What Umol or PPFD do you run in mid to late flower?

I am currently running mine at 550-700 at the canopy but most of the canopy is about 650umol. Is this the maximum without Co2? What's the maximum you run in mid to late flower? Im thinking i can push them abit farther but ive slowly been trying to optimize everything. At 700umol my DLI is 30.24. I believe i can push it to 35 without Co2 and keep it below 40. thoughts?

Milky Weed said:

I am currently running mine at 550-700 at the canopy but most of the canopy is about 650umol. Is this the maximum without Co2? What's the maximum you run in mid to late flower? Im thinking i can push them abit farther but ive slowly been trying to optimize everything.

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Push them. If u look up Mammoth Lighting on the gram...they talk a lot about ppfd and how it correlates to yield. I just did my first run with LEDs and I wish I pushed them much harder. Grow and learn! I had them around 1000 ppfd most of flower and saw no ill effects. I'm going to shoot for 1500 next round. Just have to dial everything else in. I run C02 btw

FamMan said:

Push them. If u look up Mammoth Lighting on the gram...they talk a lot about ppfd and how it correlates to yield. I just did my first run with LEDs and I wish I pushed them much harder. Grow and learn! I had them around 1000 ppfd most of flower and saw no ill effects. I'm going to shoot for 1500 next round. Just have to dial everything else in. I run C02 btw

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Yes i see people pushing 1000ppfd all the time with Co2, maybe next run ill consider it, but this run is without co2. just trying to be careful and thanks for your imput man

Milky Weed said:

I am currently running mine at 550-700 at the canopy but most of the canopy is about 650umol. Is this the maximum without Co2? What's the maximum you run in mid to late flower? Im thinking i can push them abit farther but ive slowly been trying to optimize everything. At 700umol my DLI is 30.24. I believe i can push it to 35 without Co2 and keep it below 40. thoughts?

Click to expand...

The paper at this link may be of interest. Couple what you read there with the comments from Dr. Bruce Bugbee (check out his videos on YouTube) and, loosely speaking, the more light you throw at cannabis the better but…

There's always a "but", eh?

Bugbee states, unequivocally, that light is the driving force behind yield and, interestingly, he also says that the quality of the light (the spectrum) is secondary to the amount of light. Strangely, however, a paper that his team published in August reveals that yield drops once your percentage of blue light exceeds 10% so there appears to be a conflict.

Putting that aside, he makes it clear that increasing PPFD is only one part of the equation needed to increase yield. Overall, there are nine environmental factors that have to be taken into account to increase yield. The nine are listed in one of his videos and he equates pumping up the light without improving the other aspects of the grow environment is like putting a huge engine in a car and running it on the Bonneville Slat flats - that works just fine but you need to improve the brakes and the suspension, as well.

The attachment is part of the Chandra paper. It's a great reference but should be considered in the context of the entire paper.


This graphic is from the company that makes "Photone" AKA Korona, a program that measures PPFD using a smartphone. I'm surprised to see DLI's that are so low but the programmer who wrote the app stands behind it.


This document is from LI-COR, a company that makes PAR meters and, thus, is a competitor to Apogee which is the company that Dr Bugbee owns. It's the best summary of the light issue that I've seen.

My personal take on this is to integrate high DLI with "high" temperatures and ensure that VPD is as close as possible to optimal. Having said that, though, I also see the law of diminishing returns, as Bugbee talks about and as the Chandra paper illustrates. At issue is that, once PPFD goes over 700, the rate of increase in yield begins to diminish. No surprise, that. You'll see comments on various forums about it not being cost effective but that's very situationally dependent. For a commercial grower, increasing electricity cost by 10% could result in a yield increase of 20% (for example). For a personal grower, it's a different issue.

My current grow - though it might be dying from an unsolvable Mg deficiency - is Gelato autos at a DLI of 36. I'm at that level because, until a few hours ago, the grow has been suffering for over a week so I've kept DLI in the high 20's and low 30's. Were it not for that I'd have pushed them in the low 40's or, perhaps, higher. My ambient temp is only in the high 70's and my VPD is 1.0± so conditions are pretty good. Except for the fact that they're been ravaged by a nute issue.

I hope this is helpful.

