It’s a winter-time fact in most parts of the country: greenhouses, even those that might be attached to the house or garage, need some kind of heat source (of course, supplying appropriate light is equally important). My first, only partially realized, greenhouse up in the rain forests of Washington’s Olympic Peninsula needed heating just to keep the humidity down. But in most parts of the country, cold is the problem. You may have built your greenhouse with visions of supplying your family fresh, year-’round greens. But winter growth and germination are difficult when soil temperatures seldom climb out of the low 50 degree level. Sure you can use a heating mat to encourage germination. But even the hardiest green grows slowly — very slowly — when nighttime air temperatures plunge.