I don't generally grow radishes, because they aren't a favorite plant. However, some short-season corn I tried this year proved to be seriously short-season, like 50 to 60 days, and I couldn't bear to see empty garden beds this soon. I planted 18 square feet two Tuesdays ago, another 18 square feet the Tuesday after that and a final 18 square feet this Tuesday. The first batch is already on its fourth or fifth set of leaves; the second batch is beginning its first set of true leaves; the third should be popping up on Saturday.
It's fun. I had no idea planting radishes could be such a good time! No idea yet how they will turn out. I thinned the first batch an inch apart and then got smart on the second batch, thinning them two inches apart. It'll be interesting to see how they fare.
According to the Farmer's Almanac, our average first frost here is September 19. I've seen radishes survive six inches of snow, but does anyone know how low a temperature they can take without snow to insulate them?