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

Jim Metzger metzger.72 at OSU.EDU
Wed Oct 15 13:17:47 EST 1997

At 01:47 PM 10/15/97 +0100, OZ wrote:
>In article <34447570.7E42676B at>, Jan Andersen <root at>
>>Very interesting. It seems as if you may be right :-) Still, you don't give
>>any numbers for watt input/lumen gained, and as far as I remember, this
>>was the interesting thing - the industrial 'cold white' tubes won big here.
>I checked the y-axis. The figures were the same for all the tubes. The
>book is away now but from memory it was something like:
>Absolute Power mw/nm/1000lumens.
>Hmmm, you may be right. The problem is that a lumen is NOT frequency
>independent. IIFC yellow/green (being the most sensitive to the *human*
>eye) counts much more towards the total lumens than say red or blue. On
>top of that the *energy* in a blue photon is 150% more than a red photon
>so if you are talking about *energy* output then lumens is a useless
>measure. If you are talking about plant requirements then it is really
>bad, weighting the yellow/green highly when the plant rates it very low.

Yes, Oz is right on this point: lumens, lux, and foot candles are really
misleading terms for plant growth and development. However, for most
purposes, it is also equally misleading to consider plant responses in terms
of energy. In order to discuss the "best" wavelength for plant growth one
must think in terms of photons and photon fluxes.

On the point about yellow/green being useless to plants---it's not as bad as
most people believe.  Action spectra for photosynthesis show that even green
light is signficantly utilized for photosynthesis.  The point here is that
much of the visible spectrum is harvested for plant growth.  Moreover, how
well a plant grows, or how good it looks is more dependent on how closely an
artificial light source mimics the visible solar spectrum.  At the moment
there are no such commercial lamps (except for sulfur plasma lamps).  The
greenhouse folks use high pressure sodium lamps not as a replacement for
solar light, but as a supplement, that is, to boost photosynthetic rates in
the winter when light is low.  In terms of the number of photons produced
per input watt, they certainly beat any fluorescent hands down.  Even when
considering the less than ideal wavelengths of those photons, it is much,
much cheaper to use HPS lamps than fluorescents, regardless of type, to
economically boost photosynthetic rates.  As I mentioned yesterday, it takes
so many fluorescent fixtures to raise light levels comparable to that of HPS
lamps that more light from the sun is blocked than produced. In contrast,
when providing ALL of a plant's light, cool white fluorescents have
traditionally been the choice because of their broader spectral output (see

>It is unlikely that the phosphors are very different in their
>efficiencies so one can get some sort of measure of the distortion
>between *energy* and *lumen* output from the following lumen outputs.
>[Oz hauls out aged tome again.]
>LUMEN outputs for a 6' 75W tube:
>blue    1400
>Green   5600
>Gold    2900
>Red      270 (not a missprint).
>So using lumens as a measure is not good.
>>A comment about the hi-tech greenhouse growers: they are of course
>>highly competent in their trade. But this doesn't guarantee that they know
>>much about this subject -
>Perhaps not. However these lamps are also used in univerity departments,
>knee deep in physicists and biologists who are VERY aware of the
>importance of spectral power distribution. Your comments do not apply to
>them at all.

Of all the growth chambers that I've seen over the past 20+ years (from
around the world, mind you), the vast majority use cool white fluorescents
plus some incandescents (to maintain "natural" red/far-red ratios); I have
NEVER seen a gro-lux type of bulb used for this purpose. Occasionally, I
have seen growth chambers with a combination of HPS, mercury, and metal
halide lamps. I should also say that the general experience of most
horticulturists and plant physiologists has been that the combination of
cool whites + incandescents produces the most "natural" looking plants.  So,
going back to the original question, for hobbyists, cool white lamps are
going to produce the best results at the most economical rate!
>>So, I'm still not convinced by your arguments.
>Yet, perhaps?
>'Oz     "Is it better to seem ignorant and learn,
>         - or seem wise and stay ignorant?"
Jim Metzger
Department of Horticulture and Crop Science
The Ohio State University
2021 Coffey Road
Columbus OH 43210

Voice:	614-292-3854
FAX:	614-292-7162
E-mail:	metzger.72 at osu.edu

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