This past fall I created some low-tunnels, kind of the poor man's version of a high tunnel. I used 18” pieces of rebar set 4’ apart and placed an 8’ piece of PVC pipe over each section of rebar. I then used cheapo 4mil clear plastic from Lowes to keep out the weather. Each piece of PVC protected about 3’ of ground, and I used 4 pieces, so a total of about 12 feet ended up covered. At either end, I cut the plastic kind of long, twisted it into a big, 4” rope and covered it with something heavy like a cement block or big piece of junk cement.
Sometime in the fall – I guess it was around early September – I started planting spinach every couple weeks. I covered the soil with the plastic starting in late October November and kept planting spinach until around that time. The smallest plants only had one pair or true leaves in the fall.
I had lots of spinach to harvest for Thanksgiving and then pretty much covered the bed and forgot about it over the winter. This WAS a fairly mild Michigan winter - I don't think it ever went below 0F. The pictures I just took show how well spinach can grow in cold weather. The other picture is of mache that I grew in big coldframes.
The thing I like about low tunnels is that they can be about any size you like and anyone can go to Lowe's or Home Depot and buy everything they need to throw one together. You don't need a big yard or a store bought high tunnrl to begin growing greens (and I guess other stuff) out of season.