Look up the Chandra paper and you'll see the source of that data. The money shot is here:
And it's used to justify those sorts of light levels.
When growlightmeter published the first version of that chart, I asked if they had supporting research and the reply was that I should check the footnotes on each page on their site. There were no footnotes. I don't know if they've updated their site but Chandra is the big study on net P and PPFD.
I read that paper soon after I started growing and I followed the light levels that they're recommending. But what didn't make sense to me was how everyone doing research was saying to use scads of light but the Chandra paper was showing something completely different.
Raise your hand if you're harvesting net photosynthesis.
Anyone?
No, and that's what made me think that there was more to the story.
As the author in the attached "Frontiers paper" says, testing the gaseous output of a sample of leaves in a grow chamber is, perhaps, not a valid method of trying to determine the yield of a plant. Yes, you can see how much net P you'll get from a handful of leaves so if you want to harvest net photosynthesis that's very helpful.
Again, raise your hand if you're harvesting net photosynthesis.
No one? Then why is it being thrown up on a site about growing cannabis?
I have no reason to believe that the graphic is not offered in good faith. But I have no reason to follow it because I'm not interested in net photosynthesis. I assumed that net P was a valid proxy for crop yield and it wasn't until I started looking a little deeper when I realized that was wrong.
Per the Frontiers paper, attached, from the Discussion section.
"Cannabis Inflorescence Yield Is Proportional to Light Intensity
It was predicted that cannabis yield would exhibit a saturating response to increasing LI, thereby signifying an optimum LI range for indoor cannabis production. However, the yield results of this trial demonstrated cannabis’ immense plasticity for exploiting the incident lighting environment by efficiently increasing marketable biomass up to extremely high—for indoor production—LIs (Figure 7A). Even under ambient CO2 , the linear increases in yield indicated that the availability of PAR photons was still limiting whole-canopy photosynthesis at APPFD levels as high as ≈1,800 μmol·m−2·s−1 (i.e., DLI ≈78 mol·m−2 ·d−1 ). These results were generally consistent with the trends of other studies reporting linear cannabis yield responses to LI (Vanhove et al., 2011; Potter and Duncombe, 2012; Eaves et al., 2020), although there is considerable variability in both relative and absolute yield responses to LI in these prior works. The present study covered a broader range of LI, and with much higher granularity, compared with other similar studies.
The lack of a saturating yield response at such high LI is an important distinction between cannabis and other crops grown in controlled environments (Faust, 2003; Beaman et al., 2009; Oh et al., 2009; Fernandes et al., 2013). This also means that the selection of an “optimum” LI for indoor cannabis production can be made somewhat independently from its yield response to LI. Effectively, within the range of practical indoor PPFD levels— the more light that is provided, the proportionally higher the increase in yield will be. Therefore, the question of the optimum LI may be reduced to more practical functions of economics and infrastructure limitations: basically, how much lighting capacity can a grower afford to install and run?" (emphasis mine)
So why the recommendations for a PPFD of 600 or somesuch? For one, it's good business — I'm not going to recommend that growers get anywhere near "the limits" using my product because I know that a significant percentage of this people will step on their metaphorical d*ck and I will get the blame. It's much better for a company to make recommendations that result in uniformly content customers than to make a recommendation that ends up with even a small percentage of really pissed off customers.
Check out the light level recommendations from Mars, Vipar, Spider, etc. To their credit, they are recommending pretty high light levels. That's changed in the three years that I've been growing and hats off to them for doing that. On the other hand, conventional wisdom and across a large number of cannabis sites, you'll see recommendations for much lower levels of light.
600 PPFD will not result in a bad crop. It will result in a good crop. Per the Frontiers paper, we can see the results of 600 PPFD and other values because they provide the formula for the yield curve. I've created the table below, based on that yield curve (figure 7).
Yup, a 50µmol increase from 600 to 650 resulted in a 5% greater harvest. OK, that's just 5%. Turn the dimmer up a little bit more. Kinda fun, eh? More light more weed! Of course there are limits (there are no solutions, just tradeoffs) but, as long as light is the limiting factor, yield and "quality" (the ratio of dried flower weight to dried weight of total above ground mass) will tend to increase.
That's one reason why companies are improving their lights. Check out the Spider G4500 PPFD map. That's a thing of beauty. It puts my three year old Growcraft to shame and the difference in PPFD will result in bigger harvests. The Vipar XS 1500 Pro is kick ass and so is the Spider SE 7000. Great lights in the respective tent size that provide a very high output of even light.
And I put my money where my mouth is. My data from yesterday with samples from 14 colas. Growcraft X3 at 306 watts. Hang height is 13"±. There are no signs of light stress.
And daily light data from drop to flip. Puck is a Rapid LED Royal Blue puck. Veg light and flower lights are Growcraft X3 lights. Early in the grow, I had a few leaves start to taco so I dropped some light for a day or two (probably 100µmol). Other than that, nada.
The net P graphic is accurate and of value for growers who are interested in maximizing photosynthesis. Growers who are interested in the impact of light on the size and quality of their crop would do better reading the Frontiers paper.
Or, say "Fuck it" and turn it up to 11*.
*Yeah, I understand the meaning of the phrase but, heh, I'm on a roll…
. He said it was right as he was going to workout which is usually of an evening(I was working out too haha); so i'm hoping it was only like from 4-9pm. Anyways this is what they looked like when I got home: droopy dryish leaves, flimsy branches that were more resistant.
