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Greenhouse crops are fruits and vegetables grown inside a building or enclosure to protect them from the elements and permit a longer growing period. Growing plants in greenhouses can be more expensive than planting in fields, but in some areas it may be a necessity due to scarce water supplies or environmental conditions. A greenhouse may be a permanent structure with glass or plastic windows for light, or temporary structures made from frames and plastic film.
Early greenhouses were wood or metal-framed buildings with glass windows, often attached to a home. The sun provided both the light and heat needed to grow vegetables or flowers in a more controlled environment, or when temperatures outside fell below freezing. Seeds could be germinated early in the spring and some vegetables could be grown late into the fall, particularly if an additional heat source was provided such as a wood stove or steam radiators.
The cost of glass and its ability to break led to a gradual development of breakage-resistant plastic greenhouses in the 20th century. Plastics were much lighter, could be made with the addition of pigments to block some of the sunlight, making them translucent, and could be molded in shapes to permit curved windows or domes. These structures were often permanent buildings used for specialty vegetables, herbs, or flowers, and were built to a smaller scale for residential or small commercial growers.
Commercial greenhouse crops were not considered cost-effective until the late-20th century. Demands for agricultural land for residential development and rising fuel costs for transporting fruits and vegetables made smaller greenhouse-based manufacturing more effective. A growing interest in organic products, or those products grown without artificial fertilizers and pesticides, provided a customer base willing to pay more for higher-priced greenhouse crops.
Large-scale commercial greenhouses began using large metal-framed structures and plastic film, which were sometimes built directly over the field crops. These structures were relatively easy to build and could be disassembled and moved to other locations as needed. Most of these structures were not heated with auxiliary heat, which would have been too expensive, but were used in areas where adequate sunshine could provide radiant warmth to extend the growing season.
Water conservation also became a growing concern, as potable drinking water supplies were more limited. Greenhouse crops can be grown using water conservation techniques such as drip or mist irrigation, which provides an optimum amount of water needed for plant growth. The controlled temperatures and humidity possible in a greenhouse permit the use of much less water per harvest than required in fields.
Environmental control in a greenhouse can be performed with controls linked to temperature and humidity. If temperatures rise above desired limits, windows or skylights could be opened manually or controlled by electric motors. Humidity controls could provide the same function to prevent dampness that might encourage mold. These improvements added to the cost of greenhouse crops, but provided a more consistent product quality.