Horticulture Principles and Practices

FIGURE 12–28 Electric-driven ventilation fans
used in stages to provide the required ventilation rate
necessary to maintain set point temperatures. (Source:
Dr. AJ Both, Bioresource Engineering, Department of Plant Biology and
Pathology, Rutgers University, New Brunswick, NJ 08901)
FIGURE 12–29 Evaporative cooling pads used to
cool the greenhouse when (mechanical) ventilation alone
is not sufficient to maintain the desired set point temperature.
The (wetted) pads are installed inside the ventilation
inlet opening and outside air is drawn through them
by ventilation fans installed in the opposite sidewall (not
visible). Evaporative cooling is only successful when the
relative humidity of the outside air is below saturation
(100%). The drier the incoming air, the larger the
temperature drop accomplished. Note that evaporative
cooling increases the humidity of the greenhouse air.
(Source: Dr. AJ Both, Bioresource Engineering, Department of Plant Biology
and Pathology, Rutgers University, New Brunswick, NJ 08901).
Intensity
On a clear summer day, plants in a greenhouse may receive as many as 12,000 footcandles
(or 129 kilolux) of light, an excessive amount for most crops, since by 3,000 footcandles
(32.3 kilolux), most plant leaves in the direct path of incoming light are light
saturated and cannot increase their photosynthetic rate. Whole plants can utilize about
10,000 foot-candles (108 kilolux). Sunlight intensity is also seasonal, being less in winter
and more in summer. Crops have different preferences and tolerances for light intensity.
For example, foliage (nonflowering) plants are scorched and fade in light intensities
above 2,000 to 3,000 foot-candles (21.5 to 32.3 kilolux). African violets lose green color
at an even lower light intensity (1,500 foot-candles). Poinsettia plants are a darker shade
of green when light is reduced. Also, plants such as geranium and chrysanthemum require
shading in cultivation to prevent petal burn. Shading to reduce light by about 40 percent
from midspring to midfall is helpful to prevent chloroplast suppression in most greenhouse
plants. Greenhouse light intensity can be manipulated in several ways.
Shading In addition to fans and evaporative cooling systems, there are certain times
during the year (early fall planting season and late spring harvesting period) when
excessive heat buildup in detrimental to plant growth, fruit development, and ripening.
Some modern greenhouses have retractable shade cloths that automatically deploy when
radiation load on the greenhouse exceeds what the cooling systems can handle
(Figure 12–30). In some designs the shade system is fixed (Figure 12–31). Shading the
greenhouse to reduce excessive light is accomplished in several ways. Glass greenhouses
may be sprayed with paint (whitewash) in summer to reflect light and reduce its intensity.
Commercial shading paints may be purchased. A screen made from fabric may be
more convenient to use if only a section of the greenhouse requires shading. Durable
fabrics include polyester and polypropylene; they are available in different densities of
424 Chapter 12 Controlled-Environment Horticulture