VPD (vapor pressure deficit)

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Aqua Man

Aqua Man

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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.

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 (kilopascals) 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.

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
 
Last edited:
oldskol4evr

oldskol4evr

12,202
438
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.

View attachment 910696

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.

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.

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
thats a lot of damn typing bro,great work
 
FourthCity

FourthCity

782
143
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.

Again, like aquaman said, dont use this chart to calculate VPD, the numbers are not accurate. Once you have used the algorithm to calculate your vpd the chart is useful for getting a feel of what direction temp and humidity need to move to in order to reach the desired level. Its a little hard to see in the post so I made another version zoomed on the operational temps with Aquaman's info for the correct ranges on top.

vpd sweet spot.jpg
 
Aqua Man

Aqua Man

Staff
Supporter
21,312
638
Again, like aquaman said, dont use this chart to calculate VPD, the numbers are not accurate. Once you have used the algorithm to calculate your vpd the chart is useful for getting a feel of what direction temp and humidity need to move to in order to reach the desired level. Its a little hard to see in the post so I made another version zoomed on the operational temps with Aquaman's info for the correct ranges on top.

View attachment 911280

Good stuff.

This chart would be indicative of HPS. It's taking into account that leaf temps are 2f cooler than air temps. With led this difference can be as high as 10f reported and I have seen personally 8f difference. This is why it's important to calculate your leafs temps for each grow room. You can end up with very different needs based on this.
 
Beachwalker

Beachwalker

7,039
313
Just found this, I've been looking at VPD info for days, but this is the most easily understood explanation Ive found yet! This oughta have a pin put in it! Thank you for posting this
 
MIMedGrower

MIMedGrower

17,207
438
Again, like aquaman said, dont use this chart to calculate VPD, the numbers are not accurate. Once you have used the algorithm to calculate your vpd the chart is useful for getting a feel of what direction temp and humidity need to move to in order to reach the desired level. Its a little hard to see in the post so I made another version zoomed on the operational temps with Aquaman's info for the correct ranges on top.

View attachment 911280


I think you just proved this silly chart was put out by a controller company to sell stuff to those who dont know better.
 
needshelpguy

needshelpguy

674
93
Again, like aquaman said, dont use this chart to calculate VPD, the numbers are not accurate. Once you have used the algorithm to calculate your vpd the chart is useful for getting a feel of what direction temp and humidity need to move to in order to reach the desired level. Its a little hard to see in the post so I made another version zoomed on the operational temps with Aquaman's info for the correct ranges on top.

View attachment 911280
Been waiting for you to do this hahah nice man cheers
 
C

Cushdy

17
3
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.

View attachment 910696

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.

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.

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

Actually haven't finished reading your post yet, but total thumbs up from me!!
VPD is that vital missing element I have to master. 1 of the things I'm having problems with on this grow. Is I forgot to defoliate b4 I flipped them. Downside I believe is the reason 2 much r/h around the floor level. Despite good air movement? So ok a Bit over kill I know but im using 4 temp/ r/h devices. The dehuey has 1 built in, but I've got 2 hanging in the room with probes hanging also so each device is giving 2 readings, plus yep 1 sitting on the floor. Talk about micro climates still existing!! Ok so obviously when extraction kicks in, & as I'm using a decent unit it sucks the shit off the floor so r/h & temps drop. But still what I'd consider too much variation in environmental change going on?! I built an 8ft x 8ft by 7ft high room. But as I'm using co2 it was all about getting the heat level right versus using the best type of lighting I could afford to achieve that. so I switched to using CMH ballast's this time. Hoping that these will do for now? So using 2 x630w de, 3x 315w , only 1 Phillips 3k so far (need$ want more!) For cost I paired a unis regulator with a trolmaster beta 8 controller, hooked up to an ndir sensor. This doses the girls according to what they want if using fuzzy logic mode. The ppms are always spot on. I played around with the cycles endlessly trying to balance it all ? But @ the mo we're in week 5 of flower & I've found that by letting the co2 run for 60 mins. Then 20 mins into the co2 cycle the fresh intake comes on bringing fresh air down floor level, in the middle via ducting, I read this was better than up anywhere randomly above the lights? My thoughts are to get underneath the canopy stir up any co2 sitting about? Theres also 2 oscillating fans @ opposite corners on/ off every 15m, with 2 little oscillating fans that I can clip to the girls or leave on the floor? I read it's all about a tornado effect you want to get with air movement. So I set the extraction to clean it all out for 20 mins after the co2 /intake. Now all I need to learn & get my head around is the VPD & fine tune everything? Lol hopefully its sooner rather than later. So I found this . But as I'm not so bright as you guys. I've not attempted to verify it against the actual physical chart like @Aquaman posted?
Is seems to be the real deal??
But I have to really get the IR temp gun to get the vpd exactly right & get the chance to take a variety of readings around different canopy levels. Also need to get the distance of these lights right? As pushed it too hard & stupidly damaged couple up & coming beautiful buds. Fortunately caught it early enough for it to be ok..ish. well in my book that cant & should never happen, but I'm still learning!? Do you think that the reason I've always got this higher level of r/h all around the floor level is coz of not thinning out the extra greenery while they were still in veg? I didn't wanna strip them out once in flower incase I went too hard & stressed them into hermies or worse!? I'm growing in a quality soil with clay pebbles at the base, good ratio of perlite. B4 I had used 5 gallon felt pots but I didn't like that idea after I had novice issues that meant they were in veg too long than I had planned. This time things are more under control I'm using 3 gallon felts & better sized plants with LST. Unfortunately because the veg room has limited height . Using a food from Holland called BaC got all the root stim/ bloom stim, boosters, 1 component grow- 1,, bloom etc plus decent reliable Cf stick & couple Ph pens. Make sure water temp is in the window when I feed the girls. Also better late than never I realised being outside on a concrete base garage floor their buts were getting too cold! so they're all on bubble wrap now for insulation & hopefully they're all happier for it? I'll finish reading your article to the end, but knowing me I'll still be bothering you with questions?? Its all about this vpd I know it now & actual information from guys like you who have the real knowledge is invaluable to me. I can take a few pics or do a vid if you think that'll help? Any advice you can give me I'd be well grateful. This is my passion & life 4eva!
 
