Update on growing lettuce all winter!
Last fall we put up an attached 8x12 glass greenhouse and I posted here about wanting to grow lettuce all winter in an unheated greenhouse. Since the temps go down in the teens quite frequently here, and we get very few days of sunshine, I had to find a way to keep the plants from freezing. Bubble wrap gave us about a 5 to 7 degree difference from outside but that wasn't enough. There is only room for two large garbage cans full of water, but that helps to keep the temperature just above freezing. I put row covers on at night to give me another 5 degrees above the outside temps. I even tried putting hunks of soapstone on our woodstove during the day and tucking them in among the plants at night to release heat slowly during the night. But that only lasts a few hours.
A couple weeks ago, my husband suggested putting our extra gutter cable into the raised bed. I zig-zagged the cable across the bed on top of the soil. What a difference this has made! The temp just above the bed hasn't dropped below 40 since. The lettuce plants had really slowed down with the low light, but now the combination of lengthening days and the warmed soil has worked wonders. The plants are perking up and showing some growth again. On a really cold night, I still put on the row covers to hold in the heat. Right now it's 25 outside and 50 in the raised bed.