HORTICULTURE

Light Quality

Once acclimatized to the reduced light intensity of the interior, the plantscape may still prove unsatisfactory if the light quality is incorrect. Light quality is the color of light emitted by a particular source. The sun emits all colors of light, some of which the human eye can perceive and others that are imperceptible to humans but beneficial to plants. The green-yellow light most comfortable for humans is of little use in pho­tosynthesis by plants. They depend on light from the blue and red bands of the visible light spectrum. Visible light is only a narrow region of the radiant light spectrum (Figure 8-1). The unit of measurement for light wavelengths is the nanometer.

As long as both humans and plants can derive their light energy from the sun, the needs of each are satisfied. Indoors, however, where light energy is usually created by artificial means, the quality of the light can vary considerably. Light preferred by an interior plant specialist con­cerned with the health of plantings may cause the skin tones of human beings to appear ashen and deathly. In similar fashion, an interior deco­rator may specify a quality of lighting that gives the human complexion a healthy glow while making nearby plantings appear brown and dead. Clearly, someone seeking a career in interior plantscaping needs to know the types of lamps currently available and the quality of light they provide. Categories and examples of lamps that have some use in inte­rior plant illumination are shown in Table 8-1. Table 8-2 compares the lamps in all areas important to interior plant survival. You should study these tables thoroughly before proceeding further.

Selection of the proper lamp for the illumination of an interior planting will depend on the answers to several questions:

Visible light spectrum
(Wavelength in millimicrons)

figure 8-1. Electromagnetic spectrum and spectral distribution of visible light (Delmar/Cengage Learning)

Lamps for Interior Plant Illumination

Tungsten Filament Incandescent Lamps

• standard (the familiar household lightbulb)

• reflector (spot or flood lights)

• parabolic aluminized reflector (a weather-resistant type of floodlight with a more precise beam)

• incandescent plant lamps (not proven to be any better than the standard incandescent)

Fluorescent Lamps

• cool white

• warm white

• plant lamps

• wide spectrum plant lamps High-Intensity Discharge Lamps

• mercury

• metal halide

• high-pressure sodium

• How extensive is the planting?

• Are the plants to be encouraged to grow or merely to be maintained at their current size?

• Will the plants receive any sunlight? If so, how much and for how long?

• How far will the artificial light source be from the plants?

• What types of lamps are being used for general lighting of the area and what is the intensity of surface illumination provided?

For example, consider the plantscape of a typical office. The plants may be permanently located or in movable planters. Their functions may be to serve as room dividers, establish a mood, or relieve a cluttered desk top. The ceiling may be 8 or 10 feet high. Side windows or a skylight may admit some natural light. In such a setting, cool white fluorescent lighting would be ideal for both general lighting and the growth of the plants. People, plants, and furnishings look natural beneath cool white light due to its excellent color rendition, and the plants receive the right quality of light for photosynthesis. If additional task lighting is needed, small desk lights should be used. Special effects such as shadows or textural highlights can be created with incandescent lights installed beneath the plants and directed upward. (These are called uplights.) Some benefit will accrue to the plants from the addition of lighting at the base. However, if supplemental lighting is needed for photosynthe­sis, it is most efficient when applied from overhead because chloroplasts are concentrated in the upper leaf surface.

A Comparison of Artificial Lighting Sources for Interior Plantscapes

(Continued)

A shopping mall presents different problems. The corridors may have ceilings too high to permit the use of fluorescent lamps, which are not good for illumination when the ceiling is much beyond 10 feet. There may be skylights as well as decorative architectural lighting. Overall illumination by mercury or metal halide lamps would be best. As in an office, supplemental or decorative lighting might also be desirable for special effects or the health of the plants. When uplights are used, they should be installed directly into the planters and waterproofed. When supplemental lights are added for overhead illumination, they should be positioned to light the plants fully without shining in the eyes of viewers.