A greenhouse is a large structure, with the walls and roof most often built entirely of glass or plastic. It is a house for plants, which is often filled with equipment like screening installations, heating, cooling, lighting, etc., that helps to maintain a controlled environment perfect for plants. Greenhouses are used for growing all kinds of plants, including flowers, fruits, and vegetables. These structures are often used to grow saplings in the late winter and early spring, which are later planted in the open in warmer weather. Pollination of the plants is done naturally by using bees, or artificial pollination may be done, depending on the requirements. Greenhouses also protect plants from weather phenomena such as blizzards or dust storms.
Have you ever wondered how do greenhouses work? The widely spread (and believed) explanation is based on the varying transparency of glass to solar and thermal infrared radiation. This misconception explains that solar radiation enters the greenhouse through the glass, and heats up the interior, which is true. It then says that all the heated surfaces emit longer wavelengths that are unable to pass through the glass, and hence, this radiation stays within the greenhouse. Consequently, this is how the greenhouse retains heat. However, if this were true, why would polyethylene greenhouses also retain heat, for polyethylene is almost as transparent to thermal infrared radiation, as it is to solar radiation.
How Greenhouses Works
The correct explanation to the above question is that the working starts with the heat carrying rays of the sun, which enter the walls and/or roof of the greenhouse, and heat up the interior portions. Plants generally grow well in a humid and temperate heat associated environment, as well as conditions that provide them with a good water source. The sun’s rays are in the form of short range infrared waves, which get converted to long range infrared waves. The temperature of the ground rises as it absorbs this radiation. The heat is also absorbed by the plants, which helps in the process of photosynthesis. The heat from the ground is transmitted to the layer of air next to it, which expands and becomes lighter than the air above it. This heated lighter air rises, and is instantly replaced by cooler denser air. This convection cycle continues throughout the day. The roof and the walls of the greenhouse retain this heated air, and thus, the air in the greenhouse stays warm all day long. In an open space, the heating of air is spread over a large mass, and is diluted. In the nights, the temperature in a greenhouse stays warmer than the air outside, as heat stored during the day is available throughout the night.
Many conditions need to be monitored and maintained, in order to grow and sustain different types of plants in a greenhouse. It must have a good ventilation system, to avoid overheating. The plants need to be watered, and periodically checked for pests or disease. Special care may be required during cold winters seasons, to preserve plant health, especially for warm-weathered plants. One may need to make arrangements to provide additional heat and light to the plants. But it also allows for the cultivation and production of plants, which would not naturally grow during the winters. They can also ensure a year-round supply of some staple vegetables like lettuce.
In addition to these advantages, greenhouses play a very important role in providing a constant food supply to countries at high latitudes. The greenhouse complexes in Almeria, Spain are one such example. They are among the largest of its kind in the world, covering almost 50,000 acres of land.