Concerning DLI and VPD and

[Halloweed]

Hello all. I'm curious as to whether I should go by my DLI, or go by the PAR recommendations? I can turn up my lights more but I think it will increase the dli too much.

Also, is it better to go by your VPD or the recommended temperature and humidity? I'm just starting to go by the dli/vpd so I wasn't sure what's better.

Thanks

 

[Blastfact]

Both,,, DLI is the amount of photons the plants get in a light cycle. VPD is the proper combination of temp and humidity. If you try to run DLI and you lose control of VPD then you need to work on your environmental controls and support systems. Most important is VPD or as close as you can get to it. Back off the lights if you can't control enviroment.

 

[Halloweed]

Both,,, DLI is the amount of photons the plants get in a light cycle. VPD is the proper combination of temp and humidity. If you try to run DLI and you lose control of VPD then you need to work on your environmental controls and support systems. Most important is VPD or as close as you can get to it. Back off the lights if you can't control enviroment.

Oh I see. I may need to adjust my temps some.its nice to know to focus more on DLI and VPD. Thanks.

 

[Blastfact]

Oh I see. I may need to adjust my temps some.its nice to know to focus more on DLI and VPD. Thanks.

No focus on VPD more than DLI. VPD is leaf transpiration. Which is temp and humidity and how they relate to each other. If you can't control heat and humidity you run a greater chance of burning the leaves as you try to get DLI or higher DLI.

 

[Week4Bytch]

The numbers out there are recommendations for a ton of different growers from home to the professional. So take them with a grain of salt. Just read your plants is best but, yeah, I get it, we want the best lighting available to them. For the home grower I found DLI around 35 tops is doable without burning your plants. But I haven't been really checking that lately. Been paying attention to PPFD. After a few grows under your belt you'll end up not really using these tools as much. Until you get a new light or tent. Then things change when you add Autos to the equation...

 

[Blastfact]

The numbers out there are recommendations for a ton of different growers from home to the professional. So take them with a grain of salt. Just read your plants is best but, yeah, I get it, we want the best lighting available to them. For the home grower I found DLI around 35 tops is doable without burning your plants. But I haven't been really checking that lately. Been paying attention to PPFD. After a few grows under your belt you'll end up not really using these tools as much. Until you get a new light or tent. Then things change when you add Autos to the equation...

You have to know PPFD to calculate DLI. To get it all in balance is a job. And one does learn over time where you need to be.

 

[Week4Bytch]

No focus on VPD more than DLI. VPD is leaf transpiration. Which is temp and humidity and how they relate to each other. If you can't control heat and humidity you run a greater chance of burning the leaves as you try to get DLI or higher DLI.

This ^

 

[Florida_Mike]

i'm just going to throw this out there:

as far as i'm concerned, you can toss all that ppfd and vpd crap right where it belongs (in the toilet) and still grow the biggest, baddest, dankest weed on the block. i know bc i've done it in the past and i'm about to show here in 2025 how (some of) us old timers were doing it 25+ years ago without any of that over-hyped, over priced garbage.
i'll challenge any grower on this site to a grow off with the only difference (aside from lights and media etc) being whether one uses vpd, ppfd, par meters and all that bs. i'm quite rusty and in an experimental mindset right now and trying to get my legs back under me, but even with that being said; if i didn't win, i can guarantee you that i'd hold my own, even if i am going off from 22 year old foggy memories. gimme a few cycles to make the transition between HPS and LEDS and get my system dialed in and i'll be damn near unstoppable.

now, i'm not recommending or advocating for a newb (or anybody for that matter) to try this without proper instructions and guidance by a competent person or mentor. i'm just saying it's a straight up bs claim that you have to dial that shit in and follow it to a tee, to be a very successful grower. it's a patently illinformed and misguided belief and i'll try and dispel it whenever i can.

while i concede that it is based on peer reviewed science, it's not without it nuisances, detractors, skeptics and critics. from the web:


Debates or Misunderstandings:

Some growers find VPD guidelines too rigid or impractical in real-world conditions.
Individual strains or species might thrive outside the "ideal" VPD range.
Overemphasis on VPD may distract from other critical factors, such as light intensity, nutrient availability, or root zone health.

