THE TRUTH ABOUT WHAT VPD RANGE YOU SHOULD USE.

So a couple months ago, I asked the question here What is VPD and why should I care? Got some good info.
Did a shit load of research. A software engineer, I wrote some. See my screen shot.
I test a probe from my friends at Sensor Push, and use that to control smart plugs to AC, Humidifier and Dehumidifer in my tent. So far I can keep it in the range I specify (see the picture) 99+ percent of the time (the figure you see in the picture is from the log data. The sensor reads, and I write a record, once a minute).

This is still in the playing stage, but I've got a couple test autos in there showing PHENOMENAL growth.

I will share this on github if anyone is out there that can use that and maybe contribute!

ALL THAT TO SAY THIS.

OK, guys, from experience please. What is the optimal VPD range? I hear a LOT of people say .6 to 1 for veg, and up to 1.5 for flowering.
Now, I understand that higher VPD means lower humidity for flowering. OK. We only care about humidity because of mold. Mold grows in the dark. We don't care about VPD at night.

I watch this video (can I put this here? nice PhD lady explaining VPD in a way that makes sense, and she says it's 1.0 no matter what. Considering the science (which mine is shaky) that really makes sense, it's like the pressure is in balance at that point. Honestly, I'm not a professor, but I don't think physics changes during flowering stage, which is basically what someone with different ranges is suggesting.

Isn't it? Chime in. I hear a LOT of interest in the answers from those in the know.

heres the result

I will throw in my 2 cents here...In veg and early flower...I run 1.0 to 1.4.....In late flower when the buds are really dense i am more in the 1.6 range....never had mold or bud rot...this is just my method and what works for me.

Running at over/under the "suggested" VPD makes the plant either uptake faster or slows it down.
By some minor manipulation of the VPD in bloom.
I am "in effect", pushing them a bit.
I do this to maximize the reaching for the potential's of the strain...

thats an explantion that makes sense. thanks man

Dr. Who said:

Running at over/under the "suggested" VPD makes the plant either uptake faster or slows it down.
By some minor manipulation of the VPD in bloom.
I am "in effect", pushing them a bit.
I do this to maximize the reaching for the potential's of the strain...

Click to expand...

I have a thought (because this is what mine does) that if you cycle between 1.0 and 1.5 you get the best of both worlds.

VPD is just how you set ur humidity basically right? relative to the temperature.

what temps are you targeting? lets say ur adding co2 to your grow. your going to want higher temps. something around lets say 85. so in this case ur targeting 85 temps you would use the VPD chart and adjsut ur humidity accordingly.

its like a wet rag. higher the VPD the faster the rag will dry out. a lower VPD at 80 degrees will evapoate a rag slower than a higher VPD at a temp of 50 degrees. so if you think about how this affects the grow there things like traspiration going thru the plant if ur VPD is pushing lots of water thru the plant (drying the rag quickly) i like to lower my nuitreints quite a bit or they seem to get nuirteint burn. so VPD plays into quite a bit other than just mold.

really cool system thing u made. im learing the arduino at the moment and im gunna make my vpd controler out of that. trying to learn this coding stuff durring corona. python basically. throw the link up on github if u dont mind id just like to have something to look at

JonCreighton said:

VPD is just how you set ur humidity basically right? relative to the temperature.

what temps are you targeting? lets say ur adding co2 to your grow. your going to want higher temps. something around lets say 85. so in this case ur targeting 85 temps you would use the VPD chart and adjsut ur humidity accordingly.

its like a wet rag. higher the VPD the faster the rag will dry out. a lower VPD at 80 degrees will evapoate a rag slower than a higher VPD at a temp of 50 degrees. so if you think about how this affects the grow there things like traspiration going thru the plant if ur VPD is pushing lots of water thru the plant (drying the rag quickly) i like to lower my nuitreints quite a bit or they seem to get nuirteint burn. so VPD plays into quite a bit other than just mold.

really cool system thing u made. im learing the arduino at the moment and im gunna make my vpd controler out of that. trying to learn this coding stuff durring corona. python basically. throw the link up on github if u dont mind id just like to have something to look at

Click to expand...

I'm still tweaking it.

I live in a hot, humid climate, esp. during the summer. With (xxx) plants and a handful of SF 4000 LEDs going, there's no telling what will happen.

Im growing no til organic so dont worry about nute burn in theory. but thats an intersting point.

JonCreighton said:

VPD is just how you set ur humidity basically right? relative to the temperature.

what temps are you targeting? lets say ur adding co2 to your grow. your going to want higher temps. something around lets say 85. so in this case ur targeting 85 temps you would use the VPD chart and adjsut ur humidity accordingly.

its like a wet rag. higher the VPD the faster the rag will dry out. a lower VPD at 80 degrees will evapoate a rag slower than a higher VPD at a temp of 50 degrees. so if you think about how this affects the grow there things like traspiration going thru the plant if ur VPD is pushing lots of water thru the plant (drying the rag quickly) i like to lower my nuitreints quite a bit or they seem to get nuirteint burn. so VPD plays into quite a bit other than just mold.

really cool system thing u made. im learing the arduino at the moment and im gunna make my vpd controler out of that. trying to learn this coding stuff durring corona. python basically. throw the link up on github if u dont mind id just like to have something to look at

Click to expand...

I'm not sure that will work

I guessing you have seen a vpd chart, but you are asking temps and rh so?
Here, if you haven't seen onehttps://www.dimluxlighting.com/knowledge/vapor-pressure-deficit-vpd-calculator/

I have a plan to build a 4x8 insulated air-sealed room in the corner of my shop. I have an a/c and a dehumidifier for it, an Uno R3 and some relays to control a/c devices. I haven't bought any sensors yet though.

