I'm thinking again (post-cold snap) that those of us in long winter day climates just need to add more winter protection to easily bring more greens through the winter than we normally can.
There are already lots of hardy greens that we can grow (if temperatures don't get below 20°F for too long), but with a bit of protection (cold frames, mini-hoophouses, etc.), we can extend our vegetable-growing seasons throughout the winter.
I harvested some last large leeks yesterday (delicious roasted), and hope my small leek seedlings (transplanted to beds in late fall) back home in the Piedmont are doing well, in spite of the cold snap.
It's been a mild winter so far, and temperatures will moderate again by the end of week. Time to sow more experimental winter varieties of lettuce and greens in our unheated hoophouse, at the botanical garden where I work!