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Plant Lighting

Emily Boros-Rausch
28 October
Hey plant pals! How often do you water your succulents and cacti in the winter? I have had two deaths this month
Richard Sexton Purple light is not what you want, what matters is the amount of light, not the color of it. Any colors except cheap white ones put out less photons.
Are they hot enough? They are used to being at 80F+, dry and well lit.
What you have there is good for ferns.
Wei Tan Richard Sexton hmm, what's the purple light for then?
Richard Sexton It makes people buy them.
It's an interesting phenomenon. In 1970 two guys in Ohio published "Lighting for plant growth" (Kent State Press, 1970) and looked out two photosynthetic pigments, Chlor-A and Chlor-B. These respond to 275nm nd 280nm as spectral peaks red and blue.
They were funded by GE then works with Philips to develop "Grow Lux" bulbs that put out 900 lumens of purple light compared to 2200 for warm white/cool white.
This is the sole source of the purple light myth*
The other fluorescent tube manufacturers followed suit, then all came out with "wide spectrum grow lux".
The problem is in the real works warm white grows plants better.
The problems with their study is they used one plant and looked at two pigments. There are at least 30 and they cover the entire spectrum. We wasted decdes on this silliness, not to mention a huge amount of money.
There is no such thing as a fancy color light source that works better than the cheap bright ones.
Think about the physics of it. Say 2200 lumens or photos hit a plant compared to 800 that happen to be purple. Plants cant see purple, they just do something even trying a photon hits them and trh more the better. They don't care what frequency they're at which is all color is.
*Not to be confused with the blue bulb myth, the mistaken believe blue headlight bulbs let you see better at night, they do not, amber ones do, in fact in inclement weather blue light cuts down visibility because it's short wave is scattered most by fog, amber (required in france) is the opposite. Why do poeple want blue HID lamps? So theur friends know they spent the money. The can make then in white, nobody wants them. I wish I was making this up. Source: Dan Stern/NHTSA Lighting standards guru.
Alexandra Orlova Not to nitpick but red is usually ~680nm and blue is closer to 460-ish. Chlor a & b do respond to these respective peaks so it's not entirely wild to base your lights on that principle, they could suffice if it's *the only* color you have. But agreed that in principle a full spectrum is better, so many extra carotenoids and spare pigments to soak up the fringes :)
Emily Boros-Rausch To be honest I never even paid attention to the light colour, just got gifted a grow light, and it seems to work for me 🤷🏼‍♀️
Richard Sexton I was giving those numbers off the top of my head. I'm impressed they were as close as they are. But thanks.
No "full spectrum" isn't better. Whatever is brighter is better, lumens matters not spectrum.
Say youhave a 1000W mercury vapor lamp with it's hideous and totally "wrong" blue light:

Richard Sexton Now, compare that to sunlight:
Richard Sexton No lighting source known has th same spectrum as sunlight. Now, if a 1Kw Hg bulb is inferior, a) why does it work so well? b) how many watts of "superior" spectrum does it take to do better than this in terms of a) power b) cost. You can get a $75 HID sodium light in most hardware stores. It's only 75 watts. It'll blind you tough.
In terms of LED's count the photons they put out, these are given as lumens. The most, wins, regardless of the color.
Alexandra Orlova I don't understand what the argument is, that stronger = better regardless of wavelength? When I say "full spectrum" I mean cool or warm white, which might be the best imitation of sunlight, even if it's not exact. Having too much of a certain color can also trigger different morphological changes in a plant. Too strong of a light can cause oxidative damage too.
As long as you're using a light with a wavelength somewhere within 400-700nm (which is the "photosynthetically active range") it will be able to use *something* from it, be it exclusively blue or red, etc, it comes down to excitation energy that the photons have.
I just mean to say that all other things equal, including power and your resulting lumens, a plant under full spectrum will benefit more than just a blue or purple color because there's a lot of accessory pigments that help absorb things in peaks other than r/b like chl a & b.
Alexandra Orlova The reason mercury vapor lamps work despite not having much red is because the energy from the green range is still more than in red, and therefore enough to "excite" the photosystems in chl a to the first energy state and to then proceed with the cascade of reactions that is photosynthesis. That's still pretty good, just leaves behind a lil extra energy