Criticism and Alternative Perspectives

While both concepts are rooted in science:

Some growers argue they are overly hyped, especially in controlled environments where factors like CO₂ enrichment or specific cultivar adaptations may lessen their importance.
Others contend that practical experience and observation sometimes yield better results than strictly adhering to PPFD or VPD charts.

Conclusion

Both PPFD and VPD are grounded in solid, peer-reviewed science and are valuable tools for optimizing plant growth.
However, their application should be flexible, informed by both scientific understanding and real-world experience. Not every grower or system benefits equally from strict adherence to these metrics.

 

[Blastfact]

i'm just going to throw this out there:

as far as i'm concerned, you can toss all that ppfd and vpd crap right where it belongs (in the toilet) and still grow the biggest, baddest, dankest weed on the block. i know bc i've done it in the past and i'm about to show here in 2025 how (some of) us old timers were doing it 25+ years ago without any of that over-hyped, over priced garbage.
i'll challenge any grower on this site to a grow off with the only difference (aside from lights and media etc) being whether one uses vpd, ppfd, par meters and all that bs. i'm quite rusty and in an experimental mindset right now and trying to get my legs back under me, but even with that being said; if i didn't win, i can guarantee you that i'd hold my own, even if i am going off from 22 year old foggy memories. gimme a few cycles to make the transition between HPS and LEDS and get my system dialed in and i'll be damn near unstoppable.

now, i'm not recommending or advocating for a newb (or anybody for that matter) to try this without proper instructions and guidance by a competent person or mentor. i'm just saying it's a straight up bs claim that you have to dial that shit in and follow it to a tee, to be a very successful grower. it's a patently illinformed and misguided belief and i'll try and dispel it whenever i can.

while i concede that it is based on peer reviewed science, it's not without it nuisances, detractors, skeptics and critics. from the web:


Debates or Misunderstandings:

Some growers find VPD guidelines too rigid or impractical in real-world conditions.
Individual strains or species might thrive outside the "ideal" VPD range.
Overemphasis on VPD may distract from other critical factors, such as light intensity, nutrient availability, or root zone health.

Criticism and Alternative Perspectives

While both concepts are rooted in science:

Some growers argue they are overly hyped, especially in controlled environments where factors like CO₂ enrichment or specific cultivar adaptations may lessen their importance.
Others contend that practical experience and observation sometimes yield better results than strictly adhering to PPFD or VPD charts.

Conclusion

Both PPFD and VPD are grounded in solid, peer-reviewed science and are valuable tools for optimizing plant growth.
However, their application should be flexible, informed by both scientific understanding and real-world experience. Not every grower or system benefits equally from strict adherence to these metrics.

Problem is,,, back in the day we did not have the numbers to reference. Our goal was to replicate outdoor grow conditions. But it became clear if we shifted a few known points we had better success albeit a different plant and finale outcome. I for one wish we would have had the charts 30 or 40 years ago. Instead of spending massive amounts of time to figure out the numbers/grow patterns with primitive equipment and control. Also hybrid cross breeding was not the norm back then. Was for the most part indica to indica and sativa to sativa breeding. Then when the bone heads started direct hybrid sativa and indica breeding it really helped to know percentage. Because we grew sativas different than indicas. So if it was a sativa plant you wanted more humidity and more diffused light. Then indicas wanted a more desert/steps/highland clear bold and brash light and less humidity. It's just a cluster now days as steering ones grow is a poooo fest. But the smart people figured some stuff out so we can look at a VPD chart and even bring leaf temp in and we know if we have a sativa dominant plant we know we need to have a bit more humidity at any given temp. More to the right of the scale, less humidity for a indica. And as the breeders breed the weed out of weed. We are starting to see high humidity breeds that aren't mold/mildew resistant and lower humidity breeds that aren't drought tolerant. So it's nice for new growers to have a guide IMHO.

