VPD Explained

NotAquaMan 100

NotAquaMan

In Bloom
Ok i was gonna sit down and write out a whole article but i have been lazy so instead instead I'm going to copy paste because im lazy. This is not my work so let start. You may find this very long but its worth the read. Some may want to skip ahead. I have added here and there to what i feel is missing but im going to skip alot of the more in depth stuff to keep this simple.

WHAT IS VPD:
VPD stands for Vapor Pressure Deficit. All gases have vapor pressures, but when we’re growing, we’re interested in water vapor. Vapor pressure deficit is the difference between the pressure of water vapor in 100% saturated air at a given temperature (basically a leaf’s vapor pressure) and the air’s actual vapor pressure. A high VPD value raises a plant’s transpiration rate and increases nutrient movement through the the plant’s xylem, while a low VPD value slows the movement of nutrients through the plant.

1573830809288.png



WHY SHOULD WE CARE:
VPD control is related to the following:
  1. Increasing or decreasing metabolic rate
  2. Improvement in yield quality
  3. Determining plant stresses
  4. Pathogenesis (more on this later)
  5. Carbon dioxide injection (more on this later)
Growers should care about VPD because it impacts yield quality, overall plant vigor, and nutrient utilization. Managing VPD lets me get away with using fewer nutrients, which improves my bottom line. I’m also seeing increased trichome production in the plants, which naturally follows better health.
You reach expert growing level when you learn to manage humidity and VPD. Everybody spends their time managing temperature, nutrients, and whatever else, but the last little thing you learn to manage is humidity, and it is significantly more finicky.
 
NotAquaMan 100

NotAquaMan

In Bloom
HOW TO STEER PLANT GROWTH:
In order to stay on the same page, I should point out that relative humidity (RH) andVPD are inversely related. This means that when relative humidity (RH) is high, VPD is low, and vice versa.
When growers want to know how to steer plant growth, they are interested in maximizing growth. So let’s start with how the plants react to changes in VPD:

  1. The bulk flow of water changes within a plant’s xylem as VPDchanges.
    1. If you have a high VPD, meaning that the RH is low, the plant will increase its transpiration rate and start pulling water faster from the substratein an effort to stay cool and moist.
      1. If the VPD is too high, the plant will become stressed, leading to inefficiencies.
    2. In the same vein, if the VPD is too low, meaning that the RH is high, the transpiration rate will decrease, slowing the flow of water through the plant.
  2. Nutrients follow the flow of water through the xylem and into its various tissues. Nutrientslike calcium primarily move with the bulk flow of water through the arteries of the plant.
    1. Therefore, as VPD rises (and the bulk flow of water increases), nutrient uptake will also rise.
    2. If VPD falls (and the bulk flow of water decreases), nutrient uptake will also fall.
  3. If you’re injecting carbon dioxide, you want the plants’ stomata to stay dilated for as long as possible. Ideally, the stomata would be fully dilated at all times to maximize carbon dioxide use. Plants open and close their stomatato regulate moisture loss.
    1. If you have a high VPD, or low RH, your plants are going to close their stomata to reduce water loss. When the stomata close, you’re not getting adequate gas exchange, and you’re not making the most of your carbon dioxide.
    2. If you have a low VPD, or high RH, plants will open their stomata and let in more carbon dioxide.


Effective VPD control is about balancing gas exchange. There is a “Goldilocks” zone where the plant is getting everything.
If your VPD is too low, then your plants aren’t going to acquire enough nutrients, slowing growth; if your VPD is too high, you’re going to stress the plant and the stomata will close, rendering your extra carbon dioxide ineffective. Like everything else with growing, there’s a Goldilocks zone. One VPD is too high, one VPD is too low, and one VPD is just right. There are charts of a VPD curve with a three way graph of humidity, temperature, and growth. There’s a sweet spot along the center of the chart.

vpd chart.jpg



DO NOT USE THIS CHART ITS REFERENCE ONLY
A VPD chart for a hypothetical plant, image courtesy of Argus Controls. The far left side of the graph is too wet for the plant,
and nutrient uptake is inhibited. The right of the graph is too dry for the plant and stomata close, inhibiting CO2 uptake.