What are you using to measure PPFD?

Delps8 said:

The paper at this link may be of interest. Couple what you read there with the comments from Dr. Bruce Bugbee (check out his videos on YouTube) and, loosely speaking, the more light you throw at cannabis the better but…

There's always a "but", eh?

Bugbee states, unequivocally, that light is the driving force behind yield and, interestingly, he also says that the quality of the light (the spectrum) is secondary to the amount of light. Strangely, however, a paper that his team published in August reveals that yield drops once your percentage of blue light exceeds 10% so there appears to be a conflict.

Putting that aside, he makes it clear that increasing PPFD is only one part of the equation needed to increase yield. Overall, there are nine environmental factors that have to be taken into account to increase yield. The nine are listed in one of his videos and he equates pumping up the light without improving the other aspects of the grow environment is like putting a huge engine in a car and running it on the Bonneville Slat flats - that works just fine but you need to improve the brakes and the suspension, as well.

The attachment is part of the Chandra paper. It's a great reference but should be considered in the context of the entire paper. View attachment 5018584


This graphic is from the company that makes "Photone" AKA Korona, a program that measures PPFD using a smartphone. I'm surprised to see DLI's that are so low but the programmer who wrote the app stands behind it.
View attachment 5018585

This document is from LI-COR, a company that makes PAR meters and, thus, is a competitor to Apogee which is the company that Dr Bugbee owns. It's the best summary of the light issue that I've seen.

My personal take on this is to integrate high DLI with "high" temperatures and ensure that VPD is as close as possible to optimal. Having said that, though, I also see the law of diminishing returns, as Bugbee talks about and as the Chandra paper illustrates. At issue is that, once PPFD goes over 700, the rate of increase in yield begins to diminish. No surprise, that. You'll see comments on various forums about it not being cost effective but that's very situationally dependent. For a commercial grower, increasing electricity cost by 10% could result in a yield increase of 20% (for example). For a personal grower, it's a different issue.

My current grow - though it might be dying from an unsolvable Mg deficiency - is Gelato autos at a DLI of 36. I'm at that level because, until a few hours ago, the grow has been suffering for over a week so I've kept DLI in the high 20's and low 30's. Were it not for that I'd have pushed them in the low 40's or, perhaps, higher. My ambient temp is only in the high 70's and my VPD is 1.0± so conditions are pretty good. Except for the fact that they're been ravaged by a nute issue.

I hope this is helpful.

What are you using to measure PPFD?

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I use a dr.meter led lux meter and divide the lux by 60 (dictated by the quality of my lights) to get Umol. Thank you for the research, but unfortunately i think that is all ran with Co2 involved because they like to run optimized studies. Im looking specifically experience of how far i can push without Co2. Are you running with or without Co2? thanks for input also and it is helpful! I also follow Dr.Bugbee, and he mentioned a DLI of 40 is Really pushing a plant light wise, and you generally want Co2 if your going to push it above that.

I wanted to post and get everyones general experience with that and if your at 35 without Co2 that holds up very well. I have been trying to dial in the nutrients before i try any higher, because nutrient uptake is affected by light input. Or atleast water uptake is, and that affects how much nutrients are going through the plant if using soilless media and liquid nutes.

I typically top out at 850 or so during peak flower. Without CO2 supplementation I start to have issues over that number with the strains I run.

Idk about pfd/umols I've a lux meter, at 18/20" it's 450/600k lux across the canopy, during the stretch period a few tops were only 8/10" from the light with no obvious signs of issues.

2x240w + 300w.

Star Dog said:

Idk about pfd/umols I've a lux meter, at 18/20" it's 450/600k lux across the canopy, during the stretch period a few tops were only 8/10" from the light with no obvious signs of issues.
View attachment 5018847
2x240w + 300w.

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I think you mean 45k and 65k lux, but i get your point. Im running mine about 45k lux max, late flower I’ll push it up towards 50k. Thanks everyone.

Milky Weed said:

I use a dr.meter led lux meter and divide the lux by 60 (dictated by the quality of my lights) to get Umol. Thank you for the research, but unfortunately i think that is all ran with Co2 involved because they like to run optimized studies. Im looking specifically experience of how far i can push without Co2. Are you running with or without Co2? thanks for input also and it is helpful! I also follow Dr.Bugbee, and he mentioned a DLI of 40 is Really pushing a plant light wise, and you generally want Co2 if your going to push it above that.

