We're enjoying winter salads again this year, thanks to a cold frame covered by a hoop house. We planted some cold-hardy greens in September and October in the cold frame, and covered the cold frame with a hoop house in November. The hoop house moderates the temperature inside the cold frame, so even if the temperature outside may be in the single digits, on a sunny day the inside of the cold frame can hit 50 degrees or higher.
This may be the last year for this style hoop house, though, because the winter winds and snow take their toll on the 6 mil polyethylene sheeting which comprises the hoop house. The sub-zero temperatures also tend to crack the joints that connect the PVC tubing of the hoop house, resulting in the ribs breaking loose and punching holes in the polyethylene. I'm using some galvanized steel fittings this winter on a couple of the joints, which seem to be working better, but still aren't ideal.
Next summer may require some brainstorming to come up with a different method, since it is nice having fresh greens all winter. They make the store-bought greens, even the organic variety, seem bland in comparison.