A map showing suitability for winter survival of ornamental trees and shrubs has been prepared for Canada. It is based on the estimated suitability indices (from 0 to 92) for 640 localities. It includes 10 zones with a spread of 10 units; each zone is subdivided into 2 sub-zones. In addition, probability curves indicate the chances of survival within zones and sub-zones of any plant for which the index of hardiness is known.
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Ouellet et al. (1967) studied this question.