OOops I didn't mean to post 2 postings......The system was a little slow in processing my first post.

Depends on what plants you are trying to grow. If you are germinating seeds then I would shoot for 70-80 degree days...the night temps are what could slow down germination, and if you wait too long for warmer nights then daytime temps will be too great for good germination..If you are growing tropicals I would wait until consistant low temps of atleast 55 although I havent seen damage from my citrus plants at temps near freezing...most perinials can take a wide range of temps although some are finiky...what I am trying to say you need to know what your plants like and need...from my limited experience plants can tolerate the cold conditions quite well when they are sheltered from wind agnd frost in a greenhouse..and most plants will struggle more when temps in the GH become 110...good luck and enjoy your new toy

Steve