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

Jan Andersen root at
Tue Oct 14 07:34:22 EST 1997

ken wrote:

> Scott wrote:
> >
> > 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.
> >
> Scott,
>          The trouble with incandescent gro-lights is they start with a
> lower lumen per watt rating (just becasue they are incandescent) and
> then they use subtractive filtering to get a daylight lamp.  This
> lowers the efficiency even more.  However, as you say, they do not
> require auxilary equipment.
>         I don't know the particulars (like sd curves, etc.), but SoLux
> halogen/incandescent lamps offers several selections of daylight lamps.
> Check out their webpage, and happy gardening :) Ken     goken at basec.net

Many years ago, when I studied physics at the University of Aarhus (in Denmark)

I talked with one of the physics professors (we shared an interest in orchids).
had made some measurements of the light spectra of several types of lamps. The
astonishng result was:

The worst kind of lamp was the ordinary light bulb _and_ the special 'plant
(in terms of efficiency).

The best ones were the industrial type 'cold white' flourescent tubes - they
by far the most efficient.

There are two reasons for this: 1) plants don't really utilise longer
than yellow light for photosynthesis (ie. you can't use red, yellow and 'green'
2) All (nearly) flourescent tubes are made with mercury gas inside - this
ultraviolet light, which is converted to visible light by means of an emulsion
at the
inside of the tube - a 'gro-light' has in addition to that a filter that takes
away some
wavelengths, which by necessity means that a 'gro-light' tube is less efficient
one without filter. So why spend a lot of money on such equipment - the only
'value' it has is that it look mysterious, really.


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