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enclosed glass house used for growing plants in regulated temperatures, humidity, and ventilation. A greenhouse can range from a small room carrying a few plants over the winter, to an immense heated glass building called a hothouse or conservatory, covering acres of
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a special structure used for the year-round cultivation of plants and seedlings; the roof is made of a translucent material. In middle and northern latitudes hothouses are also used for preserving and propagating thermophile plants, especially those from tropical and subtropical zones. In plant selection hothouses are used to shorten the time needed to develop new varieties and hybrids of agricultural crops; two or three generations of seeds are obtained annually instead of one. Scientific institutions use hothouses for various biological investigations. In 1913 only 4 hectares (ha) were occupied by hothouses in what is now the USSR; in 1974 hothouses occupied about 4,700 ha.
In some hothouses the plants are grown in nutrient soil on the floor. In others the plants are cultivated on racks, that is, trough-shaped wooden shelves filled with soil. The substrate may be a fertile soil mixture or an artificial medium (seeHYDROPONICS). On the average about 0.25 cu m of soil is required per sq m of hothouse.
Hothouses that can be used throughout the year (winter hothouses) are glassed enclosures. Those that are operative only in spring, summer, and part of the autumn (spring hothouses) have either glass or plastic coverings. Hothouses may be single-unit or multiple-unit and may be single-slope, double-slope, or multiple-slope. In a single-slope hothouse the glass surface of the roof faces south at an angle of 33°–45°; this type of hothouse is of limited use. In double-slope hothouses the translucent surfaces are usually directed toward the east and west at an angle of 29°–33° in large single-unit winter hothouses or at an angle of 20°–22° in spring hothouses. Most common are multiple-unit structures consisting of adjoining double-slope units, which, instead of interior walls and partitions, rest on posts. The roof of a multiple-slope hothouse consists of four or more surfaces directed toward the east and west.
A hothouse may have a framework. The framework may be sash, prop-and-beam, arched, vaulted (domical), guyed (suspended by cable), or a combination of any of the above systems. Frameless structures are paneled, inflated, or both paneled and inflated.
Hothouses should be placed on level land or land having a slight southerly slope. Terrain with the water table higher than 0.8 m to the soil surface is not suitable. The site should be protected from prevailing winds by shelterbelts of fast-growing trees or by a fence. To maintain the translucence of the roof a hothouse should be located at a considerable distance from sources of air pollution. The site must also provide for good routes of approach.
The principal parts of a hothouse are the foundation, the framework (walls and posts), and the roof. The foundation is made of stone or a reinforced-concrete slab. The roof and the upper parts of the side and end walls are made of glass; the lower parts of the walls are made of reinforced concrete, brick, or stone. Winter hothouses have a metal, wooden, or—less frequently—reinforced-concrete framework. The ventilation may be natural or forced. Natural ventilation is provided by a small window or transom in the glass roof. Hothouses for raising seedlings are equipped with reinforced-concrete or wooden racks. In lightweight stationary spring hothouses reinforced-concrete piers serve as the foundation, and the framework is constructed of wood and of metal and plastic pipes.
The heating source may be solar, biological (from biological fuel), or technical (from hot water, steam, electricity, or thermal industrial wastes). In Kamchatka, the Northern Caucasus, and Transcaucasia heat is provided by underground thermal springs. Biological fuel is used principally to heat spring hothouses with plastic coverings. The most common system of heating is hot water; heating generators and radiators are also used. Heating and ventilating units are used to heat spring hothouses with plastic coverings.
The availability of technical equipment has resulted in high labor productivity. Hothouses are equipped with a mechanized or automated system for regulating the microclimate. Hydroponic hothouses are also equipped with a complex set of machines and devices for maintaining the plant nutrient regimen. Hothouses used for experimental selective breeding have a more complex system of automation for regulating heat, light, and other regimens. Such hothouses are similar to phytotrons, that is, chambers in which the temperature, air humidity, and illumination are strictly maintained according to a prescribed program. Excavators, dump trucks, loaders, mixers, conveyors, and other machines are used for laborious earthwork and the preparation of biological fuel. Crop rotation is practiced for economical use of space.
Hothouses are constructed according to standard designs ereated by the All-Union State Institute for Agricultural Design and other design organizations.
REFERENCESNormy tekhnologicheskogo proektirovaniia teplits i teplichnykh kombinatov dlia vyrashchivaniia ovoshchei i rassady. Orel, 1971.
Spravochnik po ovoshchevodstvu. Leningrad, 1971.
Ovoshchevodstvo zashchishchennogo grunta. Moscow, 1974.
G. V. BOOS