Alexandra Orlova That's great, I get that more lumens = more energy. But if you consider power to be equal I think that it's better to have multiple peaks instead of just one.
The plant has accessory pigments that are specifically made to deal with other parts of the spectrum and are in turn responsible for different things such as secondary metabolite production and gene regulation. Having just one color can result in morphological differences in the same type of plant even. That's just life, plants have evolved to consider more than just strength of light

Richard Sexton "I don't understand what the argument is, "
No you don't. Let me try to simplify it: the most lumens wins. Where it's 100% green or a mix of orange and purple. The color - which is just frequency of the photon, doesn't matter, the total number of photos hitting the plant is all the matters.
Each one initiates a quantum chemical reaction as they bump electrons in esoteric phytochemicals around.
So, it can be all blue like mercury vapor or all red like HPS or a series od peaks lke HID metal halide.
The point is none of that matters, what matters is the number of photos that hit the plant regardless of the spectrum of the source.
It is very incorrect to say WW or CW are "wide spectrum", they're mostly green and bluey green respectively.
Wide spectrum is most closely approximated by GE Chroma 50 tubes.
"he reason mercury vapor lamps work despite not having much red is because the energy from the green range is still more than in red, "
You're falling into the trap is only considering red and blue photosynthetic peaks and ignoring thr other 30 which are widely distributed across the spectrum. Besides the two green ones there are other green ones, brown ones, blue ones, red ones, yellow ones and more. Lookin at only tei is how we ended up with purple lights!
We seen green because the light that hits is is absorbed except for green which bounces back. So consider what happens with brown or purple of red leaves? What colors are they absorbing?
Note also green plats have green pigments on the inside and other colors on the outside which act also as "suntan lotion". In the fall the plant recycles the green pigments (they cost a lot 0f nitrogen to make) and leaves the other pigments there. That's why we see yellow and orange and red in the fall. There's a lot written on this.

Alexandra Orlova Yes plants have two big absorption peaks in red and blue and then other side ones, are you missing my whole spiel about accessory pigments?
I'm not denying they exist, I'm emphasizing that, and hence, the advantage to using a light source that has the opportunity to utilize as many of those peaks as possible

Alexandra Orlova If you straight up blast a strong 680nm red at a plant it will grow. But all the other pigments and shindigs in the absorption spectra to the left of that are basically doing nothing, which the plant *will* adjust for, but you're not taking advantage of the plant's full metabolism in this case.

Richard Sexton Some plants have two big peaks there, not all.
Spectrum has nothing to do with it though. Intensity matters, change that and pants grow faster and bigger.
Change spectrum and they're probably grow less, That's the bottom line.
Sunlight doesn't work best because it has the right spectrum it world best because there's so much more of it.
Richard Sexton " If you straight up blast a strong 680nm red at a plant it will grow."
Please read Lighting for plant growth. You can do this with any color and it works.
ANY color.

Alexandra Orlova I'm hypothesizing the scenario of "IF INTENSITY IS EQUAL" then considering wavelengths :) As you might do anyway with a plant setup, no??

Alexandra Orlova ""If you straight up blast a strong 680nm red at a plant it will grow."
Please read Lighting for plant growth. You can do this with any color and it works.""
Um, yes? I'm not denying that?
Richard Sexton Intensity is never equal so it's a specious argument. Anode decay in tubeds means they're dimmer all the time.
I think you understand now intensity matters not spectrum though, yes?
If you want to show spectrum matters conduct a real world test, it's all hypothetical so far. That's how we got stuck with purple lights.
You can probably show red or green or blue LEDS do something. Problem is cheaper white ones will always work better.
Prices change each week so it's specious argument really.
Lumens per dollar is the only metric to consider when buying plant lights. Any argument there?
Alexandra Orlova Ok, you know what man, I tried, it doesn't seem like we see eye to eye and that's OK.
You can measure PPFD in most light sources. That's how you can standardize intensity for your plants across different lights.
Even if you have two LEDs of equal intensity and one is blue but the other is red you already have a difference in the energy that the light carries. This is another reason why spectrum can matter
Richard Sexton Nobody grows plants under red or blue led's.
If you can show one spectrum works better than any other why don't you do that and published your results?
I worked on exactly this for about 20 years and wasn't able to, then I noticed all sorts of silence papers that said: "We used warm white because it seems to work best.'
No mater how much I spent on any bulb and I tied them all, I was never able to find one tht worked better than 99 cent warm white and that observation has been published may times.
The human eye and plants don't see the same. We can see all the different color lights poeple like to use, plants just seem dim and bright, and bright wins.
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