Horticulture Principles and Practices

Fluorescent Lights
Fluorescent lights are the most efficient of all sources and most recommended
for houseplants. They are very energy efficient, cost less than the other types to operate,
emit little heat, and can be placed close to plants without scorching them. They are available
in a variety of colors, which adds to their decorative use at home.
Fluorescent tubes are also designed to emit different qualities of light. The spectrum
of light usable by plants includes the violet-blue and red wavelength. Thus, it is
important to take note of the spectrum on the label. Daylight fluorescent tubes provide
mostly blue light and little red light. They are suitable for foliage plants. The best fluorescent
tube lights for plants are those that provide a reddish hue, especially if flowering
species are being grown. For extra light, the wide-spectrum light may be used. Extra
light is desired by plants such as orchids, cacti, and pelargoniums. This requirement may
be satisfied by using the very high output (VHO) fluorescent tubes. Cool white light,
though poor in orange-red quality, provides excellent conditions for foliage plants to
develop rich colors, branch more, and have a slow rate of stem elongation, resulting in
fuller and more attractive plants. Unlike incandescent lights, which burn out suddenly,
fluorescent lights age and lose intensity slowly. They have to be replaced after about four
months of use.
Skylight
A skylight is not a light fixture but an architectural design strategy to allow more natural
light to reach the interior of a room through the roof. For best results, the shell
covering the opening in the roof should be constructed out of a material with translucent
(not transparent) glazing. Translucent material allows the incoming solar radiation to be
better distributed over a larger area without hot spots.
Caution: Even though light is very important for plant growth, it is better to provide
too little than too much light. The danger of overexposure to light is greatest in summer.
Note that when you place a plant in a window, only one-half of it, at best, receives
full sunlight. Intense light may bleach or scorch the foliage of plants. Glass in a window
is a filter of light preventing most of the ultraviolet rays from reaching the plant. When
growing sensitive plants, one should be aware of their needs. As already indicated, variegated
plants (e.g., Hedera helix) cease to variegate but instead produce dark-green
leaves under light intensity lower than optimum. Other light-related disorders are
discussed in Chapter 4.
11.3.5 TEMPERATURE
Houseplants generally prefer temperatures of between 18 and 24°C (64.4 and 75.2°F)
for good growth and development. This condition often prevails in the average home
in temperate climates. For most foliage ornamentals, a room night temperature of
21°C (70°F) is satisfactory, whereas growth is stalled at temperatures of 15°C (59°F).
Flowering houseplants do well at 15°C (59°F) night temperatures. Even though plants
may tolerate less-than-optimum temperatures above or below (10 to 30°C [50 to
86°F]), the danger to houseplants lies in the fluctuations in temperature. Night outdoor
temperature may drop below freezing (0°C or 32°F), whereas indoor temperature
may be 28°C (82°F) or higher. A change in temperature of more than 20°C (36°F)
is detrimental to houseplants. As such, plants that are positioned close to windows or
on windowsills run the risk of exposure to drastic temperature changes (warm inside
and freezing outside) and may die as a result. Summer temperatures of 26 to 32°C
(78.8 to 89.6°F) are tolerable for most indoor plants, provided the humidity is maintained
at a high level. It is advisable to invest in a thermometer, preferably a
maximum-minimum type, which helps determine the temperature fluctuations in
the room.
11.3 Caring for Houseplants 367