Last spring we built a cold frame in our front yard to extend the growing season of our vegetables, and we were eating arugula from it in late December. But during January through early March the cold frame was buried under a couple feet of snow, including multiple weeks where we also experienced biting subzero temperatures.
Today the snow finally melted enough to scrape off the top of the cold frame and peer inside, and to our surprise arugula, spinach, radishes and cabbages are blithely continuing to grow looking no worse for the lack of light and the record low temperatures!
Wow. It worked beautifully, and beyond my expectations!