Greenhouses are transparent glass or plastic made houses used in cold climate countries to preserve plants in a relatively warm state. The glass walls of green houses allows the sunlight to come in rising the temperature, but when it radiates as heat it is not allowed to go out; as a result the inside of a green house stays warm and humid. Greenhouses are designed to control the balance of temperature, moisture, and light to suit the growth requirements for plants as diverse as orchids, cacti, tomatoes, and citrus trees.
Small domestic greenhouses are frequently lean-to structures built against an existing wall. They consist of a glass-paned sloping roof and three supporting glass-paned sides. Large, conventional commercial greenhouses have A-shaped roofs over two sidewalls and two end walls. Another extensively used commercial greenhouse is the ridge-and-furrow house, which consists of several units placed side by side with no partitioning between them. A third and low-cost greenhouse consists of a Quonset-shaped frame with one or two layers of plastic film stretched over it; the double layer can save up to 40 percent in heating fuel.
Natural sunlight, in temperate regions, usually is sufficient to meet the light requirements of plants in spring and fall, but in winter it is sometimes supplemented by artificial light to spur growth. In summer, the panes often are covered with slat shades or whitewashed to reduce light. Much of the heat during the winter is derived from the sun. Supplemental heat comes from steam, hot-water, or hot-air circulating systems. During the hotter months, heat is reduced by whitewashing the panes, opening ventilators, or blowing air through with fans.
Humidity is controlled primarily by the quantity of water used in the soils. When greater humidity is required, extra moisture is supplied by watering the floors. Pot-grown plants, such as orchids, are frequently humidified by setting them on racks above moistened cinders.
Green houses are useful tools for botany and agriculture but Greenhouse effect is a natural phenomenon causing global warming. We hope there will be more greenhouses and less greenhouse effect in the world in not so distant future.