VPD IN DIFFERNT STAGES OF GROWTH:

Ideal KPA ranges for different stages of growth.
Seedling/clone 0.4-0.8
Veg 0.8-1.1
Early flower 1-1.4
Late flower 1.3-1.5
As a matter of fact, most growers use some form of VPD control already, without even knowing it. When you put clones under a dome, you’re keeping the RH high and the VPD low. This, in turn, slows transpiration to a crawl, greatly reducing the stress on the cuttings, which need time to form roots. Typically, most growers will keep their vegetativehumidity a little bit higher as well, which reduces stress.

Domes are a form of VPD control.
Most growers are concerned about the flowering cycle because that’s where the magic happens. You want to keep your VPD relatively high (low RH) during the flowering cycle. If you assume an average flowering cycle of 8 weeks, start with a moderate VPD (medium RH) during the first 3-4 weeks of your flowering cycle, then increase your VPD (lower your RH) towards the end of flowering. This reduces pathogenesis.

One thing you can do when a plant is stressed, say from moving from one room to another, is to raise the humidity. This lowers the transpirational stress and eases their transition into whatever phase or room you have set up. Additionally, HID lights can be stressful for plants, and VPD control gives you the ability to reduce their stress. If you have a dry environment and bright lights towards the top, you’ll see canopy leaves fold in like a taco. Plants do this to reduce light capture and reduce their internal temperature. If you see this happening, you need to ease up on the plants and reduce their stress.
 
NotAquaMan 100

NotAquaMan

In Bloom
CONCERNS WHEN USING VPD:
Pathogenesis is a big issue, which we’ve touched on briefly. The biggest drawback to running a low VPD (high RH) is that you can run into a lot of problems with pathogens if your rooms aren’t clean. As a result, many growers reduce their humidity as much as possible. Some growers brag that their humidity is as low as 20%, which is really bad for the plants and slows their growth.
Homogenizing a room’s environment is a struggle. In my experience, there are always new micro-environments forming in your room due to the nature of working with living organisms. Keeping on top of it all takes a lot of effort.

Good ventilation/circulation is necessary for VPD control.
Accurate sensor readings are also a problem I keep running into. Keeping the environment at your desired setpoint of temperature and humidity can be tricky. Having the right equipment and the right room layout can make a big difference.

WHAT EQUIPMENT DO YOU NEED TO EFFECTIVELY CONTROL VPD:
You’re going to need a humidifier for starters. You want to be able to inject humidity into the room without causing any problems such as being too close to one plant. If you have your humidifiers spraying plants directly with vapor, you will end up with undesirable microclimates which could favor pathogenesis. Personally, I think that ultrasonic humidifiers work best.

You are going to need a way to measure the leaf temperatures in order to accurately calculate VPD. This is where the online charts cause many growers problems and botrytisis becomes of real concern when not taking leaf temps into account. A simple $15 Infrared Temp gun will do the job quite well.

If you’re going to manage VPD, you’ll also want a controller that integrates your humidification and dehumidification systems. You want your controllers set up in such a way that when the lights are off, the humidification setpoints for the dehumidifiers are different if possible. An RH of 10-15% lower at night is ideal but not required.

Paying attention to RH after the lights go out is a big concern. As temperature drop the RH increases (ergo relative humidity) Slowing the temperature drop will aid in the dehumidifiers ability to keep the humidity in range. I would recommend checking humidity from 20-40 mins after lights out to ensure RH is not spiking.

Temperature are also important to control using a temp controller that controls both heating and cooling is ideal. This could be done by controlling fans, heaters, ac etc.

If you don't have all the fancy stuff listed you can still use VPD to to make adjustments to your setup that will improve the VPD for your stage of growth.

It’s important to note that plants are their own internal humidifiers, depending on how many plants are in a room and what stage of growth they’re at. Small plants have less surface area and transpire less. Small plants in a big room will require humidity injection to keep the humidity up, whereas plants at full size don’t need as much humidity injection because they’re already transpiring at an increased rate. When you hit the final stages of growth, you may have to run dehumidifiers to take water vapor out.

Realize that at this level you are doing some serious high performance fine-tuning of your gardening operation. You could be adding a few percent to the final weight of your yield, but it’s going to take some work and you are going to need the proper equipment to measure and control your garden at this level.

The fan system is required because we know botrytis and other fungi are always waiting to pounce. Botrytis establishes itself best between 50 and 70°F, in still air having humidity above 55%RH. We especially want to avoid condensation; this means watch out for uncontrolled temperature drops between daytime and night.

