The year we grew them we harvested them after frost as I recall. If you want to harvest them during the winter and your ground is likely to freeze much, I'd probably mulch the bed well so that they are easier to dig. I have done that with other winter veggies and it has seemed to work better for most as far as maintaining quality over root cellar storage. I have found that sometimes the snow is so deep that it's not worth trying to find the veggies - then they get dug and eaten in early spring.