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 1960s 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 th...