Horticulture Principles and Practices

However, in winter, greenhouse vents are closed for more efficient heating. Because
plants use carbon dioxide (animals and other combustion processes produce the gas as
by-products of respiration or burning of fuel), the supply of this gas in an airtight greenhouse
is limited, which in turn decreases the rate of photosynthesis. Under such conditions,
the greenhouse environment may be enriched with supplemental carbon dioxide
by burning natural gas in an open flame or using compressed gas or dry ice (solid, frozen
carbon dioxide that is used in other laboratory procedures as well). Propane gas burners
may also be used to generate carbon dioxide. The goal of carbon dioxide fertilization
is to raise the concentration of the gas to about 1,000 to 1,500 ppm. This high level of
carbon dioxide must be provided along with bright light (e.g., bright daylight) for it to
be beneficial to plants. In addition to light, heat is a limiting factor to the effectiveness
of carbon dioxide fertilization. Lettuce plant weight increased by more than 30 percent
at 1,600 ppm of carbon dioxide. Certain crops have been known to flower earlier under
carbon dioxide fertilization. Some of the equipment used to provide additional carbon
dioxide may release undesirable toxic gases such as carbon monoxide and ethylene
during combustion of the fuel; such gases are toxic to both plants and animals.
Carbon Dioxide
Fertilization
Deliberately increasing the
carbon dioxide
concentration in the air of
the greenhouse to increase
the rate of photosynthesis.
12.4 GREENHOUSE PESTS
The use of pesticides (especially those that are volatile) in enclosed places is a potential
health hazard to humans present in those areas. Every effort should be made to minimize
the introduction of pathogens and weeds into a greenhouse. Weeds may grow in greenhouses
with gravel or porous concrete floors. Apart from being unsightly, weeds may
harbor insects and other pests. Weed seeds may be blown into the house or carried in
potting media and impure crop seed. If required, only chemicals approved for greenhouse
use (e.g., Roundup) may be used. Common greenhouse diseases include root rot,
damping-off, botrytis blight, powdery mildew, and root-knot nematodes. Disease incidence
can be reduced through strict observance of sanitation. Pots and all containers
should be sterilized before reuse. After each crop cycle, the benches should be scrubbed
and sterilized. Unsterilized media should not be allowed into the greenhouse. It is recommended
that a greenhouse operator maintain a regular schedule of preventive programs.
Since many greenhouse plants are grown largely for aesthetic uses, any blemish
reduces the price that can be obtained for a product. A wide range of pests are found in
the greenhouse, including aphids, fungus gnats, thrips, mealybugs, leaf miners, mites,
and whiteflies.
Sometimes the grower has no choice but to use pesticides. On such occasions, the
safest products and those recommended for greenhouse use must be selected. After a
spray or fog application of a pesticide, the greenhouse must be aerated before people are
allowed to work in the facility. Greenhouses may attract rodents (especially mice and
rats), depending on the activities taking place in the facility and how it is kept. Baits and
traps may be placed at strategic places to catch these rodents.
An effective approach to controlling pests in the greenhouse is the adoption of
greenhouse-specific integrated pest management (IPM) programs. Such programs entail
a combination of strategies, namely, sanitation, physical control, biological control, and
use of pesticides. It should be pointed out that IPM will work properly in a greenhouse
if the facility is used for a specific plant or crop or can be partitioned into sections for
specific activities. IPM programs may include regular fumigation, washing and disinfecting
floors, spraying, and other sanitary precautions. Plant debris should be removed
and disposed of without delay, since debris may harbor pests.
One way of keeping plants healthy is to provide adequate growth factors (nutrients,
light, temperature, water, and air). Healthy plants are more equipped to resist attacks
from pests. To prevent entry of flying insects, ventilators may be covered with fine mesh
screens. Other physical control measures include the use of yellow sticky traps. These
traps have a dual purpose—monitoring the level of insect infestation and reducing their
12.4 Greenhouse Pests 435