Okay, so you're talking hard freeze rather than just frost. I've grown collards back in PA that I harvested in very late December, a couple days before New Year's. I cooked them up as part of a traditional Low Country New Year's Day meal. They survived frost and some light snow before that last harvest. I've also over wintered spinach that I planted as a fall crop. Mulched the garden bed with chopped straw and leaves in December when the plants went dormant. Once the bed thawed in early March the plants sprouted new leaves and I could begin light harvesting earlier than if I had grown from seed. That was five years ago before I started spending winters in SC with mum. I might see if I can find some spinach seed here to plant as a winter crop. It doesn't get cold enough for the ground to freeze so I think it should work. If I can keep the deer from eating my crop.
We're at 14 feet.