I wonder how fast and how far natural selection can change perennials' or reseeding annuals' cold-hardiness?
I would expect (especially for re-seeders) more of the hardier individuals to survive and drop seed. In subsequent years, recombination and selection among the surviviors ought to tend to produce even greater hardiness.
I would expect the perennials to be selected mainly by disapearing in adverse years, but, if they also drop seed, some hardier individuals might come back (or you can re-establish them from your saved seeds).
My guess is that this kind of selective drift can only change a species' hardiness by a few zones, or make it a little more resistant to surprise late frosts or an early thaw followed by frosts. Does someone know more about this? How much does hardiness vary within a species or within a cultivar?
It's another reason to save your own seed. Your own seeds ought to be at least somewhat adapted to your climate and soil and pests and methods. If a bad winter (or summer) kills everything in the ground, but you've saved your own seeds, re-establishing them from YOUR strain should retain any local adaptations and let you continue adapting them to your climate.
