This is the first time that I have been to your web site, and would like to offer a tip to the readers.
I made a root cellar by digging a 10'x10'x4' hole and lining it with railroad ties and a roof made from 2x6s and sheet styrofoam. I use this to overwinter potted zone 5 and 6 plants (roses mostly). The first year that I had the cellar, I put an indoor/outdoor thermometer with the remote bulb into the cellar. Even at -20 air temp, the root cellar stayed a nice +20.
I took the top off last weekend (April 14th) and the pots were still frozen. This weekend I will put them into the greenhouse and avoid all of the freeze/thaw cycles.
Don't leave the cellar closed up too long, as that is a perfect environment for mold. The shrews and voles love chewing on the plants, and I lost a lot the first year. Now I put in a dozen moth balls and a couple of mouse poison traps. That put an end to that problem. If you have dogs, put plywood over the roof to stop them from punching holes in the Styrofoam. Steps in the corner make it easy to get in and out.
On the whole, it works very well, with a 90% survival rate. -AlaskaDust