Greenhouse, structure, built mainly of glass, for the cultivation and protection of young or delicate plants. It enables plants to be grown where the climate would normally be unsuitable, makes it possible to “force” plants so they blossom or fruit out of season, and provides suitable conditions for the raising of young plants. Not all greenhouses are heated, but even unheated houses help protect plants from frost. Proper ventilation is vital to prevent the greenhouse from becoming too hot in summer and to reduce the risk of fungus diseases that flourish in such damp atmospheres. Shading may also be necessary to keep the temperature down.