is simply trial and error. I had recently moved most of my herbs and vegetable seedlings outside, and I lost a few of them last night due to unseasonably cool weather. I've grown several of these plants before, and I have a good idea about their heat tolerance, but not so much about their cold tolerance. I know they're replaceable, but I don't feel like spending money at a nursery and I don't have the patience for reseeding them this late in the game. Gah.