I have been looking into LEDs for growing and keep coming back to the same question. What minimum number of wavelengths are required to grow for the entire lifecycle?
I have seen videos on YouTube by some guy named Random Ryan that shows serious problems when he tried to grow cherry tomatoes under a white LED light vs a no-name two color blurple LED light. The blurple light started out competitive but ran into serious problems as the plant moved from the small shrub stage into more of an adult veg cycle. His conclusion was that blurple is not as good as white LEDs. I would agree with his conclusions because the results he shows are quite dramatic. Although the experiment was quite excellent, he did not take it to the next step to determine what wavelengths are missing. In fact, he didn't even really describe the spectrum he actually got from either light. Great start but I had more questions than answers at the end.
I have also been reading about the Emerson effect that indicate the need for two colors of red (one above 680nm, the other below 680nm) are needed for best growth. This has been well documented since 1957 when it was first discovered. When I look at the spectrum for Samsung LM561C that everybody seems to like here, I find there is really very little power below 680nm.
So we get back to the original question of spectrum. What is the best choice for really excellent healthy growth? Does anybody have pointers to research done to find the four or five (or six or seven...) discrete wavelengths needed to achieve excellent growth? Ideally, it would also indicate the relative optical power levels for each color to optimize energy and LED selection. I would also be grateful for pointers to universities or labs that are looking into this area.
And yes, I'm one of those guys who thinks about all the various details on what could be possible. The more detailed the research, the better it is. I suspect the LM561C is a pretty good start but could be taken up a notch or two with the addition of a few extra colors.
And another question - Which color temperature on the LM561C is best? I am thinking the 3000K is best but I can't really find any comparative studies to show if this is correct or not.
-r2k
Minimum required spectrum
That is a simple question that has a VERY complicated and not that satisfying answer. Here's the summary of what I know:
- The best measure of plant specific light intensity we have is PAR, which is measured as total energy between 400 to 700 nm, this is the gold standard used in all horticultural research papers that I've read
- The ideal PAR sensor, used in greenhouses, research and by hobbyists that can afford it, would give equal weight to all photons between 400 to 700 and ignore everything outside of that range
- We know of the average photosynthetic response of plants to different wavelengths of light (McCree curve) but this does NOT tell us of the plants actual light wavelength requirements or the ideal plant lighting profile
- Different plants have different lighting requirements, even different varieties of the same plant will have different requirements; for example red leaf lettuce sometimes needs light in the UV spectrum to actually turn red, another example is some potato cultivars do well with 24 lighting, others do very poorly
- It is a myth that plants don't use green light efficiently, look at the spectrum of light put out by the sun, there is a huge peak in the green light range... why would plants evolve to ignore that energy? Even with the McCree study it was shown that the relative action of green light was only 9% less than average and that light in the green spectrum (500 – 600 nm) is nearly as effective as blue light for a considerable number of plant species. The possible reason why you see so many burple lights is because of this myth and the fact that green LEDs are slightly less efficient (see "the green gap")... you can literally grow things in pure green light
- In the end what matters the most is light intensity (PPF / PPFD / PAR) and duration to create a daily light integral (DLI), and having a broad spectrum light whose spectrum fits mostly in the 400 - 700nm region
- Burple (red & blue) lights are painful to look at, they make all your plants look ugly and makes it hard to diagnose, and in fact may conceal, problems with the health of your plants. Sure you may be squeezing an extra 5% efficiency out of your light but does that really matter when disease or pest wipes out your entire crop?
Youch!! Nice summary, I wish I could pick your brain more, so I probably might as I muddle through this.
I think they would if they could and it makes sense that they did, but one possible answer would be that the photosynthetic reactions don't run that way, so there is no chemical reaction pathway to use. That's only my hypothetical guess and cheezy answer and I have no clue if it has any basis in fact.why would plants evolve to ignore that energy?
I guess I need to find, read, and re-read the McCree study before I shoot off my mouth any further. Now that I know it exists and that's what it is called, I will find it. Thanks.
At this point in my ignorance and search, I am not so worried about the state of technology as much as I want to know about the fundamental requirements of plant growth.and the fact that green LEDs are slightly less efficient (see "the green gap")
Yes, but I am not above clicking on the CFL or warm white LED bulb (or even tungsten bulb) that I have stashed in the corner of the grow box for just that purpose.Burple (red & blue) lights are painful to look at, they make all your plants look ugly and makes it hard to diagnose, and in fact may conceal, problems with the health of your plants.
Blurple light can't be any worse to deal with than what you get from HPS. My biggest problem in that area seems to be the slow loss of 20/20 vision as I gain grey hair. I have an optometrist for that problem.Once again, thanks for your response. It is appreciated and any other input from other members will also be helpful.
Heres some sources for my post above, covers the McCree curve, PAR, and DIL:
https://www.inda-gro.com/IG/sites/defau ... isited.pdf
https://fluence.science/science/photosynthesis-guide/
https://www.extension.purdue.edu/extmed ... -238-W.pdf
You're probably talking about growing something similar to tomatoes, but in terms of growing something like lettuce these are the parameters I have read most studies use or recommend, I have sorted them by what I prioritize and is easy to manage:
- Light 17 µmol/m²/d DLI w/ airflow; decrease to 14 µmol/m²/d DLI if tipburn
- pH 5.6 - 6.0, with 5.8 being optimum
- EC / TDS 1150 - 1250 μS/cm (575 - 625 TDS) above source water
- Water temp 75 - 77 °F*
- Air temp 75 / 65 °F, Day / Night
- Relative Humidity 50 - 70%
Things like strawberries, swiss chard, basil and pac choi seem to prefer an EC of 1.5 - 2.0, a pH around 6.0 and a DLI of 15 to 20
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PurpleGunRack
- LED Enthusiast
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3000K is the simple answer.
Other than creating different photosynthtic processes in the plant, the different wavelengths also signal the plant to do certain things at certain times.
In the blue spectrum plants use up to 70% of the available light, in the orange/red spectrum it uses up to 100% of the available light,
The wavelengths beyond the 400-700 range create very little photosynthesis, but are effective in signalling the plants.
For instance Far Red can be used for 15 minutes after lights out to make the plants go into respiratory mode a lot quicker, the same thing happens naturally at sunset.
Full spectrum is very important, and many of the blurples use green diodes to achieve which made me wonder big time when I first got them and also bought a green light torch for checking on the flowers during lights out - I never do that anymore
If you want you can supplement your 3000K boards/strips with the beyond PAR spectrums, RapidLED have some kit options for this, and digikey have a huge selection of these diodes as well
Other than creating different photosynthtic processes in the plant, the different wavelengths also signal the plant to do certain things at certain times.
In the blue spectrum plants use up to 70% of the available light, in the orange/red spectrum it uses up to 100% of the available light,
The wavelengths beyond the 400-700 range create very little photosynthesis, but are effective in signalling the plants.
For instance Far Red can be used for 15 minutes after lights out to make the plants go into respiratory mode a lot quicker, the same thing happens naturally at sunset.
Full spectrum is very important, and many of the blurples use green diodes to achieve which made me wonder big time when I first got them and also bought a green light torch for checking on the flowers during lights out - I never do that anymore
If you want you can supplement your 3000K boards/strips with the beyond PAR spectrums, RapidLED have some kit options for this, and digikey have a huge selection of these diodes as well
