We actually know surprisingly little about what triggers daylily dormancy, unfortunately, or even what kind of dormancy they have (e.g. do they have a "true" dormancy that requires some condition to release it). It doesn't necessarily have to be temperature that triggers it, it could be daylength. It could be a combination of factors. If it is temperature it doesn't necessarily have to be freezing temperatures. I've had seedlings grown in the house on the windowsill go dormant. However, with me there's probably no guarantee they didn't get too dry, I'm bad about remembering to water indoor plants and they were on a warm sunny windowsill. Drought can definitely send daylilies into dormancy! I think quite often dormant daylilies are written off as dead.
Do you have a rough idea of when in the year they started to die back, and also what would be the minimum temperature the seedlings were exposed to?
BTW Stout considered evergreen to be dominant in hybridizing.
