Google said after Nov. and through the winter; a friend's friend in n.h. is supposedly harvesting his now. What do you do? Have you had any luck harvesting them through the winter?
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.