What is VPD (Vapor Pressure Deficit) and How To Calculate It

What is VPD, how to calculate it, and what is the optimal range?
08 January 2021
13 min read
What is VPD (Vapor Pressure Deficit) and How To Calculate It

Contents:

When growing indoors you are basically controlling every single aspect of the growing conditions, such as the relative humidity and temperature. These elements may end up affecting how your plants perform their basic processes and that’s where VPD comes in. Vapor pressure deficit is a technique used to adjust the temperature and humidity to optimal values, boosting your plants' performance, and achieving maximum plant growth.

1. What Is VPD?

VPD means vapor pressure deficit and refers to the amount of water in the form of vapor in the air. As you may know, air is made up of many gases including (approximately) 78% nitrogen, 21% oxygen, and 1% other gases; one of these other gases that compose air is water vapor, known as vapor pressure.

 

Vapor pressure deficit: what is it?

Vapor pressure deficit refers to the amount of water in the form of vapor in the air.
 

Air can only hold a certain amount of water vapor at a certain temperature before it starts turning back into liquid form (in the form of rain for example), this maximum amount of water vapor that air can hold is called SVP aka saturation vapor pressure, and, as the air gets hotter, that amount increases (so the SVP increases).

When the air cools down the saturation vapor pressure decreases, this means that the air cannot hold as much water vapor as before.

That’s why, for example, there is dew all over after a cool morning; because the temperature cools down the air and it can’t hold as much water vapor as it was holding. 

2. How VPD Affects Your Cannabis Plants

If you were looking for this article it probably means that you already know that VPD is important when growing indoors. Because you have control over the growing conditions, you might be able to provide almost perfect conditions for your plants to grow in and get overall better results but can also have bad consequences if not used properly...  so how does VPD influence cannabis plants?

Stomata opening

As VPD increases, the plant’s stomata get smaller because they’re trying to reduce water loss. This means that they won’t die from dehydration due to excessive transpiration but photosynthesis may be slower.

 

Vapor pressure deficit: stomata

VPD can be a useful tool but can have bad consequences if not used properly.

CO2 uptake

As the stomata get smaller they absorb less CO2. Whereas if VPD decreases and stomata open, they absorb more CO2. This process allows you to control how much CO2 your plants get for proper photosynthesis.

Transpiration

Based on the properties of water, it tends to diffuse from areas with a high concentration of moisture to areas with a low concentration.

So as VPD increases, your plants will be able to transpire faster due to the difference in vapor pressure between the leaf and the air. 

Nutrient absorption at the roots

Additionally, as the VPD increases and transpiration increases, the roots will be able to absorb more nutrients.

Plant stress

On the other hand, as the VPD increases, there are more forces acting on your whole plant (from the roots to the leaves) so your plant will experience more stress.

As you can see, VPD can also have bad consequences if not used properly. It’s a powerful tool that should be adjusted for every stage of plant growth. You might be thinking that you’re improving your harvest but in fact, it might not be the deal.

3. How To Calculate VPD

Calculating the VPD in your grow space is quite simple, you just need to know the temperature, relative humidity, and the saturated vapor pressure (SVP) for the temperature so here’s a chart with the relation between temperature and SVP.

 

Temperature (°C) / SVP

TempSVPTempSVPTempSVPTempSVPTempSVP
1 °C6577 °C100213 °C149719 °C 219725 °C3167
2°C 7068 °C107314 °C159820 °C233826 °C3361
3°C7589 °C114815 °C170521 °C248627 °C3565
4°C81310 °C122816 °C181822 °C264328 °C3779
5°C87211 °C131217 °C193723 °C280929 °C4005
 6 °C93512°C140218 °C206424 °C298330 °C4242

 

Once you’ve gathered the information you need, you need to apply it in the following formula:

 

VPD= ((100-RH) ÷ 100) * SVP

 

For example, if the temperature in your grow room is 26°C you’ll have to look for the SVP in the table so for 26°C the SVP is 3361. Now that you have the saturated vapor pressure value you just need the relative humidity which in this example will be 65%, so now it’s just a matter of doing the math.

 


VPD= ((100-65) ÷ 100) * SVP

VPD= (35/100) * 3361

VPD= 0.35 * 3361 

VPD= 1176.35 Pascals

 

Now that you have the value in Pascals you have to convert it to hectopascals, luckily it’s super simple, it’s just a matter of dividing it by 100 so:

 

1176.35 Pascals ÷ 100 ≈ 11.8 hectopascals

 

Here’s a table that will help you visualize what would be optimum values (blue), less-than-ideal (light blue), and bad values (pink).

