-20F. Ouch. It only gets down to around 5F here, and I haven't seen lower than something like 10F in my garage (you're using the lights inside, right, so it will be a little warmer than the lowest temperature outdoors), but my T8 and SOME CFL bulbs really haven't had any issues in the cold. You just have to look closely at the package for the minimum starting temperature. -20F is the starting temperature I have on all my fluorescents.
Old timey fluorescents had issues below 40F, but the newer bulbs and ballasts have fixed this. My T8s and CFLs that are rated to light down to -20F do start out a bit dim in the cold, and need a few minutes to reach full brightness (but they do this to some extent when it is hot too).
Oh, and you may want a fixture that has something covering the bulbs (or, I use shoplights, with a plastic safety tube over the bulb). Either way, this insulates the bulb a little, which lets it heat up faster, so it reaches full brightness faster.