You will also need some type of computer system capable of running a modern spreadsheet program. This is not rocket surgery, but you (or someone you know) will need to know how to use some basic features of a spreadsheet. This is useful to display the logged files from a data acquisition setup, as well as for calculating VPDs and other moisture quantities. Consider it the entry stakes to quantifying and visualizing the performance of your growing operation.



HOW TO CALCULATE YOUR OWN VPD:
If you don't like math your in luck here is a formula you can put into a spreadsheet to do it for you. I use excel personally.

Enter the formula on the next line into spreadsheet cell A10 (copy and paste it).

=3.386*(EXP(17.863-9621/(A7+460))-((A6/100)*EXP(17.863-9621/(A5+460))))

You will type-in 3 values into 3 other cells:

  • Cell A5: The air temperature (A5 in the formula)
  • Cell A6: The air %RH (A6 in the formula)
  • Cell A7: The leaf temperature (A7 in the formula)
Cell A10 will then give you the total VPD for that grow room condition.

Example:

Room temperature= 80°F

Room %RH= 47%

Assumed leaf temperature= 75°F

VPD= 1.34 kPa (a little too dry for best growth)

Calculating Individual Vapor Pressures

For those interested in further exploring water vapor pressure.

Enter the formula on the next line into spreadsheet cell A20 (copy and paste it).

=3.386*(A17/100)*EXP(17.863-9621/(A16+460)))

You will type-in 2 values into 2 other cells:

  • Cell A16: The air temperature (A16 in the formula)
  • Cell A17: The air %RH (A17 in the formula)
Cell A20 will then give you the water vapor pressure for that temperature and %RH combination.

Examples:

1)

Room air temperature= 80°F

Room air %RH= 47%

Water vapor pressure= 1.67 kPa

2)

Leaf temperature= 75°F

%RH of the air inside the leaf = 100%

Water vapor pressure= 3.00 kPa

These 2 examples show the “long way” to calculate the VPD given in the VPD equation section above this one: Subtract the room condition from the leaf condition to come up with the room-to-leaf water vapor pressure deficit (3.00 – 1.67 = 1.33 kPa).

Ok well that the long/short version and I hope this helps. If ya have any questions I will do my best to answer
 
Jewels 2.1K

Jewels

Tilts at Tables
Mathematically, the leaf temp. is theoretically lower.
(For anyone interested) The physical change requires an energy input. As the water changes from a liquid to gas, it takes some heat with it.
The leaf is now cooler, because the process not only requires, but, also steals heat.

Thanks for posting that up Aquama,,,er,, , @NotAquaMan
I like seeing math explained in long sentences. When someone has a firm grasp on a concept, they can make contrasts and correlations, providing insight that the more unfamiliar audience can relate with.

Of note, are the speculations on the relationships between high vpd and C02.
It is my understanding that the stoma, in a CO2 rich atmosphere, would naturally present smaller. Whereas a (natural aspirated) wide open stoma can effectively collect more CO2, a CO2'd stoma would operate more efficiently, collecting ample carbon whilst operating with a more conveniently sized, smaller stomal opening.
Conveniently sized, meaning - "it is fricken hot and dry in here, I seem to be getting all the CO2 I need, so I will just pucker up my pores a bit here and keep giv'n er shit . "

I always kinda assumed that is why you see more 'boom' with CO2 + higher temps.

At those high temperatures is a plant forced into a high-intensity situation,.,? and it is equipped to deal with the potential water loss, by collecting all the carbon it needs from the smaller opening? Should I inject flashlights and Purel into my body? Am I making this up ?


I really like this stuff. Interesting.
The ultra-goldilocks zone.

I remove a lot of heat. I move a lot of air. Coldest leaf I found was 27C. I only find leaf temp measurements varying by 3C all over, with an average of 30 across the canopy. Seems anything over 1500 ppfd or 34C will bleach them out @ 30% rh
 
Bruno8437 309

Bruno8437

In Bloom
Ideal KPA ranges for different stages of growth.
Seedling/clone 0.4-0.8
Veg 0.8-1.1
Early flower 1-1.4
Late flower 1.3-1.5
Maybe I missed it but what is KPA? Is that kPa, kilopascals?

HOW TO CALCULATE YOUR OWN VPD:
If you don't like math your in luck here is a formula you can put into a spreadsheet to do it for you. I use excel personally.

Enter the formula on the next line into spreadsheet cell A10 (copy and paste it).

