Gardeners know intuitively that plants need light. All green plants require light to
perform photosynthesis, the process by which they manufacture their food in the form of
carbohydrates. (We’ll discuss photosynthesis in detail next week.) Some plants can
withstand quite shady conditions while others require direct sunlight, but all need at
least some light. The quantity and quality of light also trigger many plant responses,
such as flowering and dormancy. The amount of light
that reaches a plant can be measured in several ways.
1. Light
intensity is a measure of how bright the light is. Intensity diminishes as the
distance from the source of the light increases. For example, the intensity of light
reaching an object six inches from a fluorescent bulb is much lower than the intensity if
the object is only three inches away from the bulb. When the source is as powerful as the
sun, a distance of a few inches doesn’t much matter—but when you’re trying
to grow seedlings under fluorescent lights, you need to keep them close to the light
source (within an inch or two) to provide the maximum light intensity. And though the sun
might seem bright on a clear winter day, the intensity of that light is much lower than it
is on a sunny summer day.
2. Light quality
refers to the color (or wavelength) of light. Sunlight contains the full spectrum of
colors; artificial lights vary in their dominant colors. Fluorescent light is high in the
blue range of the spectrum, which is good for vegetative or leaf growth. Incandescent
light is high in the red end of the spectrum. Since red light stimulates flowering, you
might be tempted to use incandescent light bulbs for your plants. Don’t! They produce
too much heat and can burn tender plant tissue. Fluorescent "grow lights" emit a
spectrum of light that more closely mimics sunlight, and can be useful if you are growing
plants year-round under lights. If you are just starting seedlings, regular
"cool-white" fluorescents are fine.
3. Light duration
refers to the amount of time a plant is exposed to light. This is usually represented
by the number of hours of light in a 24-hour period. Indoors, light duration is important
if you are growing plants under fluorescent lights. Seedlings, for example, seem to do
best if you keep the lights on for about 16 hours per day. Outdoors, the number of hours
of sunlight depends on your latitude as well as on what season it is. At the equator, days
and nights are both about 12 hours long. In the summer, the number of hours of sunlight
increases as you head away from the equator. Summer days are longer (they have a greater
number of hours of sunlight, that is) in Vermont than they are in Texas. The opposite is
also true; in the winter, the number of hours of daylight decreases the farther you get
from the equator. (For a brief time in the summer, gardeners in parts of Alaska get to
work outdoors at any hour of the day or night because the sun never fully sets. The
opposite is also true—for a brief time in the winter, the sun never fully rises.)
Plants vary in their light needs—some require lots of
sun, some prefer shade. Understanding a plant’s light requirement is the first step
in choosing the best planting site, or the best location for a new houseplant.
Knowledgeable nursery people should be able to tell you a plant’s preferences, and
there are many resource books with that information.
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Houseplant Blues
Many new indoor gardeners get
discouraged because their new plants lose some leaves, thinking they just don't have a
green thumb.
But remember, most houseplants are
native to the tropics, and many begin their lives growing outdoors or in greenhouses.
These plants will often lose leaves as they acclimate to their new, indoor growing
conditions.
Try to come as close as possible to
the recommended growing conditions--if the label says the plant needs some direct sun,
don't put it in a dark corner! |
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