In the Southeastern U.S, we have ample sunshine (over 10 hours a day, except for about a week and half around the winter solstice) for growing vegetables year-round.
In 'normal' winters, we can grow kale and collards that sail through normal freezes, as well as garlic and onions.
In exceptionally cold winters (like the last couple of years), unprotected hardy greens have suffered significant frost damage. But even this year, mustards have re-emerged looking pretty leafy in protected walled gardens like the kitchen garden next to the visitor center at the Garden where I work.
I've been delighted with my winter greens experiment in the unheated hoophouse at the Garden --totally amazing. I'm planning to sow a short season sequence of greens in the pots and bags of soil mix, as well as chard and beets, just to see what happens.