=3.386*(EXP(17.863-9621/(A7+460))-((A6/100)*EXP(17.863-9621/(A5+460))))
Still being a little dense here but I'm assuming that the output of the formula is the VPD in kPa, correct?
For the formula, do the temperature units matter (F vs C)?

Great write up. I appreciate the thorough explanation and the Goldilocks analogy is spot on.
 
Caddis 75

Caddis

In Bloom
Good information!
In school, things relating to math and science that we “won’t need”, should of concentrated on that part.
The smoking weed part didn’t require as much research, but do to continued overwhelming interest, we’re here now. :headwall:
Dumb as a stump, but my tolerance is scholarly! 🤓
 
NotAquaMan 100

NotAquaMan

In Bloom
Mathematically, the leaf temp. is theoretically lower.
(For anyone interested) The physical change requires an energy input. As the water changes from a liquid to gas, it takes some heat with it.
The leaf is now cooler, because the process not only requires, but, also steals heat.

Thanks for posting that up Aquama,,,er,, , @NotAquaMan
I like seeing math explained in long sentences. When someone has a firm grasp on a concept, they can make contrasts and correlations, providing insight that the more unfamiliar audience can relate with.

Of note, are the speculations on the relationships between high vpd and C02.
It is my understanding that the stoma, in a CO2 rich atmosphere, would naturally present smaller. Whereas a (natural aspirated) wide open stoma can effectively collect more CO2, a CO2'd stoma would operate more efficiently, collecting ample carbon whilst operating with a more conveniently sized, smaller stomal opening.
Conveniently sized, meaning - "it is fricken hot and dry in here, I seem to be getting all the CO2 I need, so I will just pucker up my pores a bit here and keep giv'n er shit . "

I always kinda assumed that is why you see more 'boom' with CO2 + higher temps.

At those high temperatures is a plant forced into a high-intensity situation,.,? and it is equipped to deal with the potential water loss, by collecting all the carbon it needs from the smaller opening? Should I inject flashlights and Purel into my body? Am I making this up ?


I really like this stuff. Interesting.
The ultra-goldilocks zone.

I remove a lot of heat. I move a lot of air. Coldest leaf I found was 27C. I only find leaf temp measurements varying by 3C all over, with an average of 30 across the canopy. Seems anything over 1500 ppfd or 34C will bleach them out @ 30% rh
The IR production of the lighting, evaporative cooling based of transpiration but also forced evaporation from air flow. I feel are the 3 biggest

Something like HPS under normal grow room condition (if you can call it that) would be roughly 2f cooler. Where as LED without added IR would be roughly 5-8f cooler, personally I have seen as much as 10f cooler.

That why I feel people should know thier leaf temps. Its pretty commonly accepted 75-77f leaf temps ideal for photosynthesis. But that could mean very different room temps to help achieve that.

As for CO2 and temps. I'm not convinced yet that a higher temps is better. I'm still leaning towards it allows the plants to tolerate higher and more airid conditions. I run a small sealed room with co2 and haven't seen that it actually improves growth with higher leaf temps 75-78f which is what I usually run.
 
Last edited:
NotAquaMan 100

NotAquaMan

In Bloom
Maybe I missed it but what is KPA? Is that kPa, kilopascals?



Still being a little dense here but I'm assuming that the output of the formula is the VPD in kPa, correct?
For the formula, do the temperature units matter (F vs C)?

Great write up. I appreciate the thorough explanation and the Goldilocks analogy is spot on.
Yes kilopascals.

Yes it will matter use fahrenheit for this calculator.
 
Jewels 2.1K

Jewels

Tilts at Tables
The IR production of the lighting, evaporative cooling based of transpiration but also forced evaporation from air flow. I feel are the 3 biggest
Respiration?
Radiation, convection,
Makes sense
mechanisms-of-heat-loss-n.jpg

igcse-biology-edexcel-217-232-6-638.jpg
igcse-biology-edexcel-217-232-8-638.jpg
igcse-biology-edexcel-217-232-9-638.jpg

Temp can become limiting factor.

Formula 1 cars all struggle to be the first to the finish line. The playing field is levelled by issuing a standard-sized intake on every engine. They can do whatever they want with that car, but they all need to pull air through the same sized hole.
The ratio of air to fuel is difficult to manipulate. @ WOT there is only so much air fitting through that hole (cfm) and there is the correct amount of fuel to mix with that air.

