Canada's plant hardiness zones revisited using modern climate interpolation techniques

Canada's plant hardiness zones are well known to Canadian gardeners. The original hardiness indices and zones were developed in the early 1960's through regression models of several climatic parameters and plant survival data from numerous locations across the country. Since that time Canada's climate has changed and climate interpolation techniques have improved. We have remapped Canada's plant hardiness zones using data from the period of the original analysis (roughly 1930-1960) and for the 1961-1990 period using thin plate spline interpolation methods. Trials of bivariate and tri-variate splines were undertaken and evaluated using withheld data. A trivariate function of position (longitude and latitude) and elevation performed best. Standard errors of the surfaces were about 0.5°C or less for temperature variables and 5 to 28% for rainfall depending on the month (winter months being the worst). The creation of a new digital elevation model (a regular grid of position and elevation) of Canada enabled the mapping of each variable required for the plant hardiness formula at spatial resolutions of 1 km to 10 km. These models better capture the spatial variation in climate than previously possible and hence should provide a stronger basis for applications such as the determination of plant hardiness zones. Comparisons of the zones between the two time periods are consistent with what is known about climate in Canada. The hardiness index has declined or has stayed stable in eastern Canada and has increased in western areas. The results also suggest that more station data are required in western Canada to better capture the inherent spatial variability of climate, particularly precipitation, in mountainous terrain.