Canada's Plant Hardiness Site

Extreme Minimum Temperature Models

The United States Department of Agriculture (USDA) has developed a plant hardiness map based on average annual minimum temperature. Their map averages the lowest annual minimum temperature over a standard time period (e.g., 1991-2020), thus showing the lowest average temperatures that can be expected each year in the United States, including Alaska, Hawaii, and Puerto Rico. For display, average minimum temperatures are classified into 10-degree F (5.6 degree C) zones and 5-degree F (2.8 degree C) subzones. For details on this map please go to https://planthardiness.ars.usda.gov/. We have developed a minimum temperature hardiness zone map for Canada that generally follows the USDA approach (McKenney et al. 2006). Please note that there may be differences between our minimum temperature maps and those produced by the USDA due to slight methodological differences between organizations.

There is interest by some in Canada's horticulture industry to compare the USDA minimum temperature zones to Canada's plant hardiness zones (which are based on 7 climate variables). Some plants are traded between the two countries and there is sometimes a need to make such comparisons. We carried out a study to this end, concluding that there are no simple conversions between the two systems (McKenney et al. 2006). For example, when the two maps are overlaid, Canadian hardiness zone 4a overlaps with 9 different USDA zones, ranging from 3a to 7a.

References:

McKenney, D. W., Pia Papadopol, M. H., Campbell, K., & Lawrence, K. (2006). The generation of USDA-equivalent extreme minimum temperature models and a comparison with Canada's plant hardiness zones. Canadian Journal of Plant Science, 86(2), 511-523.