An older term for the method of growing in tunnels is “cold frame” farming. It acts like a greenhouse, but uses plastic sheeting instead of glass over a metal frame. During the day, the sun heats up the air and then the soil inside. The soil stays warm once the sun goes down. According to Jim Crawford, who for more than 40 years has owned and operated a certified-organic farm in south-central Pennsylvania, it can be 70 to 80 degrees Fahrenheit inside during the day while it is 20 to 30 degrees outside. And at night, the outdoor temps can fall to zero, but the soil temperature remains above freezing.
The tunnel method became popular about 30 years ago, when a do-it-yourself kit became available to farmers at a reasonable price. In 1991, Jim ordered two kits and constructed two 100 by 21 foot tunnels – large enough for a tractor to enter, plow furrows and spread fertilizer and composts. Four years later, a storm destroyed them. In 1997, he built two replacements in a more sheltered location. In 2012, he put up a third, larger tunnel – 180 by 30 feet – and in 2016 he built a fourth. The larger ones cost $26,000.