What is the optimal wavelength of light for growing plants (cacti indoors in a northern illinois climate)?

Stan Goodman sgoodman at netvision.net.il
Tue Oct 14 06:38:43 EST 1997


In message <pspopeXSPM-1310971949310001 at 165.124.30.53> -
pspopeXSPM at shopchicago.com (Scott) writes:

I am trying to accelerate the growth of my baby cacti by increasing their
light exposure with fluorescent lamps.  I am wondering what would the
optimal wavelength for promoting growth (i.e. photosynthesis).   My
question, in part, stems from interest in growth lamps. 

I have seen inexpensive (<$10) incandescent "plant lamps".  However,
fluorescent lamps produce 4x visible light than their incandescent
counterparts.  BTW, I don't want to use fluorescent plant lamps because I
do not want to use special fixtures.  Modern compact fluorescent lamps
work quite well.


Pivotal question: 

Which is better for growing cacti: incandescent plant lamps or compact
fluorescent lamps?




Fluorescent lamps are better. The light output of incandescent lamps is what
is called "black-body radiation"; at the temperatures of incandescent lamps,
this sort of spectrum will be heavy in red, infra-red, and longer wavelengths.
The spectrum of the fluorescent powders with which the inside of fluorescent
lamps is coated are more sclosely matched to the visible spectrum: this only
one reason why fluorescent lamps give more light, watt for watt than
incandescent lamps, and why they run cooler and consequently last longer.

Plants, for the most part, are uninterested in red and infra-red radiation.
They do their photosynthesis thing with the light from the sun; in order to do
this most efficiently, they are most responsive to light at the wavelengths of
the peak output from the sun, namely the middle of the visible spectrum, i.e.
green -- not coincidently the color of peak visual acuity of the human (among
others) eye, which is as efficient as it is because it, like plants, is at its
best for the wavelengths most available in sunlight. The same evolutionary
pressure has made eyes and plants most sensitive to green light, which is why
the grass is green.



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