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

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