Horticulture Principles and Practices

FIGURE 12–21 Moving tables allowing for maximized
space efficiency of the greenhouse growing area.
Note the heating pipes used to provide "bottom heat" to
the seedlings grown on the tables. (Source: Dr. AJ Both, Bioresource
Engineering, Department of Plant Biology and Pathology, Rutgers
University, New Brunswick, NJ 08901)
12.3 INTERNAL ENVIRONMENTAL CONTROL
Greenhouses are controlled-environment facilities because the user is able to adjust at
least some of the plant environmental growth factors to meet specific needs. The factors
and their control are discussed in the following sections. Modern greenhouse climate
control is highly automated. Sensors are installed for measuring factors including
temperatures (of the air, water, and growth media), humidity, wind speed, wind direction,
solar radiation, carbon dioxide concentration, and rainy conditions. The information is
fed into computers with climate control software to regulate the greenhouse conditions
to suit the crop or plant cultural activity.
Greenhouse climate is controlled primarily through devices as open motorized
windows or vents, ventilations fans, circulations fans, heaters, heating circles cooling pads
and misters, thermal shades, sprayers and evaporators, and carbon dioxide generations.
12.3.1 TEMPERATURE
The daytime temperature in greenhouses is usually higher than the nighttime temperature.
Room temperature in most greenhouses is maintained at about 13 to 18.5°C
(55 to 65°F) (at night) and 18.5 to 27°C (65 to 80°F) higher during the day. Maintaining
temperature is the next most expensive operational cost after labor. The greenhouse
is essentially a giant solar collector. The sunlight energy that enters the greenhouse
during the day is trapped (greenhouse effect) and used in heating up the contents of the
facility.
How Heat Is Lost
Temperature control is perhaps the major reason for greenhouse use. Greenhouse temperature
depends on the heat balance or net energy between the greenhouse system and its
surrounding environment. The goals of heating a greenhouse are to provide heat at
the appropriate time and in the appropriate amount, distribute it effectively through the
facility, and conserve it. The ideal situation is to maintain a stable air temperature in the
greenhouse by adding heat at the same rate at which it is lost. Fuel cost is a big contributor
to high overhead in greenhouse enterprises. Therefore, heat loss should be minimized.
Heat is lost from greenhouses by three ways: conduction, infiltration, and radiation.
Conduction Conduction heat loss is the principal mode of heat loss and occurs
through the material used in framing and glazing the greenhouse. Metals conduct heat
Conducting Heat Loss
Heat loss by transmission
through a barrier such as a
greenhouse glazing
material.
12.3 Internal Environmental Control 405