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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
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
YL Y OF MA
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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- Spring '08