Take a look in my gallery. Mine have inadvertently ended up spending at least a few frosty nights out. Likely not any lower than about 30F or so, as the animals' water wasn't frozen over or anything like that. What I've done, if you look in my gallery, is take a plastic bin and put in some metal range burner covers (big black enamel-coated steel), hopefully these act as a heat sink of sorts. Gravel, or anything dark and dense could give you just enough, just a few degrees-worth of warmth seems to make a real difference. So far, of the 2nd stage, I've got a 100% germ-to-sprout ratio. Dey be small, but dey be strong, mon!
I think it really depends on how cold your area really is. But know that some of the best stuff is grown in counties in the true north of Cali where there
must be some frosty nights even if they are more coastal, and grow they DO! Oh, also, mine are about 30' off the ground on our west-facing back deck, on the southern bit of it for the best exposure. There are ways to warm things up so you protect your plants, but you have to know what you're dealing with, first.