I saw this idea from a Mother Earth news article. The original article ran in an edition from the 70’s. It’s not a
root cellar, they called it a cold box. It’s just a 3’x5’ box, 3’ in the ground.
I built mine out of 1/2” plywood, wrapped the box in sub grade
water proofing liner. Put plastic on the ground under it (thinking it would help prevent moisture) and covered it with
pea stones.
I live in SE MI, zone 6. Last winter I have potatoes, onions, cabbage, squash, garlic and a few other things. Most of everything lasted us till mid-late January.
This year I added the foam panel insulation I had left over from my house siding
project. Adding an extra r-4 value, I’m hoping it will give me a couple more weeks!
So far, I think it’s a really great, inexpensive idea if you have limited space and are looking to hold some hardy crops a little longer.