 

Vapor pressure deficit: how to calculate

VPD table in hectopascals.
 

As you can see, in the conditions mentioned above the result was 11.8 hPa which should be okay but by adjusting the temperature and humidity a little bit you could improve your conditions, resulting in bigger and healthier plants.

4. Ideal VPD For The Stages Of Plant Growth

So now that you know how to calculate the VPD and how it can affect your plants, it’s time to know the ideal VPD. In general, the ideal VPD is between 0.8-1.2 kPa (8-12 hPa).

As you may already know, cannabis plants have different stages of plant growth so to get the perfect plant growth it’s essential you adjust the conditions to get the best VPD for each stage.

 

Vapor pressure deficit: how to calculate

VPD table in kilopascals.
 

Have in mind that it’s highly recommended to test the conditions before applying it to your entire grow room to avoid problems and this chart has been modified to fit temperatures of up to 30 °C. You can look for the complete chart online.

In this chart, dark blue represents optimum values, blue less-than-ideal, light-blue acceptable, and pink an unacceptable VPD.

Ideal VPD For Clones

Due to clones being baby plants they cannot handle a lot of stress because they still haven’t fully developed their roots so you should aim for a high humidity and a VPD closer to the lower end which would be as close as 0.8 kPa as possible.

Ideal VPD For The Vegetative Stage

In the vegetative stage, your cannabis plants should be bigger and sturdier so you can reduce the humidity a little bit to increase the VPD.

By doing this you’ll be also increasing water and nutrient absorption, just make sure you don’t increase the VPD too much because the stomata will close and your plants will absorb less CO2 which is super important during the vegetative stage.

 

Vapor pressure deficit: ideal vpd

The ideal VPD when transitioning should start at 1.0kPa and slowly increasing until you reach 1.2-1.5kPa for the flowering stage.
 

This happens because it’s one of the main ingredients for the compounds that make the structural parts of your plants, so the ideal VPD for the vegetative stage is around 1.0kPa.

Ideal VPD For The Flowering Stage

Once your plants have reached the flowering stage, they are strong enough to withstand a higher VPD but the flowers are quite sensitive so you need to avoid high humidity while still increasing the VPD, so the ideal VPD for the flowering stage should be around 1.2-1.5kPa.

5. How To Change The VPD

There are several ways to change the VPD, this can be done by changing:

  • Temperature;
  • Humidity
  • Or light intensity.

So here are a couple of ways you can do it easily!

Temperature

Increase VPD: Running a heater or reducing the AC results in an increase of the VPD because the air heats up and can hold more water vapor.

Decrease VPD: Running the AC results in a decrease of the VPD because the air cools down and won’t be able to hold as much water vapor.

Humidity

Increase VPD: Running a humidifier decreases the VPD because the amount of water vapor in the air has increased.

 

Vapor pressure deficit: how to change vpd

Increasing or decreasing the humidity is a way to change the vpd in your grow room.
 

Decrease VPD: Running a dehumidifier increases the VPD because the amount of water vapor in the air has increased.

Light Intensity

Increase VPD: Increasing the intensity either by lowering the lights or adding more lights results in an increase of the leaf temperature, thus increasing VPD.

Decrease VPD: Decreasing the light intensity either by moving the lights further or removing light fixtures results in a decrease of leaf temperatures, thus decreasing the VPD.

6. In Conclusion

The Vapour pressure deficit may seem only suited for professional growers but everyone can do it as long as you have the proper equipment. This is the best way to provide perfect conditions for your plants to grow in, resulting in healthier plants and bigger yields.

If you have any tips that will help beginner growers take their grow cycles to the next level, feel free to leave a comment in the comment section below!

 

EXTERNAL REFERENCES

  1. Plant responses to rising vapor pressure deficit. - Grossiord, C. & Buckley, T.N. & Cernusak, L.A. & Novick, K.A. & Poulter, B. & Siegwolf, R.T.W & Sperry, J. & McDowell, N. (2020). 
  2. An ecological study of vapor pressure deficit. - Huffaker, & Barton, Carl. (2021). 
  3. Prediction of Vapor Pressure Deficit in Greenhouse Environment.  - Shamshiri, Redmond. (2014). 
  4. Relative humidity or vapor pressure deficit. Ecology. - Anderson, D.B.. (1936).  

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08 January 2021
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