High tunnels, also known as hoop houses, are simple polyethylene-covered unheated structures that typically do not use fans for ventilation. Tunnels can be covered with one or two sheets of plastic; those covered with two have an air layer in between, thus offering better insulation and, consequently, more cold protection (and wind protection). High tunnels are used to extend the growing season earlier into spring and later into fall. Determinate and indeterminate tomatoes (Lycopersicon esculentum) can be successfully grown in this production system, yielding a potentially profitable "out of season."