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

Don Klipstein don at Misty.com
Tue Oct 14 22:05:51 EST 1997

  All the different figures being posted having to do with what
wavelengths chlorophyl works best at:

  There is Chlorophyl A and B.  One has a peak in the very deep red near
680 nm, the other has a peak a little shorter but still in the deep red.
Both also have peaks in the deep blue/violet-blue range, but these peaks
are secondary.  You have to stimulate both A and B to grow your plants
well, so at least half your red output should not be much longer than 670
nm.  In addition, the red peaks are asymmetric; shorter red and orange-red
wavelengths work well, but near-infrared wavelengths over 700 nm have
close to zero useful effect.
  Although chlorophyl utilizes blue light, it does not utilize it as well
as red light.  But there may be other processes requiring some blue light.
Fluorescent grow lamps typically have all or nearly their phosphor output
in the mid-red and deeper red wavelengths, and usually depend on the
mercury spectral output for whatever blue you need.  Those with special
needs for violet-blue light use "actinic" fluorescent lamps.  Such needs
are mostly for live coral in reef aquariums.

  Ordinary compact fluorescent lamps and "triphosphor" fluorescent lamps
will work well - especially if the rated "color temperature" is in the
range of 2700 to 3000 K.  These lamps produce lots of orangish red light
at 611 nm.  Plants will love these, although they may look distractingly
bright to you compared to a similar wattage of fluorescent grow lamps.
Watt for watt, these are slightly less effective for plants than grow
lamps since 2700-3000 K triphosphor lamps have some green phosphor output.
But they're widely available and will work.
  Those 32 watt 4-foot T8 (1-inch or 25 mm.) lamps are almost entirely
triphosphor and mostly quite economical.  Sylvania "designer", "interior
design", and anything else with a color code beginning with D8, as well as
General Electric lamps with color codes beginning with SPX are triphosphor.
So are just about all compact fluorescents, and Philip's "octalume"
and "ultralume" lamps.

  Some greenhouse owners use high pressure sodium lamps!  These produce
mainly yellow and orange light.  Apparantly, the short end of chlorophyl's
red peak extends to the orange-yellow - at least enough to take advantage
of the very high efficiency of sodium lamps.

 - Don Klipstein (don at misty.com)

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