A New Plant Hardiness Zone Map - and the Controversy Has Only Just Begun!

Cercis close-up.jpg (46548 bytes)

Close-up of a branch of eastern redbud (Cercis canadensis), a native tree that may have a slightly larger area of hardiness (as far as Ottawa?) than once thought. Author photo.

On May 11 Larry Sherk, chief horticulturist at Sheridan Nurseries, and I travelled to Ottawa for the offi-cial unveiling of Canada’s new Plant Hardiness Zone Map. Larry had done the horticultural and plant work on the first map, issued in 1967. At that time my involvement was tiny, as part of a team of three (with Larry and the late Art Buckley) who actually assigned the whole range of woody plants to the various zones.

My initial comments on the new map are on the http://www.icangarden.com/ website, dated May 12. Since that time I’ve had the opportunity to talk to nurserymen, gardeners and other horticulturists across the country. While it’s really too early to gauge opinion--because most people do not even know of the new map’s existence--my sixth sense tells me that once the reaction comes it’s going to be incredible. Lindsay Davidson of Specimen Trees Wholesale Nurseries in Pitt Meadows BC, along with John Mathies of nearby Cannor Nurseries, were both almost in shock when I told them of the re-zoning of much of the lower mainland portion of their province. The area south and southeast of Vancouver, formerly zone 8a and 8b, is now 6b, meaning it’s actually the same as the small areas around Niagara and Windsor Ontario, and similar to the 6a of Toronto and Hamilton. 

A further question arises about a small area on Vancouver Island, near Victoria, that is showing up now as zone 6a! That’s almost tropical territory! How can these be? Anyone who has been in both areas will testify to the much wider range of plants that grow in the entire Vancouver and Victoria areas, com-pared to anywhere in Ontario. 

Other anomalies exist right here in southern Ontario. With the old map most of the area around the west end of Lake Ontario from just east of Toronto to Niagara, as well as along Lake Erie and even up to Sarnia was listed as 6b. Tiny areas around Windsor and Niagara Falls were said to be in 7a. On the new map most of this area is now shown as 6a or 5b, with the aforementioned tiny areas being 6b. In other words, zone 7 has disappeared from the map in Ontario!

I did consult with Ken Farr who did considerable work on the map, including choosing the indicator plants for each of the zones. His reply regarding the area on Vancouver Island was as follows: “I'm only speculating at this point, but a large variance in the 30 year wind velocity factor at that extreme coastal location might have contributed.” 

Ken further commented: "’Hardiness’" is by definition subjective. Individual plants, like individual gardeners, often appear eccentric. I'm thinking of the Magnolia acuminata that survives year after year be-side the William Sanders Building at Ottawa’s Experimental Farm, most years producing flowers and even fruit. Certainly out of zone, but a survivor. Chamaecyparis nootkatensis ‘Pendula’ is showing up more and more here in Ottawa, supposedly a Zone 6 plant but hanging on and spreading. I'm also see-ing the “odd” Hydrangea macrophylla around town and suppliers are pushing it. An extreme example I could offer is a Cercis canadensis [redbud] planted immediately east of Prince of Wales Drive in the Dominion Arboretum. Not only is it in the prevailing west wind, but directly in the salt spray, all winter long. It often dies back, but the roots survive year after year, and resprout.”

One further example about which Ken is likely not aware is a decent specimen of redbud that thrived for years there, near the University of Ottawa, but on land that was expropriated in the late 60s for highway development. It was supposed to be moved, but I am unable to find out what happened to it.

But all of this does not answer the question of how to proceed now that we have a new map. Ken’s suggestion is “If we accept that the climate has indeed changed since 1960 then it is high time to do a survey of growers throughout Canada (the available database has certainly expanded) and ask which cultivars survive, which survive with protection, and which just don't?”

I agree. We must have a survey of growers (read knowledgeable gardeners) all across Canada just as soon as possible. Like, let’s get it going this winter.

Finally, I mentioned in my earlier May 12 article that the first Canadian map had never been intended to apply to herbaceous perennials. Perennial growers have just taken it upon themselves to use it. That should not be. There are other factors (snow cover, frost depth, mulching etc.) that affect perennials that cannot be considered in drawing up such a map. I just had that driven home again when I talked with Cecilia Ryan in Labrador City. That’s a zone 0a climate where the main trees are birch (Betula), mountain ash (Sorbus aucuparia), and spruce (Picea). [Incidentally, these are respectively zone 2, 3 and 1 trees according to the old map data!] She grows wonderful perennials and has invited me to see them. Their frost-free period begins around June 20, and ends in early September, but many perennials such as peonies and delphiniums do extremely well.