 

[Florida_Mike]

Problem is,,, back in the day we did not have the numbers to reference. Our goal was to replicate outdoor grow conditions. But it became clear if we shifted a few known points we had better success albeit a different plant and finale outcome. I for one wish we would have had the charts 30 or 40 years ago. Instead of spending massive amounts of time to figure out the numbers/grow patterns with primitive equipment and control. Also hybrid cross breeding was not the norm back then. Was for the most part indica to indica and sativa to sativa breeding. Then when the bone heads started direct hybrid sativa and indica breeding it really helped to know percentage. Because we grew sativas different than indicas. So if it was a sativa plant you wanted more humidity and more diffused light. Then indicas wanted a more desert/steps/highland clear bold and brash light and less humidity. It's just a cluster now days as steering ones grow is a poooo fest. But the smart people figured some stuff out so we can look at a VPD chart and even bring leaf temp in and we know if we have a sativa dominant plant we know we need to have a bit more humidity at any given temp. More to the right of the scale, less humidity for a indica. And as the breeders breed the weed out of weed. We are starting to see high humidity breeds that aren't mold/mildew resistant and lower humidity breeds that aren't drought tolerant. So it's nice for new growers to have a guide IMHO.

very good explanation, thank you

 

[Week4Bytch]

i'm just going to throw this out there:

as far as i'm concerned, you can toss all that ppfd and vpd crap right where it belongs (in the toilet) and still grow the biggest, baddest, dankest weed on the block. i know bc i've done it in the past and i'm about to show here in 2025 how (some of) us old timers were doing it 25+ years ago without any of that over-hyped, over priced garbage.
i'll challenge any grower on this site to a grow off with the only difference (aside from lights and media etc) being whether one uses vpd, ppfd, par meters and all that bs. i'm quite rusty and in an experimental mindset right now and trying to get my legs back under me, but even with that being said; if i didn't win, i can guarantee you that i'd hold my own, even if i am going off from 22 year old foggy memories. gimme a few cycles to make the transition between HPS and LEDS and get my system dialed in and i'll be damn near unstoppable.

now, i'm not recommending or advocating for a newb (or anybody for that matter) to try this without proper instructions and guidance by a competent person or mentor. i'm just saying it's a straight up bs claim that you have to dial that shit in and follow it to a tee, to be a very successful grower. it's a patently illinformed and misguided belief and i'll try and dispel it whenever i can.

while i concede that it is based on peer reviewed science, it's not without it nuisances, detractors, skeptics and critics. from the web:


Debates or Misunderstandings:

Some growers find VPD guidelines too rigid or impractical in real-world conditions.
Individual strains or species might thrive outside the "ideal" VPD range.
Overemphasis on VPD may distract from other critical factors, such as light intensity, nutrient availability, or root zone health.

Criticism and Alternative Perspectives

While both concepts are rooted in science:

Some growers argue they are overly hyped, especially in controlled environments where factors like CO₂ enrichment or specific cultivar adaptations may lessen their importance.
Others contend that practical experience and observation sometimes yield better results than strictly adhering to PPFD or VPD charts.

Conclusion

Both PPFD and VPD are grounded in solid, peer-reviewed science and are valuable tools for optimizing plant growth.
However, their application should be flexible, informed by both scientific understanding and real-world experience. Not every grower or system benefits equally from strict adherence to these metrics.

I sometimes forget to turn on or add water to my water cooler, I use it to keep my humidity up in the garage. The humidity will drop to 20%. Plants don't seem to care. But if you look closely, the edges are serrated or starting to curl in. But they'll live. Thrive? That's another question. You can be anal about it or not give a fuck, plant is still gonna grow.