Aqua Man

Aqua Man

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Actually haven't finished reading your post yet, but total thumbs up from me!!
VPD is that vital missing element I have to master. 1 of the things I'm having problems with on this grow. Is I forgot to defoliate b4 I flipped them. Downside I believe is the reason 2 much r/h around the floor level. Despite good air movement? So ok a Bit over kill I know but im using 4 temp/ r/h devices. The dehuey has 1 built in, but I've got 2 hanging in the room with probes hanging also so each device is giving 2 readings, plus yep 1 sitting on the floor. Talk about micro climates still existing!! Ok so obviously when extraction kicks in, & as I'm using a decent unit it sucks the shit off the floor so r/h & temps drop. But still what I'd consider too much variation in environmental change going on?! I built an 8ft x 8ft by 7ft high room. But as I'm using co2 it was all about getting the heat level right versus using the best type of lighting I could afford to achieve that. so I switched to using CMH ballast's this time. Hoping that these will do for now? So using 2 x630w de, 3x 315w , only 1 Phillips 3k so far (need$ want more!) For cost I paired a unis regulator with a trolmaster beta 8 controller, hooked up to an ndir sensor. This doses the girls according to what they want if using fuzzy logic mode. The ppms are always spot on. I played around with the cycles endlessly trying to balance it all ? But @ the mo we're in week 5 of flower & I've found that by letting the co2 run for 60 mins. Then 20 mins into the co2 cycle the fresh intake comes on bringing fresh air down floor level, in the middle via ducting, I read this was better than up anywhere randomly above the lights? My thoughts are to get underneath the canopy stir up any co2 sitting about? Theres also 2 oscillating fans @ opposite corners on/ off every 15m, with 2 little oscillating fans that I can clip to the girls or leave on the floor? I read it's all about a tornado effect you want to get with air movement. So I set the extraction to clean it all out for 20 mins after the co2 /intake. Now all I need to learn & get my head around is the VPD & fine tune everything? Lol hopefully its sooner rather than later. So I found this . But as I'm not so bright as you guys. I've not attempted to verify it against the actual physical chart like @Aquaman posted?
Is seems to be the real deal??
But I have to really get the IR temp gun to get the vpd exactly right & get the chance to take a variety of readings around different canopy levels. Also need to get the distance of these lights right? As pushed it too hard & stupidly damaged couple up & coming beautiful buds. Fortunately caught it early enough for it to be ok..ish. well in my book that cant & should never happen, but I'm still learning!? Do you think that the reason I've always got this higher level of r/h all around the floor level is coz of not thinning out the extra greenery while they were still in veg? I didn't wanna strip them out once in flower incase I went too hard & stressed them into hermies or worse!? I'm growing in a quality soil with clay pebbles at the base, good ratio of perlite. B4 I had used 5 gallon felt pots but I didn't like that idea after I had novice issues that meant they were in veg too long than I had planned. This time things are more under control I'm using 3 gallon felts & better sized plants with LST. Unfortunately because the veg room has limited height . Using a food from Holland called BaC got all the root stim/ bloom stim, boosters, 1 component grow- 1,, bloom etc plus decent reliable Cf stick & couple Ph pens. Make sure water temp is in the window when I feed the girls. Also better late than never I realised being outside on a concrete base garage floor their buts were getting too cold! so they're all on bubble wrap now for insulation & hopefully they're all happier for it? I'll finish reading your article to the end, but knowing me I'll still be bothering you with questions?? Its all about this vpd I know it now & actual information from guys like you who have the real knowledge is invaluable to me. I can take a few pics or do a vid if you think that'll help? Any advice you can give me I'd be well grateful. This is my passion & life 4eva!
Thank you for the kind words. I think the humidity at floor level may be the increased watering after the plants get bigger from both transpiration and evaporation from the soil/felt pot combo

A vid may help. I will do my best to answer your questions to the best of my ability
 
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