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A polytunnel (also known as a polyhouse, hoop greenhouse or hoophouse, or high tunnel) is a tunnel made of polyethylene, usually semi-circular, square or elongated in shape. The interior heats up because incoming solar radiation from the sun warms plants, soil, and other things inside the building faster than heat can escape the structure. Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall. Temperature, humidity and ventilation can be controlled by equipment fixed in the polytunnel or by manual opening and closing of flaps. Polytunnels are mainly used in temperate regions in similar ways to glass greenhouses and row covers. Besides the passive solar heating that every polytunnel provides, every variation of auxiliary heating (from hothouse heating through minimal heating to unheated houses) is represented in current practice. The nesting of row covers and low tunnels inside high tunnels is also common.
Polytunnels can be used to provide a higher temperature and/or humidity than that which is available in the environment but can also protect crops from intense heat, bright sunlight, winds, hailstones, and cold waves. This allows fruits and vegetables to be grown at times usually considered off season; market gardeners commonly use polytunnels for season extension. Beyond season extension, polytunnels are also used to allow cold-hardy crops to overwinter in regions where their hardiness isn't quite strong enough for them to survive outdoors. Temperature increases of only 5° to 15° above outdoor ambient, coupled with protection from the drying effect of wind, are enough to let selected plant varieties grow slowly but healthily instead of dying. The effect is to simulate "moving the farm south by several hardiness zones", that is, to create a microclimate that simulates the temperatures of a location several hardiness zones south (and protects from wind as well).
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