I'm experimenting with mini greenhouses made from sheep fencing (in an inverted U shape, like a quonset hut) with plastic over them. Right now I only have New Zealand spinach under one of them, and it did freeze when the temps went down into the 20s the other night (I will be covering it with a blanket tonight), but I plan to put some bubble wrap over them for insulation. I hope to be able to extend my growing season for some of the summer plants throughout the year this way. Our daytime temps are usually well above freezing, so the ground is usually warm enough for plant roots. My peppers, eggplants, and grape tomatoes were still growing and bearing until a few days before Christmas, so I have high hopes.