Does anyone have experience using a cold frame in Zone 5? I used to have one years ago that I used for hardening off spring seedlings... but really never used it in fall or winter. My main purpose for wanting one now is simply to extend the season a bit in late fall and early winter. I grow Parma violets in pots and seem to have amassed a rather large collection. For now I leave them outdoors as long as possible until the temps get down into the 30's. Then they come indoors to spend the winter under fluorescent lights in the basement. The temperature down there usually hovers in the 50's. They bloom for me, but the flowers are always a bit undersized and the blooming period rather short. I believe they would be happier, and more productive, if they were able to stay outdoors longer into the fall. I guess my main question is... to what temperature is a cold frame actually effective? I can believe it would stay nice and toasty on a sunny day even if the temperature was well below freezing.... but would it provide sufficient protection for plants on a 20 degree night? Any info would be appreciated.