Most mature and hardened lettuce crops can survive temperatures in the mid- to high 20’s with some tolerating even lower if they have been adapted to colder temperatures (10-20 F). Lettuce contain a lot of water and when they freeze, ice crystals form within their cell walls that rupture and damage plant tissue. This low tunnel is covered with 6 mil plastic providing a few degrees of protection from the cold ( 5 degrees around the plants or or more on sunny days; source: Utah State University).