"Ray" wrote in message
...
OK, I give up.

In shade cloth, the solid material blocks the light, and the openings let
it pass, right? As an opening is an opening is an opening, what
difference does it make what color the light-blocking part is?

The color we perceive is due to the difference between the light reflected
(or that passes through translucent material) and the light absorbed. If
you shine white light on a green material, we seen green because most other
frequencies in the visible spectrum are absorbed and it is primarily green
light that is reflected. In the context of a greenhouse, then, you will
have light of various frequencies bouncing around, reflected by different
surfaces. An opaque green cloth will absorb the frequencies most useful to
plants and reflect those frequencies that are least useful to plants. This
is why, for example, leaves appear green; they absorb red and blue and
reflect green. I could go on, but I am sure those interested can find a
text on plant physiology or biochemistry.

It may help to think about the ultimate fate of the light that enters
through the openings in the shade cloth. While sunlight is nowhere near
white, lets assume that the light entering the greenhouse through the pores
in the shadecloth is white. What happens to it once inside. It bounces
around, reflected by one surface or another until it either escapes through
another pore or is absorbed. Since the plants will be absorbing red and
blue frquencies and reflecting green, the spectrum will increasingly include
primarily green colors. Similarly, if the shade cloth is green, any red or
blue light hitting it will be absorbed (and probably reradiated as
infrared), while the light reflected will be green.

Does any of this make a significant difference? I don't know since I have
not looked at, or conducted, any experiments that would tell us. I would
expect that there may be some effect on rates of primary production, and
thermal effects, but only a series of controlled experiments will tell us
for sure. I am not sure this is even important, except for those who design
and test shade cloth, since for ordinary growers and breeders, the
recommendations of how to use shade cloth will probably largely be based on
experience of how well different plants perform with different amounts of
shade cloth in different regions.

I don't know if this is helpful, but it is fun to think about.

Cheers,

Ted

--
R.E. (Ted) Byers, Ph.D., Ed.D.
R & D Decision Support Solutions
http://www.randddecisionsupportsolutions.com/
Healthy Living Through Informed Decision Making