Horticulture Principles and Practices

FIGURE 12–30 Movable internal shade curtain.
Note that curtains can be mounted in a variety of
greenhouse designs, including designs with a fair
amount of overhead installations (heating and irrigation
pipes, supplemental lighting, etc.). The curtain
fabric is selected based on the two functions it is to
perform: reflect sunlight (during the summer and
particularly the middle of the day) and heat (at night
and during the winter months). (Source: Dr. AJ Both,
Bioresource Engineering, Department of Plant Biology and Pathology,
Rutgers University, New Brunswick, NJ 08901)
weave for providing degrees of shading from 20 to 90 percent, 50 percent being the most
common. Some modern greenhouses have retractable shades that are used as needed.
Saran may be used to cover plants in sections of a greenhouse or bench. Also, aluminized
strips may be used to create an interior ceiling that can be drawn on sunny days.
Undesirable shading may be inherent in the design of the greenhouse. Materials for
construction and design of the frame, cooling systems, and plumbing and light fixtures
that are located overhead intercept incoming light. The narrower the frame material, the
better. Overall, these obstructions may reduce greenhouse light intensity by 30 percent
or more. Further, seasonal and daily light intensity reductions may be caused by the orientation
of the greenhouse and the characteristics of the location.
Plant Density Plants may also be spaced to utilize incoming light. In summer, closer
spacing should be used, whereas wider spacing is recommended in winter. Spacing–light
intensity interaction can be manipulated to control the size of plants, since the same
amount of dry matter will be produced under an available light intensity. Therefore, to
obtain larger plants, space widely; to obtain smaller plants, space closely.
Washing To increase light intensity, the glass covering of the greenhouse should be
cleaned (Figure 12–32). Dust particles on the glass plates reduce intensity by up to
about 20 percent. Washing by running water through a hose may not be sufficient. A
cleaning solution should be used (e.g., 11 pounds [5.0 kilograms] of oxalic acid dissolved
in 33 gallons [150.2 liters] of water may be sprayed onto a damp, dirty greenhouse
and hosed down with water after three days). Other greenhouse types also need
periodic cleaning.
Supplemental Lighting
In winter or darker periods (middle of fall to early spring season), supplemental lighting
may be required by certain plants for good growth and quality. When needed, it may be
supplied by using a variety of sources, categorized as follows.
FIGURE 12–31 Fixed external
shade curtain. Compared to
internal shade curtains, these systems
reduce the amount of heat
entering the greenhouse (requiring
removal by the ventilation
and/or cooling system). External
shade curtains are particularly
practical for crops requiring low
light levels. (Source: Dr. AJ Both,
Bioresource Engineering, Department of
Plant Biology and Pathology, Rutgers
University, New Brunswick, NJ 08901)
12.3 Internal Environmental Control 425