Libby, I am in zone 6 and I started mine at the beginning of April in baggies, in the house (Poochella gives a good description either here or over on cubits.org/dahlias). I have taken cuttings of quite a few already and will start putting them in pots on my deck next week to harden off. They'll remain there till it's safe to put them in the ground. I am keeping most in pots this year, as my soil is so awful- but will sink the pots and mulch around them to retain as much moisture as possible. I did this on a smaller scale last year with good results, and it made digging the tubers a whole lot easier after the frost got them. A couple of late bloomers got moved to the garage when frost threatened, and were able to be enjoyed for a couple of extra weeks.