We tried a little experiment this fall by planting some lettuce, asian green and leek seeds in flats and transplanted them into the ground in the greenhouse in mid-October, once the tomatoes were taken out. They are also covered by remay (white-cloth row cover) as well as having the protection of the greenhouse. They grew happily into their new home until sometime in November, when we experienced -16C temps and they appeared to be extremely frost affected. I thought that was the end of them, but they all came back to life once the temperatures warmed a little. Since then we have had a few more extreme freeze-thaw cycles, but each time they come back as good as new, fresh as daisies. I have been eating salad ever since. Amazing.