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  #1  
Old 05-10-2005, 01:52 PM
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First recorded activity by GardenBanter: Jul 2005
Posts: 11
Default Black or green shade net?

I read in the book, Orchids Of Asia by Teoh Eng Soon that green ranges and green PVC as cover for orchids represses plant growth. Does this cover green shade cloth rather than a black net? I'm using green shade net. What's the difference between the two colors?
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  #2  
Old 05-10-2005, 03:54 PM
?
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On Wed, 5 Oct 2005 12:52:05 +0000 in blass wrote:

I read in the book, Orchids Of Asia by Teoh Eng Soon that green ranges
and green PVC as cover for orchids represses plant growth. Does this
cover green shade cloth rather than a black net? I'm using green shade
net. What's the difference between the two colors?


Remember plant foliage tends towards green because they are pulling
the energy out of the red and blue ends of the spectrum.
I suspect, and will defer to the knowledge of those with greenhouses
and shadehouses, that use of green shade netting will lead to higher
measured light levels, but less light that is useful to the plants.

And on that thought, are there places that sell red/blue shade netting? :-).




--
Chris Dukes
Suspicion breeds confidence -- Brazil
  #3  
Old 05-10-2005, 04:41 PM
Kenni Judd
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Aluminet comes in a variety of colors. I know they have red, not sure about
blue.
--
Kenni Judd
Juno Beach Orchids
http://www.jborchids.com

And on that thought, are there places that sell red/blue shade netting?
:-).




--
Chris Dukes
Suspicion breeds confidence -- Brazil



  #4  
Old 05-10-2005, 10:37 PM
Ray
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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?

--

Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Artwork, Books and Lots of Free Info!


"Kenni Judd" wrote in message
...
Aluminet comes in a variety of colors. I know they have red, not sure
about blue.
--
Kenni Judd
Juno Beach Orchids
http://www.jborchids.com

And on that thought, are there places that sell red/blue shade netting?
:-).




--
Chris Dukes
Suspicion breeds confidence -- Brazil





  #5  
Old 06-10-2005, 01:46 AM
Ted Byers
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"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


  #6  
Old 06-10-2005, 10:15 AM
Ray
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Default

I had considered the fact that there might be edge effects - some slight
diffraction around the edges of the openings, but I doubt it's significant.
My mind still wants to go back, though - if I have a white board and a green
board creating shadows, aren't the shadows the same?

Likewise, I suppose it's possible that some light is transmitted through the
mesh material, in which case the color might be important, but again I doubt
it's significant.

--

Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Artwork, Books and Lots of Free Info!


"Ted Byers" wrote in message
...

"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



  #7  
Old 06-10-2005, 10:32 AM
Registered User
 
First recorded activity by GardenBanter: Jul 2005
Posts: 11
Wink

Thanks you guys and to Ted for that lengthy insightful piece. To quote from the book, Orchids Of Asia by Teoh Eng Soon, top orchid grower of Singapore, (Chapter 6 on Light)......

"The type of light is also important for the proper growth of orchid. Light in the ultra-violet, near ultra-violet and green ranges represses plant growth, and green PVC sheets are TOTALLY UNSUITED for roofing of orchid houses. When too much green algae collect on top of the plastic roofing, it similarly represses plant growth. The algae must be scrubbed off or the roofing replaced. The simplest approach is to employ shading which will not interfere with the normal spectrum of sunlight, such as with lath houses, white paint over glass, or black saran cloth."

Now comes the subject of the color black in shade net trapping heat efficiently more than the green one. Any takers?
  #8  
Old 06-10-2005, 01:23 PM
Ted Byers
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Posts: n/a
Default


"Ray" wrote in message
...
I had considered the fact that there might be edge effects - some slight
diffraction around the edges of the openings, but I doubt it's significant.
My mind still wants to go back, though - if I have a white board and a
green board creating shadows, aren't the shadows the same?

Yes, the shadows would be the same.

However, don't forget that in the case of the shade cloth, you have the
pores that let some of the light through. What happens to that light? It
isn't all absorbed by the plants and fixtures in the greenhouse. Much of it
is reflected, and much of what is reflected will hit the inside surface of
the shade cloth. So the shade cloth is hit by light on both of its sides.

