Taking Plant Hardiness, and What Will Grow Where, One Large Step Into The 21St Century

As predicted, the new Canadian Plant Hardiness Zone Map has created no end of controversy amongst those who have had time to learn about and study it.

Personally, after much consideration, and innumerable discussions I don’t see how it can be taken seriously. In fact, I wonder if we would not be better sticking with the 1967 version. For example, I don’t see how any amount of explanation can account for the fact that a great population area of southern Ontario, and a similar area in British Columbia south of Vancouver can both be considered zone 6b. In my opinion, after 40 years of intensive experience with what plants grow where in Canada, I do not see how anyone can resolve the problem of these two areas being the same zone and yet hardy plants that grow in the areas are hugely divergent. If that is to be the case, the map is of little use! But wait!

I’ve had further discussions with Ken Farr who assigned the zone indicator plants, and Dan McKenney, PhD. (chief, landscape analysis and applications section, Natural Resources Canada, Canadian Forest Service, Sault Ste. Marie, Ontario) who is senior investigator for the new map. Dan is in full agreement with the need for a survey of gardeners across the country, likely utilizing the World Wide Web, to establish just which plants will grow where. When I expressed my concern about errors if too many people not totally knowledgeable on the subject where included, he agreed again, saying he thought we should start off with a small study group who would set up the criteria for reporting.

This is where it gets interesting. Dan tells me he has the computer software that can map locations where any individual tree, shrub, vine, evergreen etc. might grow based on input of a group of criteria. This software has been used successfully in Australia, where the criteria for the growing area of a particular plant was inputted and then the rest of the country was checked and the software pin-pointed other areas where the climate matched. This strongly suggests it would be suitable for these locations. These kinds of mapping tools have also been used in ecological studies to help find rare species.

This concept obviously has good potential. A list of criteria important to a wide variety of individual genera and species (even cultivars) could be gathered and inputted. The software could then predict all areas in the country (regardless of zone) where this plant might grow--much depends on which climate variables are used. I think this concept bears considerable experimentation.

The one possible problem with this: if certain information was inputted for a tree, say a certain degree of moisture in its climate, that would mean the software might not pick up otherwise suitable areas of the country (because that level of moisture was not present). A specific example of this would be my weeping cedar (Cedrus deodara ‘Kashmir’). It is a west coast tree that I believe to be worthy of fairly extensive planting in the “banana belt” of southern Ontario. If inputted criteria about it were to include the type of moisture received in B.C., southern Ontario might well not show up as a possible trial growing area. And yet, keen gardeners could easily provide missing elements such as moisture.

In other words, we would not want to exclude areas of possible successful growing for any plant due to presumed “necessary climate factors.” If we did that, we would set back the experimentation with woody plants that presently goes on when gardeners try plants they happen to find in nurseries that the knowledgeable professionals, including me, say are not hardy.

However, Dan McKenney’s willingness to experiment with an entirely new line of thinking as regards which woody plants will grow where deserves considerable co-operation by those in the ornamental plant business.

There are two other points about the hardiness map. Dan tells me that it would not be difficult for him to come up with a map for all of Canada based solely on the minimum-temperature factor only, the case with the present USDA map, on which so many gardening authors seem to want to base their zone recommendations because they want to sell their Canadian books into the USA. Personally, I think this would be a retrograde step. Our old map was superior to the newest in the USA, and we should not be going back just to accommodate those in another country.

Second, to repeat the last point made in the previous commentary, neither the old nor new map is for perennials. However, Dan McKenney says that it would not be difficult to come up with range maps for herbaceous perennials in Canada, based on not only some of the present factors but other climate factors thought to be important.