Horticulture Principles and Practices

FIGURE 12–32 A machine used to clean the roof glass of
a Venlo-type greenhouse. Over time, dust and dirt accumulation
can significantly reduce light transmission into the greenhouse.
(Source: Dr. AJ Both, Bioresource Engineering, Department of Plant
Biology and Pathology, Rutgers University, New Brunswick, NJ 08901)
Incandescent Lamp Incandescent lamps have tungsten filaments. They are not desirable
for greenhouse use because, in addition to light, they generate excessive heat. Incandescent
lamps characteristically emit a high proportion of red and far red light, which produces
abnormal growth in some plants (e.g., soft growth or induced tallness). They are very
inefficient, converting less than 10 percent of the electrical energy consumed into light.
Fluorescent Lamps Fluorescent lamps are energy efficient (20 percent electrical
energy converted to light) and produce little heat. However, because they are low-power
sources, many of them are often required to produce the desired intensity. Consequently,
a large number of fixtures must be installed. These installations end up blocking incoming
natural light. They are commonly used in growth rooms where seeds are germinated
or in small greenhouses. The cool white model, which provides light of predominantly
blue wavelength, is commonly used. However, fluorescent tubes with capacities to emit
a superior quality of light for photosynthesis are available. One class, called plant growth
A, produces light in the red region of the light spectrum; another class of tubes, plant
growth B, has capabilities for emitting radiation beyond the 700-nanometer wavelength.
High-Intensity-Discharge Lamps As their name indicates, high-intensitydischarge
lamps can generate intense light (Figure 12–33). There are several types and
designs available: high-pressure mercury discharge, high-pressure metal halide, and
high-pressure sodium light. In certain types, power ratings may be as high as 2,000 watts.
The high-pressure sodium lamps are less expensive than the others to install and operate
and can generate up to 1,000 watts of power for 24,000 hours. The light quality emitted
is in the 700- to 800-nanometer spectral range. Low-pressure versions of the highpressure
sodium lamps are available. Considered the most efficient lamps for supplemental
greenhouse lighting (27 percent electrical energy converted to light), they are
available in power ratings of up to the popular 180-watt size. However, these lamps are
deficient in quality of light produced and do not emit enough light in the 700- to
850-nanometer range. Therefore, when used alone, plants often develop abnormally
(e.g., pale foliage in lettuce and petunia). When used in combination with daylight or
426 Chapter 12 Controlled-Environment Horticulture