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

Jim Ivey jivey at nospam.marine.usf.edu
Tue Oct 14 13:56:52 EST 1997

Stan Goodman (Stan Goodman) wrote:
 In message <pspopeXSPM-1310971949310001 at> -
 pspopeXSPM at shopchicago.com (Scott) writes:
 I am trying to accelerate the growth of my baby cacti by increasing
 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
 work quite well.
 Pivotal question:
 Which is better for growing cacti: incandescent plant lamps or
 fluorescent lamps?
> 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.

I've performed measurements of absorption spectra of marine plant life
and they prefer light in the blue and the red region.  Plants can use
all colors of  visible light and terrestrial plants use little more blue
light than red but not a lot more.  The absorption peaks for chlorophyll
"a" (in a plant leaf)are about 675 nm and 440 nm. I would use a bulb
that has an output similar in color to solar radiation or a specialized
grow bulb that has peaks in both the red and blue spectrum.  A cool
white or bulb with a high color temperature would give you a good peak
in the blue and blue light has higher energy for photosynthesis.  The
reason some plants are green is green is the color of chlorophyll.
Chlorohyll reflects most of the green light and does not absorb it.  

										Hope this helps,


Jim Ivey
USF Marine Science Dept.
Please remove the NOSPAM from my address to reply

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