Greenhouses protect crops from the elements and allow for longer growing periods, but can be expensive. Early greenhouses were attached to houses and used the sun for light and heat. Break-resistant plastic greenhouses were developed in the 20th century. Commercial greenhouse production became more affordable in the late 20th century, and water conservation techniques were developed. Environmental controls can be used to maintain consistent product quality.
Greenhouse crops are fruits and vegetables grown inside a building or enclosure to protect them from the elements and allow for a longer growing period. Growing plants in greenhouses can be more expensive than planting in the field, but in some areas it may be a necessity due to water scarcity or environmental conditions. A greenhouse can be a permanent structure with glass or plastic windows for light, or temporary structures made with plastic frames and films.
Early greenhouses were wood or metal frame buildings with glass windows, often attached to a house. The sun provided both the light and heat needed to grow vegetables or flowers in a more controlled environment, or when outside temperatures dipped below freezing. Seeds may have germinated in early spring and some vegetables may be grown in late autumn, particularly if an additional heat source such as a wood stove or steam radiators has been provided.
The cost of glass and its ability to break led to a gradual development of break-resistant plastic greenhouses in the 20th century. Plastics were much lighter, could be made with pigments added to block some of the sunlight, making them translucent, and could be molded into shapes to allow for curved windows or domes. These structures were often permanent buildings used for specialty vegetables, herbs, or flowers, and were built on a smaller scale for residential or small-scale commercial growers.
Commercial greenhouse crops were not considered affordable until the late 20th century. Demands for agricultural land for residential development and rising fuel costs for transporting fruits and vegetables have made smaller greenhouse production more effective. A growing interest in organic produce, or those products grown without artificial fertilizers and pesticides, has provided a customer base willing to pay more for higher priced greenhouse crops.
Large-scale commercial greenhouses began using large metal frame structures and plastic films, which were sometimes built directly over 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 they were used in areas where adequate sunlight could provide radiant heat to extend the growing season.
Water conservation also became a growing concern, as potable water supplies were more limited. Greenhouse crops can be grown using water conservation techniques such as drip or mist irrigation, which provides an optimal amount of water needed for plant growth. The controlled temperatures and humidity possible in a greenhouse allow for much less water to be used per crop than is required in the fields.
Greenhouse environmental control can be done with controls related to temperature and humidity. If temperatures exceed the desired limits, windows or skylights can be opened manually or controlled by electric motors. Humidity controls could provide the same function to prevent dampness that could promote mold growth. These improvements added to the cost of greenhouse crops, but provided more consistent product quality.
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