"best" - I'll define that as maximum rate for photosynthesis.
Autos will do great at a DLI of 45 mols. The lights-on period can vary, so you can got from 18/6 to 24/0 to get to that level and you'll end up with a great harvest, assuming other factors are equal.
Having said that, there's no "rule" that you can only give autos 45 mols. The programmer at Photone recommends 45 mols but doesn't cite any research on that (the DLI chart on their site is for photos. When I asked him about autos, he said 45 mols after seedling stage). If you look at research (Bugbee, Chandra, etc.) they discuss light levels at 900 µmols. Run the numbers on that and it's ≈ 63 mols. That's a shitton of light.
I recently switched from 25 mols for seedlings and then 45 mols to using 900 µmols in veg and flower. That's been an evolution for me as I've read more about light levels. My rationale for that amount of light is that in a non-CO2 environment, that level is considered the light saturation point.
What pushed me over the edge was going back and watching a video by De Bacco (for the Nth time). I took the two screenshots, below.
View attachment 1481496
View attachment 1481491
The host of the show has a degree in "plant biology" (I forget his actually degree) and, as far as I can tell, he does to consulting work. Putting aside my "appeal to authority", he cites his sources - Bugbee and Chandra (small world, eh?). I think those are helpful because they're so straightforward.
Having plumped for 900µmols, I'll be forthright and state that I have
not done a grow, stem to stern, at 900 µmols but the last month or so of my current Gorilla Glue auto grow has been very close to that level and I see no reason to not use that light level for future grows.
There are reasons to not use that level of light - too much of a hassle; the plants won't tolerate it; insufficient cost/benefit of yield vs cost of electricity; or inability measure light levels accurately.
I found that the plants in my current grow will tolerate ≈ 900 µmols. I use an Apogee and take multiple readings of the canopy. Below are the data from the sampling that I do, one sample per large/major cola.
320 = wattage for the Growcraft X3
18 = hours of light
# = the sequence number of the sample
The numbers at the bottom center are the average values of the sample >= 35 mols. The values to each side at the bottom are the standard deviation of those values, which indicates the uniformity of the light. Blue is< 35 mols, black is 35 mols to 900 µmols. Over time, I change the criteria for coloration.
Re. 900 µmols and too much light. I kept and eye on the colas that were DLI > 60. They were fine for the first 6 hours or so but, later that day, I saw "photo avoidance" — the colas actually bent themselves away from the light. I moved them so that their PPFD dropped into the high 800's and they did straighten out the next day.
View attachment 1481482
My rationale using that much light is that researchers have shown that cannabis will use the extra light. 900 µmols is well past where the yield curve starts to flatten but the cost of cannabis is high enough that the increased yield outweighs the cost of electricity for me even here in SoCal, which has the second highest electricity rates in CONUS. That was the finding of the only research paper I've seen on the topic, as well, though that was a commercial grow site in Ontario. It may not be cost effective for other growers - that's for the grower to determine.
The other issue is that we can measure light levels accurately. For me, if I was using a Li-Cor, SPOT ON, or Apogee PAR meter or a LUX meter that's been tweaked using a manufacturers formula (HLG), it would be reasonable to go to 900 µmols initially. If I was using any other sensor, I'd go to 800 or 850 µmols initially.
The most important issue is to not set the light level at X µmols and declare victory. In reality, using a sensor, regardless of its accuracy, is part of the concept of "listen to the plants".
Huh, how is using a PAR meter part of listening to your plants? My thinking - by measuring the light level and setting it to X µmols we're just using technology to set the light levels to the ballpark values very quickly. Once we do that, it's up to the grower to watch how the plants react and take appropriate action.
One caveat — per Bugbee, at these light levels, you're maximizing only one of the nine parameters of the grow environment. If the other eight parameters aren't functioning at a high level, you're not going to get the most out of all of those photons. Bugbee uses the analogy of souping up a race car engine but not improving the steering or the brakes. If that's the case, things may not turn out as intended.
While high light levels do work the biggest issue is to determine what light levels are needed to achieve the results that you want. Some growers get their best results at much lower levels. To my way of thinking, if you want maximum net photosynthesis, aim for 900 µmols in a non-CO2 environment, and then let the plants tell you.
