22 Plant Hardiness
Learning Objectives
- Explain plant hardiness zones.
Over the course of their evolution, plant species adapt to the climate variations of a region. Therefore, the ultimate deciding factor in whether a plant will survive in a given location (with adequate supplies of light, moisture, and nutrients) is quite simply the lowest temperature it will have to endure. Although several factors such as length of frost free period, rainfall, snow cover, wind, and soil type affect the hardiness of a plant, in temperate climates the minimum temperature during the winter is the most important element in plant survival.
Plant Hardiness Zones
Average annual minimum temperatures are determined for locations throughout North America. Plotting areas with similar average minimum temperatures yields a temperature zone map. Zones numbered 0 to 9 relate to the average annual minimum temperature calculated for that zone. The zones are divided into “a” and “b,” the “b” area representing the mildest part of the zone. Plants designated “a” with the zone number are hardy in the colder part of that zone; those designated “b” in only the milder section.
Plant hardiness ratings are determined by testing over several years at agricultural research and testing stations as well as private nurseries and gardens. A plant which is hardy to a particular zone can be expected to survive in all regions on the map which have an average annual minimum temperature equal to or greater than the hardiness zone rating for that plant.
Currently, two hardiness zone maps are widely used in North America:
- Agriculture Canada
- United States Department of Agriculture (U.S.D.A.)
In Canada, horticulturists often refer to the Agriculture Canada hardiness zone map. It is similar to the U.S.D.A. system except that the temperature range for each of the 9 zones is given in degrees Celsius instead of degrees Fahrenheit. Table 22.1 lists examples of hardiness zones for some Canadian communities.
| Location | Zone |
|---|---|
| Edmonton, Alta. | 2 |
| Prince George, B.C. | 3 |
| Ottawa, Ont. | 3 |
| Fredericton, N.B. | 5 |
| Langley, B.C. | 7 |
| Vancouver, B.C. | 8 |
A map that outlines all of the different zones is available at this link to Canada’s Plant Hardiness Zones [New Tab][1].
Horticulturists in the United States most commonly use the U.S.D.A. map. It divides the United States into 13 zones based on the average annual extreme minimum temperature with zone 1 being the coldest (-60 F.) and zone 13 being the warmest (above 60 F.). It is included in many books and catalogs, and is available at this link to the USDA Plant Hardiness Zone Map [New Tab][2].
Changing Climate Means Changing Hardiness Zones
Natural Resources Canada updated the plant hardiness zones map to include, among other factors, the effects of elevation on plant hardiness. The update provided evidence that there have been marked changes in hardiness zones in Western Canada. While the map expanded the factors affecting plant hardiness, local variability in topography, shelter, and snow cover were not captured. In an effort to increase knowledge about the effect of changing climate climate on the range of species growth in different locales, Natural Resources Canada created an interactive zone map where experts and gardeners contribute information about plant survival at this link to Canada’s Plant Hardiness Site [New Tab][3].