probabilities of plant survival in relation to average, broadscale conditions. Extreme variations, local topography and human interventions (e.g. mulching [I question this point about mulching since we're dealing with woody not herbaceous plants, but micro climates created by natural screening etc. could have an effect]) can have a significant impact on plant survival in any particular location."
This technical paper, which explains in detail the methodology used in developing the revised map, also makes a couple of references to 'future research'. For example, "Future efforts could also focus on developing a plant survival database so that the hardiness zones could be better calibrated to currently-used cultivars."
The folks in the north should be happier with this new map. It certainly details the zones in areas of the country (even Northern Ontario) that were not included on the original map. However, much of that part of the country, not surprisingly, is limited to the two coldest zones.
Only this week I had a call from Cecilia Ryan in Labrador City, Newfoundland. She told me that the snow was all gone and that everyone was out raking up. She even mentioned that her peonies were showing through the ground. She actually called to order a copy of my book Gardening Off The Ground, and I intend to call her back and ascertain just what woody plants she actually grows in that zone 0a climate. I may also consult her later this year to see if she has any comments on this new version of the hardiness map.
The updated map is a production of Natural Resources Canada's Canadian Forest Service scientists. They used the same variables and more recent climate data. They also used modern climate mapping techniques and incorporated the effect of elevation.
The new map is divided into nine major zones: the harshest is again 0, and the mildest is 8 (previously there were ten major zones, with the mildest being 9). Subzones 'a' and 'b' are assigned in each major zone except in 8, which is not subdivided.
At the announcement during the official opening of the Canadian tulip Festival in Ottawa on Friday, I talked with Ken Farr, Science Information Officer, about the new map. Ken worked extensively on the new map, and he's the one we can blame if we disagree with any of the indicator trees and shrubs named on the map.

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A small section of Canada’s new Plant Hardiness Zone Map showing the extended coverage into the northern parts, here showing Labrador City in zone 0a.

Ken is most anxious to receive feedback. Once you've examined the map (that's most easily done by checking it out on the web at: or you may e-mail him with your ideas and comments at
I think you'll be impressed with the new map on the web (it's also available on glossy paper, but the colour gradations of that version make it somewhat difficult to understand the zone boundaries), especially with the fact that you can zoom in on a particular area up to six times.
Ken Farr and I discussed various future possibilities including interactivity for web users of the map, and even the development of a data base of plant species and cultivars recommended for each zone. The one problem with this is we could easily have gardeners recommending certain plants as hardy in their particular zone, but at the same time, not actually knowing the true identity of the specific plants they're recommending!
Much discussion could and should take place on the future for this new hardiness map's data. It's a great opportunity to improve the information on just what will grow where in Canada. Thank you Natural Resources Canada (the Canadian Forestry Service), and the people at Agriculture and Agri-Food Canada who worked with them. It's just too bad Agriculture didn't have a horticulturist staff member who could have provided some specific ornamental plant input.
With the new map, we now definitely need a 'meeting of minds' to discuss every single woody plant sold in Canada, and determine the zones in which each may be recommended as hardy.
Last week I said I would write about an alternate form of control for apple maggot fly that causes such devastation to apples. This mechanical control was devised in the early 1960s by the late I.B. Lucas of Markdale, Ontario. Mr. Lucas was a lawyer and interested amateur gardener whose father had been the Attorney General of Ontario, and whose son still lives in the house where the apple orchard is. I.B. died at age 91 in 1988. Again, I haven't done what I said I would do. So, here's my answer. I have written up I.B.'s "system" on my own website-- --and if you go there, and check out the 'Commentaries' area, you'll find what I promised to write here. You'll also find the same information in the June issue of Forever Young (Today's Seniors) newspaper.


Art Drysdale is horticultural editor of Canada's oldest and fastest growing national gardening magazine, Plant & Garden. He is seen daily on Canada’s Weather Network at 23 minutes past each hour, and heard Saturdays from 8:05 to 10 AM, with a live radio broadcast on Toronto's powerful and clear, AM740 CHWO Primetime Radio.

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