Resource Book on Horticulture Nursery Management

temperature maintained during the day and the night. The difference in temperatures is
usually 5 to 6 0 Celsius.
Each kind of plant possesses a specific temperature range most suitable for its
development. However, for greenhouse crops there is a general division of cool season
crops and warm season crops.
Seed germination responds to air temperature. Some varieties of lettuce refuse to
germinate properly when the air temperature reaches 25 to 30 0 Celsius or above. Other
seeds like okra, pea and lima beans, germinate, but are easily subject to rotting during the
germination stages at these temperatures.
Sunlight: All plants require adequate light for growth but they often require shade to
protect them from scorching. Most plants appear to require at least 1000 Footcandles of
light to support growth. Higher levels improve growth up to 6000 to 10,000 Footcandles.
Most vegetable and floral crops benefit by some shade.
Short days along with much cloudy weather in some areas cause light to be a
limiting factor in greenhouse crop production during the winter, but in summer both the
day length and the light intensity provide abundant light. In fact, excess light becomes a
limiting factor in such cases. Coupled with high air temperature, high light intensities will
adversely affect the growth of some plants; thus it is a common practice to whitewash the
glass of greenhouses in the summer. Some outdoor crops are grown under tobacco cloth,
cheesecloth or coarse muslin shade to obtain more satisfactory growth. A wooden lattice
is installed for this purpose.
Carbon Dioxide: The atmosphere contains about 0.03 per cent carbon dioxide and
is the source of this essential compound for plant growth. Usually carbon dioxide is not a
limiting factor in crop production. Under conditions of bright sunlight the plant may be
able to efficiently use greater concentration of carbon dioxide than normal atmospheric
concentrations.
Relative Humidity: This factor plays an important part in the general vegetative
responses of the plant and its susceptibility to diseases. Low relative humidity of the
atmosphere is conducive to a hard type of growth, while high relative humidity causes a
soft type, if all other cultural factors are similar. Also, the moisture content of the air
directly affects the transpiration rate of the plant. Water losses from the plant are much
higher when the relative humidity of the atmosphere is low.
The maintenance of adequate humidity conditions in the greenhouse is a major
practical problem. Many techniques are used to raise the humidity. Walk ways and areas
under raised benches are kept wet. Plants are often lightly sprinkled with a fine stream of
water. In the winter steam injections appear to be feasible. This will influence both the air
temperature and the moisture content. Light shade on the glass in the summer will also
aid in raising the greenhouse humidity by lowering the air temperature.
Wind: Wind is not a direct factor in greenhouse crop production. Various fabrics
tobacco cloth, cheesecloth and coarse muslin, are easily damaged by the strong wind.
Sometimes it is necessary to use less desirable types of shade because of wind velocity.
For best results, windbreaks should be constructed to reduce the velocity of the wind.
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