"I couldn't image how any of our vegetables would survive in temperatures that drop below freezing. I had heard that some farmers use a plastic covering to extend the growing season into the winter. But while I was intrigued by the prospect of keeping the garden alive, and I was open to experimenting, I was skeptical that a sheet of plastic would keep the plants warm through the D.C. winter.
In spite of my doubts, we went ahead and built what are called low tunnels, or hoop houses, simple metal frames that stand about two feet off the ground and are covered by a clear plastic tarp. To my amazement and delight, under those simple structures, several of out crops continued to flourish, even in very cold weather - and were unusually sweet. I later learned that since sugars don't freeze, the plants produce a lot of sugars to protect themselves from the cold."
Michelle Obama in American Grown