Greenhouses. Planning A Home Greenhouse

Small greenhouses need the larger amount in winter 20

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Unformatted text preview: house will be used. In summer, 1 to 1 1/2 air volume changes per minute are needed. Small greenhouses need the larger amount. In winter, 20 to 30 percent of one air volume exchange per minute is sufficient for mixing in cool air without chilling the plants. One single-speed fan cannot meet this criteria. Two single-speed fans are better. A combination of a singlespeed fan and a two-speed fan allows three ventilation rates that best satisfy year round needs. A single-stage Cooling Air movement by ventilation alone may not be adequate in the middle of the summer; the air temperature may need to be lowered with evaporative cooling. 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 rollup screens of wood or aluminum, vinyl netting, and paint. Small package evaporative coolers have a fan and evaporative pad in one box to evaporate water, which cools air and increases humidity. Heat is removed from the air to change water from liquid to a vapor. Moist, cooler air enters the greenhouse while heated air passes out through roof vents or exhaust louvers. The evaporative cooler works best when the humidity of the outside air is low. The system can be used without water evaporation to provide the ventilation of the greenhouse. Size the evaporative cooler capacity at 1.0 to 1.5 times the volume of the greenhouse. An alternative system, used in commercial greenhouses, places the pads on the air inlets at one end of the greenhouse and uses the exhaust fans at the other end of the greenhouse to pull the air through the house. Controllers/Automation Automatic control is essential to maintain a reasonable environment in the greenhouse. On a winter day with varying amounts of sunlight and clouds, the tem- 7 UNIV ER A ST E K R E G E PA E LL UNIVERSITY OF MARYLAND AT COLLEGE PARK UNIVERSITY OF MARYLAND EASTERN SHORE Y OF MA R SIT AND YL AND YL Y OF MA R SIT O C small greenhouse is likely to have a variety of plant materials, containers, and soil mixes that need different amounts of water. Time clocks or mechanical evaporation sensors can be used to control automatic watering systems. Mist sprays can be used to create humidity or to moisten seedlings. Watering kits can be obtained to water plants in flats, benches, or pots. UNIV ER COOPERATIVE EXTENSION SERVICE ER N S H O R Planning a Home Greenhouse Fact Sheet 645 CO2 and Light Carbon dioxide (CO2) and light are essential for plant growth. As the sun rises in the morning to provide light, the plants begin to produce food energy (photosynthesis). The level of CO2 drops in the greenhouse as it is used by the plants. Ventilation replenishes the CO2 in the greenhouse. Because CO2 and light complement each other, electric lighting combined with CO2 injection are used to increase yields of vegetable and flowering crops. Bottled CO2, dry ice, and combustion of sulfur-free fuels can be used as CO2 sources. Commercial greenhouses use such methods. Figure 5. Thermostats in the middle of the greenhouse in a shaded, white, and aspirated box perature can fluctuate greatly; close supervision would be required if a manual ventilation system were in use. Therefore, unless close monitoring is possible, both hobbyists and commercial operators should have automated systems with thermostats or other sensors. Thermostats can be used to control individual units, or a central controller with one temperature sensor can be used. In either case, the sensor or sensors should be shaded from the sun, located about plant height away from the sidewalls, and have constant airflow over them. An aspirated box is suggested; the box houses each sensor and has a small fan that moves greenhouse air through the box and over the sensor (Figure 5). The box should be painted white so it will reflect solar heat and allow accurate readings of the air temperature. Alternative Growing Structures A greenhouse is not always needed for growing plants. Plants can be germinated in one’s home in a warm place under fluorescent lamps. The lamps must be close together and not far above the plants. A cold frame or hotbed can be used outdoors to continue the growth of young seedlings until the weather allows planting in a garden. A hotbed is similar to the cold frame, but it has a source of heat to maintain proper temperatures. For More Information Watering Systems For more information on environmental control systems, refer to Extension Bulletin 351 Greenhouse Heating, Circulation, and Ventilation Systems. For further discussion of hotbeds and cold frames, see Fact Sheet 633 Hotbeds and Cold Frames for Starting Annual Plants, also available from your county Cooperative Extension Service office. A water supply is essential. Hand watering is acceptable for most greenhouse crops if someone is available when the task needs to be done; however, many hobbyists work away from home during the day. A variety of automatic watering systems is avail...
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