Stumay111 said:

I have a thought (because this is what mine does) that if you cycle between 1.0 and 1.5 you get the best of both worlds.

Click to expand...

Pick one way and avoid swings in RH at all cost's.

It's changing RH that starts off bad PM attack's. More so then just high RH.

I run pre set temp zones and keep my RH at about 50-55. Only a 5 point swing and that's what deheuy cycles at..

Stumay111 said:

I don't know what a PM attack is... sorry.

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the dreaded POWDWERY MILDEW

Roy O'Bannon said:

I guessing you have seen a vpd chart, but you are asking temps and rh so?
Here, if you haven't seen onehttps://www.dimluxlighting.com/knowledge/vapor-pressure-deficit-vpd-calculator/

I have a plan to build a 4x8 insulated air-sealed room in the corner of my shop. I have an a/c and a dehumidifier for it, an Uno R3 and some relays to control a/c devices. I haven't bought any sensors yet though.

Click to expand...

I wrote software to do this (I'm a programmer). It's on GitHub for the sharing.
I am using SensorPush sensors. They have a gateway that allows my software to poll the device via an API.
I am using TLink Smart Plugs to control my devices. These also have an API that allows my software to turn the plugs on and off.

I have an 8x8 tent. I have 3 Spider Farmer 4000 lights. I live on the Gulf Coast, hot and humid.

I have 2 12000 BTU portable AC units. One runs constantly to keep temp/hum to a certain level. The other is switched on and off to maintain VPD, along with a dehumidifier and a humidifier. The trick is to find equipment that resumes after a power cut.

I can maintain my tent at 1.0-1.4 VPD all day and night. 100%.

This is a working solution. Message me if you want more details.

ricman said:

I will throw in my 2 cents here...In veg and early flower...I run 1.0 to 1.4.....In late flower when the buds are really dense i am more in the 1.6 range....never had mold or bud rot...this is just my method and what works for me.

Click to expand...

I'm wondering if you agree with Dr. Who about VPD swings. With my system, I am running up and down between 1.0 and 1.3 or so. This doesn't seem horrid to me. I cannot maintain a true constant with the equipment/environment I have. Besides, I'm holding with my theory that if plants respirate best at 1.0, exposing them to that VPD would benefit them.

Just trying to see what works for everyone ahead of time.

I think almost everyone’s temp and humidity will fluctuate to a point...A/C untits and dehumidifiers have set points or a range that they turn on and off at....creating highs and lows... so as your temp and humidity fluctuate so will your VPD... your just trying to keep it in the sweet spot as far a temp .. humidity and VPD goes... and if from time to time if your VPD is a bit out of range... it’s really no big deal...IMO.... I had monster grows 20 years ago before I ever knew what VPD was....
just one mans opinion
Good luck to you.

I will keep all posted.

Stumay111 said:

I wrote software to do this (I'm a programmer). It's on GitHub for the sharing.
I am using SensorPush sensors. They have a gateway that allows my software to poll the device via an API.
I am using TLink Smart Plugs to control my devices. These also have an API that allows my software to turn the plugs on and off.

I have an 8x8 tent. I have 3 Spider Farmer 4000 lights. I live on the Gulf Coast, hot and humid.

I have 2 12000 BTU portable AC units. One runs constantly to keep temp/hum to a certain level. The other is switched on and off to maintain VPD, along with a dehumidifier and a humidifier. The trick is to find equipment that resumes after a power cut.

I can maintain my tent at 1.0-1.4 VPD all day and night. 100%.

This is a working solution. Message me if you want more details.

Click to expand...

id love to know how you started automating you tent. mind if i pm?

Let the plants tell you what the right VPD is.

Certain chemical reactions in the plant are more efficient in a pretty narrow leaf temperature range. If the plants are getting everything they need that's when you can raise the leaf temperature a few degrees, either by slowing extraction or turning up/moving the lights (if you've got a PAR meter). If the VPD is too high or the leaf temperature gets too high (above 104f certain enzymes involved in photosynthesis lose shape and no longer work), the plant tries to protect itself and shuts down, what temperature exactly that happens depends on the cultivar, plant size and a host of environmental variables.

Impossible for me most of the time to increase my VPD ( too much vegetation in room)
I never get rot (4 ceiling fans help)
And I haven't noticed a much steeper pot weight dip during summer temperatures with higher VPD.
I'm getting excellent yields (600gr/m2+) of extremely sticky bud too.
VPD is not the be all, end all people think.

I personally think WIND velocity is one of the most important ways to prevent rot and somehow affects VPD greatly too

Here's an interesting quote with source about VPD, it explains the plants find balance with high humidity.

"A dry environment will simply pull more transpiration out of the plant itself, drawn up through the roots. If the pressure imbalance goes on too long, and the atmosphere is too dry, plant's close off their stomata as a final protective mechanism to reduce moisture loss. "

"In a relatively humid environment, the VPD is low. The vapor pressure internally and externally even out. Finding a perfect balance of humidity and temperature, guarantees water and nutrient movement, but prevents aggressive waste seen in low relative humidity environments. Plants are generally less stressed, and more receptive to CO2 in the grow room."

SOURCE: https://www.cannabistech.com/articles/mastering-vpd-control-for-plentiful-harvests/




Here's the perfect healthy plants in a low VPD environment. (taken just now as with the above graph)

Tht_Blk_Guy27 said:

id love to know how you started automating you tent. mind if i pm?

Click to expand...

sorry for the late response. sure. might take a while to reply. i don't get on here much.