Updated plant hardiness zones for Canada and assessment of change over time.

Plant hardiness systems have been developed for various regions around the world to help ensure that cultivated plants are grown at locations where suitable climate conditions prevail. In Canada, a multivariate plant hardiness index was developed in the 1960s that incorporates several temperature- and precipitation-related variables, as well as snow depth and wind speed. In the United States, the plant hardiness system involves averaging annual extreme minimum temperatures over a period of interest, with values subsequently classified into hardiness zones. Here we report on efforts to update hardiness zone maps for Canada using both the Canadian and US approaches and using climate data for the 1991-2020 period. The two hardiness systems produced generally similar spatial patterns in plant hardiness across Canada, including high index values in southern and coastal regions and low index values in northern and high-elevation areas. Detailed comparisons to previous hardiness maps indicated that, since 1961-1990, zone values have increased by between half a zone and two full zones across the country, with the largest increases occurring in western and northwestern Canada. For the multivariate Canadian hardiness system, a change attribution analysis indicated that three temperature-related variables were primarily responsible for driving the observed changes in the plant hardiness zones. The new maps are available at http://planthardiness.gc.ca .