DigitalVinyl wrote:

...When a book says peppers and tomatoes need 55 degree minimum
overnight...are they referring to soil temperature or ambient
temperature. I had been assuming the later...



They mean air temperatures and are referring to mature plants. Below 55
F, tomatoes are subject to catfacing of the fruit, where the blossom end
of the tomato is covered with scars. It's a problem for producers, but
not really for home growers. It doesn't affect the taste, only the
appearance and saleability.

I would put the limit at about 40 F for setting immature plants out. 40
F average temperatures could easily get to freezing on an occasional
clear night, but you can cover the plants for that night.