I see a lot of misinformation and confusion about VPD, so I made an infographic to explain things

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PulsePeet

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VPD Infographic



To expand on the graphic, since VPD affects how much water evaporates (transpires) from the leaves, it also affects how much the plant drinks.

How much the plant drinks affects how much nutrients it absorbs, which in turn affects photosynthesis and how much the plant grows.

If you have any more questions, I've got a ton of research on VPD so feel free to just ask some questions, also I've got more info on the guide i put together here: www.pulsegrow.com/vpd
 
Moe.Red

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What are the effects of CO2 injection at 1200PPM in a flowering plant? What should the target VPD be if different than atmospheric 400PPM levels?
 
PulsePeet

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What are the effects of CO2 injection at 1200PPM in a flowering plant? What should the target VPD be if different than atmospheric 400PPM levels?

CO2 doesn't really impact ideal VPD, but the ideal temperature to keep your plants. CO2 supplementation raises the ideal temp for photosynthesis, so you need to run your room hotter if you want to take advantage of CO2. With higher temps, you need higher humidity to hit the normal VPD targets. I'll prob write a blog post on CO2 soon!
 
PulsePeet

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CO2 doesn't really impact ideal VPD, but the ideal temperature to keep your plants. CO2 supplementation raises the ideal temp for photosynthesis, so you need to run your room hotter if you want to take advantage of CO2. With higher temps, you need higher humidity to hit the normal VPD targets. I'll prob write a blog post on CO2 soon!
To illustrate this, check this out: http://www.co2science.org/education/reports/greening/thefuture.php


Pic from the article (this isn't for cannabis plants, but an aspen tree, but illustrates the general point).
Screenshot 11
 
Grapefruitroop

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Sure with Co2and high temperature u can grow monsters and fast but in my experience everytime i tryd to flower in the 90s the quality went down big time!Big buds but....sayonara fat tricomes! I found that low temps are much better for the frostiness and the terpenes...still run 1500ppm of gas..maybe im wastin it?! but plants are still really happy in high humidity!!
Everytime i see studies and articles on co2 with plants none has been done on cannabis flowers!!!
Anyway...that pic on the VPD is really cool!
 
Cashmeh

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So my humidity has increased by 15% average since going sealed with co2 at 1800ppm. My leaf temps actually running 75 to 76f now vs no co2 at 76-77f. I put in a window unit inorder to cool the room. Im using a 1000w hid and around 12 inches from the canopy. No leaf burns, one plant has a calcium def it seems. . but shared res so im thinking its something else. . but overall. . i had the same issue with the same strain with no co2. Not really posting to fix that issue, more less wondering what vpd charts would suggest my environment would be. Im hydro at 74-75f water temps. I find the warmer the temps, the faster the metabolism. So now does the vpd charting work when comparing slow and fast growing phenos? Wouldnt a faster growing pheno demand more than a slower one. How does vpd take a plants metabolism into effect? Ive never focused on vpd honestly. Can you convince me why I should plz?

I plan on getting a dehumidifier soon. . but do i need one? I run 40-60 rh before co2. Now since im sealed im 50-70 rh. I was going to drop it down to 50rh with controller. . but thats only because i dont want budrot. I guess im not understanding something. .
 
PulsePeet

PulsePeet

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Sure with Co2and high temperature u can grow monsters and fast but in my experience everytime i tryd to flower in the 90s the quality went down big time!Big buds but....sayonara fat tricomes! I found that low temps are much better for the frostiness and the terpenes...still run 1500ppm of gas..maybe im wastin it?! but plants are still really happy in high humidity!!
Everytime i see studies and articles on co2 with plants none has been done on cannabis flowers!!!
Anyway...that pic on the VPD is really cool!
90's is too high, i don't think you want to go above the high 80s.

Thanks, we worked a lot to try and make VPD easy to understand in a visual way.

I think general studies of plants with a similar photosynthesis type to cannabis (c3) should be pretty applicable, but yeah there aren't a ton of published studies on cannabis, probable due to legality concerns.
 
PulsePeet

PulsePeet

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So my humidity has increased by 15% average since going sealed with co2 at 1800ppm. My leaf temps actually running 75 to 76f now vs no co2 at 76-77f. I put in a window unit inorder to cool the room. Im using a 1000w hid and around 12 inches from the canopy. No leaf burns, one plant has a calcium def it seems. . but shared res so im thinking its something else. . but overall. . i had the same issue with the same strain with no co2. Not really posting to fix that issue, more less wondering what vpd charts would suggest my environment would be. Im hydro at 74-75f water temps. I find the warmer the temps, the faster the metabolism. So now does the vpd charting work when comparing slow and fast growing phenos? Wouldnt a faster growing pheno demand more than a slower one. How does vpd take a plants metabolism into effect? Ive never focused on vpd honestly. Can you convince me why I should plz?

I plan on getting a dehumidifier soon. . but do i need one? I run 40-60 rh before co2. Now since im sealed im 50-70 rh. I was going to drop it down to 50rh with controller. . but thats only because i dont want budrot. I guess im not understanding something. .

You're probably not really getting the full advantage of CO2 if you're running such low temps.

If you want to take full advantage of the CO2 you're using, you need to bump leaf temps up to 82-86, and increase humidity to hit your VPD targets. As you get later into flower you can probably ease off the temps and lower humidities. To prepare for this, you should be running IPM the whole time in veg, and making sure you're keeping an extra clean room. Cannabis isn't the only thing that grows well in those conditions (mold does too).

There's an ideal temp for photosynthesis because there are two competing reactions, and you want them to be at their ideal place. With no CO2, you want temps in the mid 70s for that ideal temp. With CO2, it increases the ideal temp for photosynthesis. If you go too warm, photosynthesis actually slows down.

VPD is still relevant, and the same, regardless of whether you're using CO2 or not, it just indicates that you need to further raise humidity at the higher temp (if supplementing with CO2).

VPD doesn't have to do with metabolism, but with the opening of stomata, which in turn controls flow of water and nutrients through the plant, and flow of CO2 into the plant.
 
Deadstill

Deadstill

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This is why I prefer my propane co2 generator. It's cheap (at least it was) to fill and the extra heat isn't a problem. I run my sealed co2 grow rooms in the 90's and plants thrive, and can take a massive amount of light. Usually only hits those temps in peak summer though, otherwise they're just as happy in the mid to upper 80's. I have to cool my hydro water, though.
 

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