Although we've had a milder winter, we've still had a few significant snow storms this year. One delivered 8 inches of snow and then one a week later we got 4 or 5 additional inches. Today it's forecast to be in the high 40's to low 50's.
I've kept a close eye on the evening temperatures with my high/low digital thermometer under the covers. I know there are expert farmers/gardeners who claim that for every layer of protection you move your garden a USDA zone and a half or 500 miles to the south . Since I'm in zone 6B, that would make my gardens the equivalent of zone 9 with my double coverage. That would put me on par with the thermal belt of California's central valley. Click here to see what folks in zone 9 are growing at this time of the year. There is no way I can come close to what they're doing. Not right now.
What I think is most important is the "Sunset Zone Considerations" list. As this article explains, the difference between USDA and Sunset zones are very important. While the USDA categorizes zones by the winter low temperatures, the Sunset zones take into consideration many other important factors. The influences listed-altitude, mountains, hills, wind,continental influence, ocean influence(vastly different from east coast), etc. can have a significant effect on winter gardens. If you'd like to grow something through the 6B winter and are of the thought that a simple double cover of plastic and floating row will take you to zone 9-give it a try! You'll be sadly disappointed.
I did find, however, that my cold frame covered with a simple glass window did a much better job of keeping warm than my double covered square foot garden beds. The challenge with the cold frame is you've got to have many of them to take you through the winter. And you must remember to vent them if it's sunny and gets past 45 degrees.
If you live in a climate where the winter temperature gets below 15 degrees or colder-the lower limit for quality winter salad crops-it becomes even more important to choose the proper things to grow. By minimizing the size of your garden as we do in the square foot gardening system, you can have a different garden by protecting it from the real destroyers of crops-wind and the continual freezing/thawing of crops. With the ability to do just that with your square foot garden, it then becomes a simple matter of crop selection.