Minus 5 (that is FIVE degrees BELOW zero) this morning. The lettuce bed never fell below 22 degrees above zero, with no lights or anything providing extra heat. I do not know if the lettuce can take that for several hours. Tomorrow, when the high hits 31 I will open the tunnel and see if any lettuce is worth picking (that is my intention, anyway). Then I will pick it all.
With any luck, most of the roots will survive the winter. As it warms up in a month or so, I'll pull the mulch away from the plants so the sun shining through the plastic can warm the soil and get the lettuce growing again. I might even plant some lettuce When I cleaned out one of the other beds last week, I saw that some of the lettuce that had been cut earlier had started to regrow. This gives me hope of an early crop this spring.