Only on the more tender plants. Hardier plants can handle tenperature swings pretty well. Any thing tropical shouldnt see below 45 for good health, and a good warm up. Plants like orchids and other tender plants are probably best indoors.

Good luck!

I read on this site many experienced people comment that cold hardy palms should not be permitted to overheat. So a green house/ hoophouse etc... is good as long as you vent it on sunny days over 30*F.

If not it can reach very high temps and cause the palm to grow too much and then drop to freezing and damage the plant. The plant has natural antifreeze and its activated in the plants dormant cycle (this last part is half educated guess). Like TZ7 said (hi TZ)it largely depends on the plants we are talking about. Which plant did you have in mind.

I let my green house to hit the 80s everyday when the suns out so the citris can bloom.