At least two reasons exist to grow vegetables in greenhouses. One is to provide the warmth and shelter that plants such as tomatoes and melons need, and the other is to extend the growing season into early spring and late fall. It takes careful planning to achieve both of these objectives in the same greenhouse, but it can be done. Many home gardeners and commercial growers now use plastic-covered greenhouses called hoophouses.
Glass, Polycarbonate or Plastic Film?
Almost every kind of vegetable, from arugula to zucchini, can be grown in greenhouses. Traditional greenhouses are covered with glass, which provides good light transmission. However, it is heavy, and breaks easily. Modern gardeners who like the peak-roof shape of traditional greenhouses prefer polycarbonate sheets, which are double-walled (like a thermopane window) and are thus more energy-efficient than single glass, and are unbreakable too. A much less expensive greenhouse can be made from steel or plastic tubing covered with clear plastic film. Because of their most common Quonset shape, these are called hoophouses.
The transparent covering of greenhouses, whether it is plastic film, clear polycarbonate, or glass, heats the interior on any sunny day in summer and winter. On a winter day, when the temperature outside is 20° F. (-7° C), even an unheated greenhouse can be a comfortable 50° F. (10° C). Night-time temperatures, however, can drop fast, and may get too cold for many plants, such as tomatoes and peppers. One way to deal with this for less sensitive plants, without providing artificial heat, is to use row-cover tunnels inside the greenhouse.
The lightweight fabric called row cover can be used to provide a double layer of plant protection inside a greenhouse in cold weather. The crops are covered with the fabric, which is stretched over heavy-gauge wire hoops that form a tunnel about 12 to 18 inches high. The plants are inside this tunnel, inside the greenhouse. Daytime heat is stored in the soil, and because of the row cover fabric it does not all escape. Many plants that will tolerate cool temperatures, including lettuce, spinach, and arugula, can be grown through the winter in all but the very coldest climates. Seedlings and transplants of most vegetable crops can be started early in the spring to get a jump start on the season.
For many years commercial vegetable growers have used hoophouses because they are relatively inexpensive to construct and maintain, and they work well. Thousands of home gardeners have now followed suit with smaller hoophouses in backyards all over America. Favorite crops are tomatoes, peppers, eggplant, melons, and other vine crops such as summer and winter squash.
A hoophouse can be anywhere from 8 to 20 feet wide, and as long as needed. The "hoops" are made of steel or plastic tubing, set 4 to 6 feet apart. The plastic film is heavy greenhouse film, which has a UV inhibitor to make it last at least 4 or 5 years, after which it has to be replaced. Kits are available from several manufacturers.
A home greenhouse does not have to be heated to tropical temperatures, unless you want to grow tomatoes in the winter or things as heat-sensitive as orchids. Many plants will do just fine if the temperature is kept just a few degrees above freezing, which costs a lot less. Polycarbonate greenhouses are more energy-efficient than single glass. Plastic-covered hoophouses can be fitted with a second plastic covering, with a small fan that keeps a cushion of air between the inner and outer layers---a remarkably efficient insulator.
Greenhouse heaters have to be suitable for that use---you certainly do not want a heater that produces fumes toxic to your plants and to you. Most home greenhouse heaters are electric, or run on oil or some form of gas. Good thermostats, and automatic vent openers, are required.