I leave ours in the ground without any cover here in SW Scotland. They are in raised beds simply because our soil is cold, wet and peaty. Came through the weeks of -6 to -10 last winter; tidy off dead leaves and extra runners in spring, feed with blood, fish and bone and let them get on with life. Good crop this year 12.1kg from about 30 plants plus an awful lot eaten but not weighed first.