Greenhouse
A greenhouse is a structure where plants are grown. The primary heating mechanism of a greenhouse is convection. Greenhouses create a sheltered environment for plants by using solar radiation to trap heat. This system of heating and circulating air helps to create an artificial environment in a greenhouse that can sustain plants when the outdoor temperature is too cool or variable. Heat enters the greenhouse through its covering of glass or plastic and starts to warm the objects, soil and plants inside. The warmed air near the soil begins to rise and is immediately replaced with cooler surrounding air that starts to heat up. This cycle raises the temperature inside the greenhouse more rapidly than the air outside, creating a sheltered, warmer microclimate.
In temperate climates, greenhouses extend the growing season and protect plants from harsh weather conditions. In higher latitudes, greenhouses increase plant production by optimizing the available light. Even in hot, arid regions, specialized greenhouses have been developed that can help lower temperatures and manage water loss in plants due to transpiration. It protect crops from too much heat or cold, shield plants from dust storms and blizzards, and help to keep out pests.
Greenhouses are often used for growing flowers, vegetables, fruits, and tobacco plants. Bumblebees are the pollinators of choice for most greenhouse pollination, although other types of bees have been used, as well as artificial pollination. Hydroponics can be used in greenhouses as well to make the most use of the interior space.
Greenhouse technology is making it easier to study the potential value of medicinal plants and explore ways to increase plant yields and make plants more disease resistant. Greenhouses are even being used to preserve plant species whose natural habitat is threatened.
Contents |
[edit] Greenhouse Types
- Cold - provide protection for plants and vegetation, but the temperatures can still drop below freezing during the winter because of the absence of heat source. Cold houses can help start spring crops a few weeks early and extend the growing season in fall, but they're limited by the weather.
- Cool - relatively warmer than cold houses, cool houses keep plants above freezing and in a temperature range of between 7 to 10 degrees Celsius. Keeping the temperature above freezing will protect frost sensitive plants, like geraniums and hibiscus.
- Warm - allows a broader range of plants, but requires slightly warmer temperatures too, 13 degrees Celsius. Although the temperature range doesn't support many tropical plants, some varieties of orchids and ferns can over-winter in a warm house environment.
- Hot - designed to house tropical plants like caladium, dieffenbachia and gardenia, which need a temperature range 15.5 degrees Celsius and higher. They require the most supplemental heat and insulation and can cost more.
[edit] Heating
The heating requirements of a greenhouse depend on the desired temperature for the plants grown, the location and construction of the greenhouse, and the total outside exposed area of the structure. As much as 25 percent of the daily heat requirement may come from the sun, but a lightly insulated greenhouse structure will need a great deal of heat on a cold winter night. The heating system must be adequate to maintain the desired day or night temperature.
Usually the home heating system is not adequate to heat an adjacent greenhouse. A 220-volt circuit electric heater, however, is clean, efficient, and works well. Small gas or oil heaters designed to be installed through a masonry wall also work well.
Solar-heater greenhouses were popular briefly during the energy crisis, but they did not prove to be economical to use. Separate solar collection and storage systems are large and require much space. However, greenhouse owners can experiment with heat-collecting methods to reduce fossil-fuel consumption. One method is to paint containers black to attract heat, and fill them with water to retain it. However, because the greenhouse air temperature must be kept at plant-growing temperatures, the greenhouse itself is not a good solar-heat collector.
Heating systems can be fueled by electricity, gas, oil, or wood. The heat can be distributed by forced hot air, radiant heat, hot water, or steam. The choice of a heating system and fuel depends on what is locally available, the production requirements of the plants, cost, and individual choice. For safety purposes, and to prevent harmful gases from contacting plants, all gas, oil, and woodburning systems must be properly vented to the outside. Use fresh-air vents to supply oxygen for burners for complete combustion. Safety controls, such as safety pilots and a gas shutoff switch, should be used as required. Portable kerosene heaters used in homes are risky because some plants are sensitive to gases formed when the fuel is burned.
[edit] Ventilation
Ventilation is the exchange of inside air for outside air to control temperature, remove moisture, or replenish carbon dioxide (CO2). Several ventilation systems can be used. Be careful when mixing parts of two systems.
Natural ventilation uses roof vents on the ridge line with side inlet vents (louvers). Warm air rises on convective currents to escape through the top, drawing cool air in through the sides.
Mechanical ventilation uses an exhaust fan to move air out one end of the greenhouse while outside air enters the other end through motorized inlet louvers. Exhaust fans should be sized to exchange the total volume of air in the greenhouse each minute.
[edit] Cooling
Air movement by ventilation alone may not be adequate during summer; the air temperature may need to be lowered with the use evaporative cooling system. Also, the light intensity may be too great for the plants. During the summer, evaporative cooling, shade cloth, or paint may be necessary. Shade materials include roll-up screens of wood or aluminum, vinyl netting, and paint.
