Thanks for your input! I knew it wouldn't survive my winter out in the open but was wondering what its limits were. I do have a tall cold frame that DH constructed last year. It's pretty air tight and has twin wall poly panels in it. I put rigid styrofoam along the sides and put large rocks in it to hold some heat. He also found a handy plug with a built-in thermostat that will turn on the power to a 60-watt light bulb when a preset low temp is reached (22 degrees). It will turn off the power when an high temp of approx 34 degrees is reached. A little computer fan keeps the air circulating. I can also pile on rigid styrofoam on the top covered with a blanket to trap more heat. It did keep potted kale viable (although not growing) all winter. Since I grow most of my produce in pots, I was thinking of stashing some of the more cold-tolerant stuff in the cold frame. Growing through to New Year's might be doable.
