Actually I grow all winter in the greenhouse. Mostly lettuce, spinach, and other greens but this year I also had tomatoes. I’m in zone 8 with lowest temps about 15 so do need heat in there at times. The lowest temp I’ve seen this year is 20 at night. Most days the sun keeps the heat up and it’s only at night that it dips. By the beginning of Feb the freezing temps are very few if any although it can happen. The median last freeze here is March 28 so no plants get set out until then and then with protection of needed until mid April.
I’m zone 6 so year around might not be an option, but almost year around could be. I would like to get a big jump on seedlings for earlier and longer harvests. My father-in-law can get 55 gallon drums for free, and I’m wondering if I could use a couple of those filled with water to help heat at night.