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

Kirk Kerekes redgate at tulsa.oklahoma.net
Tue Oct 14 12:10:52 EST 1997

In article <61vopp$l6v at sjx-ixn4.ix.netcom.com>, "R Coffey"
<datepalm at ix.netcom.com> wrote:

. Actually, Stan, the grass is green because it absorbs the red and blue
. light and uses it. It reflects the useless green light. So that is
. what you see!

Yes, most plants have absorbtion peaks in both red and blue bands. There
are several different varieties of chlorophyll, each using different light
wavelengths. (If you do a web search with "spectrum" and "photosynthesis"
as the keys, you will likely stumble across some informative spectral
plots. Or jump straight to
<http://www.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html> for a lovely
page with nifty spectra, molecular models, and informative text)

The notion behind the fluorescent "gro-lamp" is quite valid: such a lamp
should use a mix of phosphors which emit only at the red and blue
wavelenths of interest, and nowhere else -- the lamp will emit less
overall illumination, but more energy in the important spectral bands. 

Whether these lamps are actually implemented correctly is perhaps a
different story. Fortunately, you can get a "student spectrometer" from
Edmund Scientific for pocket money (under $10, as I recall) which is
entirely accurate enough to verify whether a particular bulb is a
legitimate gro-lite, or just a purple lamp. A GE "Plant/Aquarium" bulb I
just checked shows a strong peak in the blue at 440nm, a seemingly
pointless peak in the green at 570 (probably to make greenery look good to
the human eye) and a broad, intense red band. This is labelled a "broad
spectrum" bulb, and indeed there is significant energy across the visible
band, not just at the peaks mentioned.

Chlorophyll 'A' has absorption peaks at about 430nm (I am reading a graph
here, so these numbers may vary from the canonical values) and 680nm,
while chlorophyll 'B' has peaks at about 480nm and 650nm. The bulb
described above would drive chlorophyll A rather well, and "B" fairly

A standard CFL lamp emits narrower spectral bands, but they do appear to
be appropriately placed for decent plant illumination, although the 680nm
wavelength is totally absent. 

A "cool-white deluxe" lamp shows the same peaks as the gro-lite, including
the broad red band. It would probably make an adequate gro-lite, although
the plants wouldn't _look_ as good. It may also be that a properly
designed gro-lite would deliver more lumens/watt in the bands of interest
than a broad-spectrum lamp, and the reduced energy costs could easily pay
for a more expensive bulb and its associated fixture.

An incandescent gro-lite is certainly the worst of all possible worlds,
and should be avoided.

Kirk & Diane Kerekes
Red Gate Ranch
redgate at tulsa.oklahoma.net

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