How Come Everything Grows Well in Tunnels...
Greenhouses work by raising the median temps and by helping to keep out a lot of the harmful effects we don’t want like hard rains, high winds, or temperature fluctuations. The greenhouse also helps to keep humidity fairly high and carbon dioxide trapped inside the greenhouse. Plants use carbon dioxide and they release oxygen. It is important to always have fresh air coming through a greenhouse.
Some people grow their entire crop in a greenhouse, while some use it to start plants before moving them outside. You can get an early start on Spring this way. Others grow cold hardy greens straight through winter with no added heat. There are commercial units available, but it’s typically not very hard to make your own using a wooden frame or PVC pipe.
High tunnels -- unheated, plastic-covered, relatively inexpensive structures -- can grow lots of food on little land, can do it nearly 12 months out of the year even in colder climates, and need fewer inputs than larger-scale, open-field farming methods. Already widely used in Europe, with growing pockets of interest in America, high tunnels help farmers farm successfully on smaller pieces of land. That makes them handy on farms (or backyards) near cities, typically places with high land prices and little or no room to expand.
High tunnels come in many sizes and can serve many functions, depending on the needs and objectives of the farmer or gardener, from 8- by-12-foot backyard models to commercial types as long as a football field. They come in kits, or handy growers can buy their own materials and build them fairly easily. However, while high tunnels are one of the more aggressive and comprehensive ways to extend the growing season, they're not the only way. Lower-cost, less-involved options include raised beds, plastic mulches, row covers and low tunnels, alone or in combination. Growers who want to extend their season could try those first without investing in high tunnels.