To see what I mean, you could try some ray tracing. Draw a cross section of
your greenhouse, with a solid floor and walls and roof that have gaps in the
line syou've used to represent them. Then, draw a series of parallel lines
hitting the greenhouse, representing incident light. Some of those will hit
the lines used to represent one of the walls or the roof. Those will either
end there or be reflected away from the greenhouse. Those that hit the
simulated pores will enter. Then continue those lines until they hit a
surface, and start drawing the path that would be taken by the reflected
light. Some of these will escape the greenhouse through the pores in the
shade cloth. Many, though, will hit the walls (shade cloth) and be
reflected back into the greenhouse. These paths will continue inside the
greenhouse until they happen to hit a pore, or until the light is absorbed
by something.

Energy is conserved. Light doesn't just enter the greenhouse and disappear.
Whatever isn't absorbed is reflected, and that reflected light must go
somewhere. The ray tracing exercise described allows one to see this, in
simplified form. A complicating factor is that any light absorbed is either
used by the plants in photosynthesis (converting electromagnetic energy into
chemical energy) or reradiated at a lower frequency (usually in the infrared
range), and this is reradiated in all directions. So, on average, half of
the light absorbed by the shade cloth will be radiated into the greenhouse
in the infrared, and half will be radiated out of the greenhouse.

Cheers,

Ted


  #9  
Old 06-10-2005, 03:53 PM
Registered User
 
First recorded activity by GardenBanter: Jul 2005
Posts: 11
Default

I found a site (http://www.diatex.fr/uk/produits/agri/ombrage.htm) selling shade nets. And this is what they have to say or warn (darn, why now?) before buying their shade nets:


Caution

Green and black shade nets behave like filters. Essential radiation for photosynthesis is reduced.
Thus, the growth is reduced. A green and black shade net decreases the lightís quantity and spectrum quality.

White shade nets :
They decrease only lightís quantity, without altering luminousí spectrum quality. As a consequence, the plantís growth is faster with a white shade net.
  #10  
Old 06-10-2005, 04:13 PM
Ted Byers
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Now comes the subject of the color black in shade net trapping heat
efficiently more than the green one. Any takers?

If attention is focussed solely on thermodynamics, the answer is obvious.
Instead of answering directly, I'll put the question back to you in slightly
different terms.

NB.: A flat black surface is absorbing light energy in all frequencies in
the visible spectrum.

NB.: A green surface is absorbing all frequencies in the visible spectrum
EXCEPT for green. Green is reflected which is why it appears green.

Which do you suppose is absorbing more energy, assuming both are subject to
the same intensity of white light?

Which will reradiate more energy out of the greenhouse as infrared
radiation?

Which will reradiate more energy into the greenhouse as infrared radiation?

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


  #11  
Old 06-10-2005, 10:17 PM
Ted Byers
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Posts: n/a
Default


"blass" wrote in message
...

I found a site (http://tinyurl.com/8znl8) selling shade nets. And this
is what they have to say or warn (darn, why now?) before buying their
shade nets:


*-Caution-

Green and black shade nets behave like filters. Essential radiation for
photosynthesis is reduced.
Thus, the growth is reduced. A green and black shade net decreases the
light's quantity and spectrum quality.

White shade nets :
They decrease only light's quantity, without altering luminous'
spectrum quality. As a consequence, the plant's growth is faster with a
white shade net.*

This seems generally OK, with a couple points that don't quite make sense.
What is said about green shade nets is obviously true. What is said about
black shade nets seems only partially true. While it is true that a black
material will reduce the quantity of light, I don't see how it can reduce
the quality of light (with respect to photosynthetically active radiation or
PAR) and appear black.

I can see how a black net can result in less PAR than a white one, due to
more of the light that is reflected inside the greenhouse being absorbed by
the interior surface of the net. But I'd hazard a guess that the effect of
a choice between white and black shade nets on photosynthesis would involve
a tradeoff between a little more PAR vs a little more heat.

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


  #12  
Old 06-10-2005, 11:38 PM
Ray
external usenet poster
 
Posts: n/a
Default

Ah HAH! I had forgotten about the reflected light. Do you think it's
that significant?

--

Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Artwork, Books and Lots of Free Info!


