Plant hardiness in Ottawa

Part I: The Macro Picture Derived from a press release issued by Natural Resources Canada and made available to the OHS through Dr. Ian Efford

Nothing is more important to gardeners than knowing whether their plants are likely to survive the winter. So back in the 1960s, scientists at Agriculture Canada developed a reference map illustrating suitable planting zones for ornamental trees and shrubs. The original map was developed using climate data from about 640 weather stations. This information was married to data about where and how well various plants survived. The scientists created a formula that correlated plant survival with climatic conditions and then drew lines on a map of Canada to represent the boundaries between what we now call plant hardiness zones. The 35-year-old map has been helping to take the guesswork out of decision-making for Canadian gardeners, horticulturists, and nursery owners for more than three decades and is routinely reprinted in gardening magazines and on the backs of seed packages.

Statistical analysis methods and computer modeling programs for predicting climatic conditions between weather stations have become far more sophisticated since then, prompting Dan McKenney of the Canadian

Forest Service to try his hand at updating the old plant hardiness zones map. Dr. McKenney was able to generate new computer models of Canada's climate that incorporated elevation effects into their calculations and maps. The results are now included in a new plant hardiness zones map and in Agriculture and Agri-Food Canada's web-based plant hardiness software, which allows people to zoom in and look closely at conditions in their area.

The revised map is much more accurate than its predecessor, both because it accounts for the influence of elevation and because it is built on much smaller units -the map on the web is built from about a 2-km grid of Canada! Forest managers, horticulturists, and conservation authorities should be able to calibrate their expectations of plant hardiness with much more precision.

The online version allows a closer check of individual areas than does the Canada-wide printed version. Check it out on the web at http://cfs.nrcan.gc.ca/subsite/glfc-arboretum/zone-maps

Ottawa's average frost-free period is from May 6 through October 5. Our zone 5 represents the northern limit for Norway maple (Acer platanoides), English oak (Quercus robur), horse-chestnut (Aesculus hippocastanum), smokebush (Cotinus coggygria), early forsythia (Forsythia ovata), and Korean spice viburnum (Vibernum carlesii), according to an insert in the May 2001 issue of "Gardening life".

A Caveat Many perennial experts believe that plant hardiness zones are not always reliable for rating the winter hardiness of perennials. This is because winter freeze-and-thaw patterns in warmer parts of the country can often kill plants that do just fine in more northerly regions where they're protected under a thick blanket of snow. It is a good idea to hedge your bets with the judicious use of mulch in the fall.

Knowing that Ottawa falls into hardiness zone 5a is helpful indeed but most gardeners know that sometimes they can grow plants rated for 5b or 6 or even, as is the case in Glenn Clark's south Gloucester garden, ornamental trees rated at zone 7! Or sometimes, zone 5 plants are iffy, at best. This phenomenon is accounted for by the existence of micro-climates and it is possible for an individual gardener to determine the micro-climate of his or her own plot.

Here are Marjorie Harris's instructions, derived from "The Canadian Gardener's Year; A Perpetual Diary For Northern Gardeners."

1. Record the official date of the last spring frost in your area given in the newspaper. Also, using judiciously-placed thermometers in your garden or by checking for signs of frost on the ground first thing in the morning, record the last frost (0 degrees C) date in your garden. 2. Record the first fall frost date in your area, given in the newspaper. Determine the first autumnal frost date in your garden by checking thermometers, looking for signs of frost on the ground, or by surveying the state of the annuals there. 3. Count the number of days between your garden's last frost in the spring and its first frost in the fall. Also calculate the number of days between the official last and first frost days. If you have more frost-free days, your micro-climate is milder (tending toward 5b or 6). If your garden has fewer frost-free days, it may be more like 4b or 4a.

If you are visiting Kiwi Gardens near Perth, you can tour the lovely private garden immediately to the south of the Kiwi site. Mr. and Mrs. James have put up notices indicating that their garden is open for touring; they ask only that visitors consider making a contribution to the Children's Hospital. Look for the donation box next to one of the perennial beds. Since many of their plants come from Kiwi Gardens, it offers a good opportunity to view a plant in situ before making a purchase next door.

Please contact the OHS or the author if you wish to republish these articles. © Ottawa Horticultural Society

Back to index of articles