Injecting CO2 is like cheating. It removes limiting factors,,, the plant can now utilize more light, the plant can now utilize more heat.

A plant ,too cold simply will not grow. Warm it up - metabolizes faster.
Warm it up a little more -it metabolizes even faster.
Yes, there is a point of where the temp will kill the plant/process- yet that plant has no choice: to ever increase the rate of metabolism,,, until it hits that wall.
 
NotAquaMan 100

NotAquaMan

In Bloom
Respiration?
Radiation, convection,
Makes sense
View attachment 8408

View attachment 8409
View attachment 8410
View attachment 8411

Temp can become limiting factor.

Formula 1 cars all struggle to be the first to the finish line. The playing field is levelled by issuing a standard-sized intake on every engine. They can do whatever they want with that car, but they all need to pull air through the same sized hole.
The ratio of air to fuel is difficult to manipulate. @ WOT there is only so much air fitting through that hole (cfm) and there is the correct amount of fuel to mix with that air.

Injecting CO2 is like cheating. It removes limiting factors,,, the plant can now utilize more light, the plant can now utilize more heat.

A plant ,too cold simply will not grow. Warm it up - metabolizes faster.
Warm it up a little more -it metabolizes even faster.
Yes, there is a point of where the temp will kill the plant/process- yet that plant has no choice: to ever increase the rate of metabolism,,, until it hits that wall.
100% agree but i would say the transpiration rates are what drives the exchange not so much the size of the stomata. I guess it actually goes hand in hand a Ideal VPD will give the most ideal gas exchange also. There are so many variables that you really need to dial in your grow to see the biggest benefits from VPD. Like you say temp and light also play a huge part. I would say every variable in a grow play a part to an extent. I can admit I have probably been a little bias on the humidity factor taking for granted that others factor are ideal when there is as you point out a lot more it.
 
spyralout 8.1K

spyralout

🌱🌿🌲🔥💨
Staff member
@NotAquaMan

Just want to make sure that this calculator works right for on the fly calculations: Pulse Grow VPD (you have to subscribe to get the excel doc):

Using the above values, I get the same value under Leaf VPD:
1589092336014.png

You can also get growth stage relative charts based on Leaf Temp difference from Room Temp:

Clone
1589092663840.png

Veg
1589092688109.png

So for example in veg, if I'm at 86F room temp and 81F leaf temp, then I should be between 60-62% RH (1-1.1 kPA)?
 
NotAquaMan 100

NotAquaMan

In Bloom
@NotAquaMan

Just want to make sure that this calculator works right for on the fly calculations: Pulse Grow VPD (you have to subscribe to get the excel doc):

Using the above values, I get the same value under Leaf VPD:
View attachment 9954

You can also get growth stage relative charts based on Leaf Temp difference from Room Temp:

Clone
View attachment 9957

Veg
View attachment 9958

So for example in veg, if I'm at 86F room temp and 81F leaf temp, then I should be between 60-62% RH (1-1.1 kPA)?
You got it
 
spyralout 8.1K

spyralout

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Staff member
I have a little CVS cool mist humidifier from last year. It worked last night, I was in the zone! But the tank is tiny, maybe not even a gallon, so on full blast, exhaust fan off, it can raise the humidity 20% within an hour, but it runs out in the middle of the night. Any suggestions for an affordable one that has more of a tank? IDK what the difference is between cool mist and ultrasonic, or are they the same?
 
NotAquaMan 100

NotAquaMan

In Bloom
I have a little CVS cool mist humidifier from last year. It worked last night, I was in the zone! But the tank is tiny, maybe not even a gallon, so on full blast, exhaust fan off, it can raise the humidity 20% within an hour, but it runs out in the middle of the night. Any suggestions for an affordable one that has more of a tank? IDK what the difference is between cool mist and ultrasonic, or are they the same?
what sized room?
 
NotAquaMan 100

NotAquaMan

In Bloom
This would likely do ya. But if in a tent its better to humidify the room the tent is in, this way you can keep running your air exchange. I also run mine on controllers.

Hmm n not allowing amazon links?
 
spyralout 8.1K

spyralout

🌱🌿🌲🔥💨
Staff member
This would likely do ya. But if in a tent its better to humidify the room the tent is in, this way you can keep running your air exchange. I also run mine on controllers.

Hmm n not allowing amazon links?
It does. Sometimes it comes out looking funny. Try posting again, straight link in the body.
 
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