Many fewer species of cacti are native to and thought to be able to survive winters in eastern Ontario than in the similarly cold winters of central Colorado. We collected 12 yr of data on 107 specimens representing 50 cactus species that have been tested outdoors in gardens in the City of Ottawa (Canada) and report which have successfully weathered six or more consecutive winters and which have not. Some species that we expected to be more successful were not, while a small number of species native to considerably warmer environments survived surprisingly well. We review general mechanisms for freeze-tolerance in plants, focusing on what is known about cacti in particular. Phylogeny does not appear to be important in determining success or failure in cold climates, so we explore other possible explanatory factors for differences in survival between Ontario and Colorado. Our data indicate that freeze tolerance of cacti in eastern Ontario may be a function of snow cover, rather than polyploidy. Colorado's greater cactus richness may also be a function of its location closer to the southwestern deserts' center of diversity, which would provide a larger pool of potential species that could expand to colder regions. More thorough studies of freeze-tolerance over a larger geographic range – albeit controlling for growing conditions, using identical clones at multiple sites, and determining precise cause of death – will be necessary to reach more definitive conclusions.
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Vol. 62 • No. 1