LOL, Ken -- I wouldn't plant them with temps as low as you describe, either; but the temps Carol is currently having can be summer days up here ...
Carol, if you aren't trying to grow other things that need more heat, I don't see where you would need to use your heater for the radishes right now. And, sad to say, if those seedlings were planted in early November, then, yes, I would start over. Do you know what variety you planted? My inclination would be to try a different packet of seed and see if that makes any difference. (but don't toss the original packet out yet if you still have seeds left -- no sense wasting them if that isn't the problem).
