I start all my tomatoes from seed and since I grow some for selling, I have a lot and have to put them in unheated greenhouse as soon a possible. As long as it's above 40, they stay outside. I actually forgot to bring one tray in the other night when it got below 40 and they did fine. They seem to get stocky and stronger than if I had babied them by keeping them inside (even though my house is cold too! )