It is difficult to separate out the two factors that might affect hardiness. One is where was the cultivar hybridized. Northern-bred plants will be more winter hardy than southern-bred plants. The other is foliage type. Dormant versus Evergreen.
The comparison cannot be made simply by comparing the winter hardiness of dormants versus evergreens. One needs to compare the winter hardiness of dormants versus evergreens that were hybridized in the north only with each other. One also needs to compare the winter hardiness of dormants versus evergreens that were hybridized in the south only with each other.
So for example, R. W. Munson registered 44 dormants according to the AHS database. He registered 752 evergreens. To test whether evergreens have the same winter hardiness as dormants we could choose (at random) a number of dormants registered by Munson and a number of evergreens (at random) registered by Munson and grow them in the same garden. We could validly compare their hardiness. Of course we do have a factor that may affect even this comparison - that is if the dormants that were registered by Munson had more northern-bred parents than the evergreens that he registered we would have the same confounding factor of northern plants being selected for hardiness (and passing that hardiness to their seedlings). Nevertheless, this comparison is better balanced for diminishing the effect of where the particular foliage-type plants are hybridized.
We might also try comparison tests of hardiness between dormants and evergreens registered by a hybridizer in the north and compare the hardiness of dormants and evergreens registered by that same hybridizer after they moved to the south (for example, Stamile). We would still have some confounding influence of the parents of dormants being more likely to have been bred in the north versus those of evergreens being more likely to have been bred in the south.
When we look at hardiness in a garden and classify cultivars as dormant versus evergreen it is likely that in practice that classification is really not the one that affects the hardiness. It is likely that basically dormants are northern-bred and evergreens are southern-bred and northern-breds are automatically winter hardy (to their home locations) while southern-breds are winter hardy to their home locations - which tend not to be in the north). The classification dormant versus evergreen really is not the one that is in fact causing any possible hardiness differences seen - the classification dormant really stands for northern-bred and therefore automatically northern winter hardy while the classification evergreen really stands for southern-bred and therefore northern winter hardiness is unknown/untested and possibly lacking.
