This winter in Indiana we had a batch of spinach that we harvested in late November and our planta is not heated.The spinach survived and by early March we were picking spinach from the same plants.
I wasn’t able to finish installing one side as we are only there for a few weeks and I had other projects. But the quality of the greenhouse meant I could leave it somewhat unfinished over the winter without concern.
We added a heater so we can grow 4 seasons now. We have a heavily treed lot so I oriented the greenhouse to get the most sun in the winter and the leaves on the trees provide shade in the summer to help with over heating.