"Ted Byers" wrote in message
.. .

"Ray" wrote in message
...
I had considered the fact that there might be edge effects - some slight
diffraction around the edges of the openings, but I doubt it's
significant. My mind still wants to go back, though - if I have a white
board and a green board creating shadows, aren't the shadows the same?

Yes, the shadows would be the same.

However, don't forget that in the case of the shade cloth, you have the
pores that let some of the light through. What happens to that light? It
isn't all absorbed by the plants and fixtures in the greenhouse. Much of
it is reflected, and much of what is reflected will hit the inside surface
of the shade cloth. So the shade cloth is hit by light on both of its
sides.

To see what I mean, you could try some ray tracing. Draw a cross section
of your greenhouse, with a solid floor and walls and roof that have gaps
in the line syou've used to represent them. Then, draw a series of
parallel lines hitting the greenhouse, representing incident light. Some
of those will hit the lines used to represent one of the walls or the
roof. Those will either end there or be reflected away from the
greenhouse. Those that hit the simulated pores will enter. Then continue
those lines until they hit a surface, and start drawing the path that
would be taken by the reflected light. Some of these will escape the
greenhouse through the pores in the shade cloth. Many, though, will hit
the walls (shade cloth) and be reflected back into the greenhouse. These
paths will continue inside the greenhouse until they happen to hit a pore,
or until the light is absorbed by something.

Energy is conserved. Light doesn't just enter the greenhouse and
disappear. Whatever isn't absorbed is reflected, and that reflected light
must go somewhere. The ray tracing exercise described allows one to see
this, in simplified form. A complicating factor is that any light
absorbed is either used by the plants in photosynthesis (converting
electromagnetic energy into chemical energy) or reradiated at a lower
frequency (usually in the infrared range), and this is reradiated in all
directions. So, on average, half of the light absorbed by the shade cloth
will be radiated into the greenhouse in the infrared, and half will be
radiated out of the greenhouse.

Cheers,

Ted



  #13  
Old 07-10-2005, 03:43 AM
Ted Byers
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Posts: n/a
Default


"Ray" wrote in message
...
Ah HAH! I had forgotten about the reflected light. Do you think it's
that significant?

--

I'll give a definitive "It depends!" ;-)

It all depends on the details of the geometry of the structure. If the
geometry of the greehouse, and the properties of the shade cloth, are such
that a large proportion of the light entering the greenhouse is ultimately
absorbed inside the greenhouse (meaning used by plants to produce new
tissue, or ultimately converted into heat), then this effect may well
overwhelm the contribution to plant production from light that has entered
the greenhouse and not yet been reflected. On the other hand, if the
combination of the geometry of the greenhouse and the properties of the
shade cloth are such that most of the reflected light exits the greenhouse,
then the effect is likely negligible.

The only way to know for sure if the effect is likely to be significant is
to construct a computer simulation; a numerical experiment if you will. The
physics, and the computational requirements for ray tracing, is simple
enough. The data management, though, would be challenging, since you'd have
to run a reasonably large number of rays, simulating a constant input of new
rays, and keep track of rays that have left the structure or been absorbed,
until the system approaches equilibrium (probably reached almost instantly
in the real world, but likely to take some time in the simulation). You can
then estimate the proportion of rays striking a given surface that have been
bouncing around the greenhouse before striking the surface in question (and
do this for a number of different frequencies, simultaneously, to get a
sense of the impact of this effect on the spectral quality of the light. Of
course, once such a simulation model has been constructed, it is easily
modified to examine the consequences of using black shade cloth rather than
white shade cloth (or any other color for that matter), both for the quality
of light reaching a given surface and for thermodynamic effects.

While I am a specialist in environmental modeling, I am not sure I want to
spend the months needed to produce an application to do this to the
standards I maintain for my own work; especially when I am working hard on
getting a new business established. If I was working at some university,
though, I'd certainly assign it to graduate students to give them a sense of
the application of science in the real world.

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


  #14  
Old 07-10-2005, 04:25 AM
Ray
external usenet poster
 
Posts: n/a
Default

Well Ted, I guess we can ignore light reflected from the plants, since it's
the wavelength they don't use. Then we have to speculate on the
reflectivity of the benches, floor, media, algae, moss, etc.

