I planted some spinach last Oct 09, I planted Bloomingdale long standing. I had 30+ below windchills and 20+ below  true air temp conditions. I just have the spinach in large containers with 6 mil plastic covering them and two heavy rag blankets I use in the garden. The spinach survived the 2 week period cold. I did not disturb the covers, and I check the plants the middle of Feb and took off the blankets off and the spinach was starting to grow and come out of hibernation. I thought I did pretty good this year for a simple set up. I am located in Nebraska.