spectral light
Visible Spectrum
Indoor growers traditionally use metal halide (MH) lamps in
the vegetative stage and high pressure sodium lamps (HPS),
or a mix of both, in the flowering stage. Metal halide lamps
have more blue light, while HPS lamps have more red light.
Many light emitting diode (LED) lamps allow you to change the
color temperature as needed throughout the vegetative and
flowering cycles. There are also MH and fluorescent “daylight”
lamps that produce a white light closer to that of the sun.
Some growers swear by using HPS lamps alone throughout
the entire cycle, which works but will often grow taller plants
with greater internodal spacing and produce a smaller harvest.
Although a plant can grow under red lights alone for its entire
life cycle, indoor growers usually prefer shorter, bushier plants
due to limited vertical space, and have found that using a fuller
spectrum of light will produce a higher yield come harvest.
“IT WAS FOUND that the dry weights, crispness,
sweetness, and shape of plants treated with the
red-blue-white LED and fluorescent lamp were higher
than in plants treated with only red-blue LEDs.”
STUDIES IN LIGHT
In a 1997 study published in the Oxford Journal of Experimental
Botany, wheat was grown under red LEDs with and without
supplemental blue lighting, and was then compared to wheat
grown under white daylight fluorescent lamps. The study found
that wheat grown under red lights alone demonstrated less
development during vegetative growth, grew longer stems
and leaves, and yielded fewer seeds than wheat grown under
white light. Wheat grown under red LEDs with supplemental
blue fluorescent lighting produced similar growth and yield
to wheat grown under white light. The study concluded that
wheat can, in fact, complete its life cycle under red LEDs alone;
however, larger plants and greater seed yields are produced
with the fuller spectrum of light.
A 1987 study published in Scientia Horticulturae studied the
effects of blue, natural, green, yellow, and red light in chry-
santhemums, tomatoes, and lettuce. It found that blue light
reduced the dry weight, height, and leaf area in the plants
compared to other light spectrums. Green and yellow light
enhanced the leaf area of tomatoes compared to natural light.
They also found that blue light produced darker green leaves
than natural light, whereas green and yellow light produced
light green leaves.
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