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

Jan Andersen root at 127.0.0.1
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).
He
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
lights
(in terms of efficiency).

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

There are two reasons for this: 1) plants don't really utilise longer
wavelengths
than yellow light for photosynthesis (ie. you can't use red, yellow and 'green'
light).
2) All (nearly) flourescent tubes are made with mercury gas inside - this
produces
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
than
one without filter. So why spend a lot of money on such equipment - the only
'value' it has is that it look mysterious, really.

/jan

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