My guess is that it's insignificant compared to the incoming light.

--

Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Artwork, Books and Lots of Free Info!


"Ted Byers" wrote in message
.. .

"Ray" wrote in message
...
Ah HAH! I had forgotten about the reflected light. Do you think it's
that significant?

--

I'll give a definitive "It depends!" ;-)

It all depends on the details of the geometry of the structure. If the
geometry of the greehouse, and the properties of the shade cloth, are such
that a large proportion of the light entering the greenhouse is ultimately
absorbed inside the greenhouse (meaning used by plants to produce new
tissue, or ultimately converted into heat), then this effect may well
overwhelm the contribution to plant production from light that has entered
the greenhouse and not yet been reflected. On the other hand, if the
combination of the geometry of the greenhouse and the properties of the
shade cloth are such that most of the reflected light exits the
greenhouse, then the effect is likely negligible.

The only way to know for sure if the effect is likely to be significant is
to construct a computer simulation; a numerical experiment if you will.
The physics, and the computational requirements for ray tracing, is simple
enough. The data management, though, would be challenging, since you'd
have to run a reasonably large number of rays, simulating a constant input
of new rays, and keep track of rays that have left the structure or been
absorbed, until the system approaches equilibrium (probably reached almost
instantly in the real world, but likely to take some time in the
simulation). You can then estimate the proportion of rays striking a
given surface that have been bouncing around the greenhouse before
striking the surface in question (and do this for a number of different
frequencies, simultaneously, to get a sense of the impact of this effect
on the spectral quality of the light. Of course, once such a simulation
model has been constructed, it is easily modified to examine the
consequences of using black shade cloth rather than white shade cloth (or
any other color for that matter), both for the quality of light reaching a
given surface and for thermodynamic effects.

While I am a specialist in environmental modeling, I am not sure I want to
spend the months needed to produce an application to do this to the
standards I maintain for my own work; especially when I am working hard on
getting a new business established. If I was working at some university,
though, I'd certainly assign it to graduate students to give them a sense
of the application of science in the real world.

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



  #15  
Old 07-10-2005, 11:33 AM
Reka
external usenet poster
 
Posts: n/a
Default

In article ,
says...

"blass" wrote in message
...

I found a site (
http://tinyurl.com/8znl8) selling shade nets. And this
is what they have to say or warn (darn, why now?) before buying their
shade nets:


*-Caution-

Green and black shade nets behave like filters. Essential radiation for
photosynthesis is reduced.
Thus, the growth is reduced. A green and black shade net decreases the
light's quantity and spectrum quality.

White shade nets :
They decrease only light's quantity, without altering luminous'
spectrum quality. As a consequence, the plant's growth is faster with a
white shade net.*

This seems generally OK, with a couple points that don't quite make sense.
What is said about green shade nets is obviously true. What is said about
black shade nets seems only partially true. While it is true that a black
material will reduce the quantity of light, I don't see how it can reduce
the quality of light (with respect to photosynthetically active radiation or
PAR) and appear black.

I can see how a black net can result in less PAR than a white one, due to
more of the light that is reflected inside the greenhouse being absorbed by
the interior surface of the net. But I'd hazard a guess that the effect of
a choice between white and black shade nets on photosynthesis would involve
a tradeoff between a little more PAR vs a little more heat.

Cheers,

Ted



Okay, I know a little about orchids, but more about orchards, a subject
that I have grown up with. (Shades of those poor newbies who think this
is "rec.gardens.orchards"!) Over here, we have problems with hail damage
on apple crops from summer to early fall. Thus, hail netting is
stretched over the trees during that time period. Very dark green,
black, or white nets have been used. It has been proven that the red
apples under white netting color up much better than those under the
other two dark colors. Now, how this can be correlated to orchids, I
don't know, but perhaps it is worth the time I took to write it. And I
am a slow typer. :-) I am assuming it could be the extra heat generated
under the darker colors that prevent better color. Red apples need
temperature swings from cool nights to warmer days without rain in order
to color up well.
--
--
Reka

This is LIFE! It's not a rehearsal. Don't miss it!
http://www.rolbox.it/hukari/index.html
 




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