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Not using CO2 - other than living in SoCal!

Agree re. CO2 and DLI 40. That works out to 700± µmols at 18/6 which is about the breakpoint on the Chandra graphic.

What are your thoughts about the DLI graphic from Photone? What I don’t get is that Bugbee says turn it up to 11 whereas the Photone chart shows that there should be a drop in DLI. I’ve never seen that discussed.

Milky Weed said:

I wanted to post and get everyones general experience with that and if your at 35 without Co2 that holds up very well. I have been trying to dial in the nutrients before i try any higher, because nutrient uptake is affected by light input. Or atleast water uptake is, and that affects how much nutrients are going through the plant if using soilless media and liquid nutes.

Click to expand...

Agree about nutes. Gotta have our ducks in row if we’re going to live on the edge!

Delps8 said:

Not using CO2 - other than living in SoCal!

Agree re. CO2 and DLI 40. That works out to 700± µmols at 18/6 which is about the breakpoint on the Chandra graphic.

What are your thoughts about the DLI graphic from Photone? What I don’t get is that Bugbee says turn it up to 11 whereas the Photone chart shows that there should be a drop in DLI. I’ve never seen that discussed.


Agree about nutes. Gotta have our ducks in row if we’re going to live on the edge!

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I think it’s pretty strange it has such a high peak near late veg and then bumps it down again for the stretch. I have never seen anyone do that so far in my research on the forums. Usually it’s a gradual increase the whole way until some point you reach your desired flower light level.

I’m not experienced enough to say the photone graph is wrong, but it’s new to me.

Milky Weed said:

I think it’s pretty strange it has such a high peak near late veg and then bumps it down again for the stretch. I have never seen anyone do that so far in my research on the forums. Usually it’s a gradual increase the whole way until some point you reach your desired flower light level.

I’m not experienced enough to say the photone graph is wrong, but it’s new to me.

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Great way to put it.

Question for you/the thread - is there a difference in DLI between photos and autos?

I’ve not seen anything address that topic and, at the moment, my 4 Gelatos autos are getting plenty of light. Interesting, though, is that I’m having a hell of a time with a Mg deficiency and falling pH. Could too much light be a factor? Typical approach is to drop TDS until pH stops falling - what about dropping DLI which would/could cause nute uptake to drop which would/could stop pH from dropping?

[hits forehead] Does that make sense? Too high a DLI could cause pH to drop? Don’t mean to hijack the thread but this issue has been plaguing me for a week, including actually losing sleep over it.

So you should run the most light towards the end?

NanoGadget said:

I typically top out at 850 or so during peak flower. Without CO2 supplementation I start to have issues over that number with the strains I run.

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I’ve found my pro grower buddy is probably right that anything over 800 without supplemental CO2 is a waste.

Three Berries said:

So you should run the most light towards the end?

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It plateaus as some point but generally yes flowering cannabis likes higher intensity light due to the hours of light being shortened

Milky Weed said:

It plateaus as some point but generally yes flowering cannabis likes higher intensity light due to the hours of light being shortened

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I'm about 2 weeks more for the cut.

Just my 2 cents - I've found that pushing them too hard during the last few weeks often leads to foxtailing.

Hot Diggity Sog said:

Just my 2 cents - I've found that pushing them too hard during the last few weeks often leads to foxtailing.

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Do you back the lights off a tad the last week or so? I figured steady higher light from mid to late flower would be best as flowering buds put on a lot of weight their last week or two. Fox tailing is interesting to me.

Three Berries said:

I'm about 2 weeks more for the cut.

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I’m a newer grower myself so I’m just learning from the experienced guys, I was going to go for steady light at about the strength im at maybe a touch more. It sounds like I’ve about maxed them out.

I'm no expert on foxtailing, but this was my observation a few runs ago when I tested pushing them really hard. My current run I'm just running not as hard. My next run I'll push them and then back off and observe. There are probably a ton of factors that contribute to whether or not foxtailing is going to happen - starting with genetics.