greenhouse Minimum temp for young plants in green house

Want to go from grow room to green house as soon as possible, then refill grow room with things that take less time than super hots.  Thinking March.  If I am reading the charts right, we have an average low of about 30 degrees and a record around 0 for most of March.  To figure out if it is economic to heat in March, the last piece of the puzzle is what the lowest night time temperature young peppers (maybe a foot tall) can remain healthy at.

Thoughts?  Have seen things survive close to freezing, but they were not happy.

From my readings (eg DeWitt-Bosland) optimal night temp should be in the 60-75F range.
You can keep temps lower but in the 40-60F range seedlings will grow poorly (closer to 40 is worse obviously).