Vapor pressure deficit (VPD), or VPD chart, is an environmental factor you may already know about or perhaps have heard about from experienced growers.
VPD aka Vapor pressure deficit refers to the ability of the atmosphere to take out moisture from your plants and the capacity of moisture that it can hold. It sounds closely related to humidity, and that’s because it is.
Often, growers only focus on the humidity of their growing environment, and although it’s important, it’s mistaken as the factor that drives the growth of cannabis.
This article will discuss the importance of VPD to plant growth, the difference between humidity and VPD, and lay out a VPD chart that you can use.
- How to calculate air VPD, aka vapor pressure deficit
- How to calculate leaf VPD (vapor pressure deficit):
VPD is a factor of the atmosphere in your growing environment that plays a pivotal role in the transpiration process of your cannabis.
It’s measured in pounds per square inch (psi) or kilopascals (kPa). All of this will begin to sound rather technical, but essentially the key thing to understand is how everything connects.
Different environmental factors in your grow area contribute to the biological processes in your cannabis plants, affecting their growth and development.
To learn how to counteract those factors and enhance your plant yield, download our Marijuana Grow Bible below:
In particular, transpiration is vital for growing cannabis plants; it’s how cannabis converts liquid water in the soil to gas that it then releases through the stomata found in the leaves.
This functions as a way to cool itself down in humid conditions, but more importantly, it allows for minerals and water to be pumped throughout the plant.
Healthy transpiration in your cannabis will result in faster growth as nutrients from the soil are being brought to the plants’ leaves which fuels photosynthesis. And for healthy transpiration to occur, you need to sustain a certain VPD level.
This is where environmental factors connect with your cannabis plants biological processes. Transpiration will only occur if you have the right VPD level in your atmosphere.
As we’ve mentioned before, VPD , or vapor pressure deficit is the ability of the atmosphere to take moisture out of your plants and the capacity of moisture that it can hold.
A VPD level greater than 1.0kPa is a high VPD and will demand too much moisture from your plants.
This will cause your plants to appear malnourished even though they have enough nutrients and water.
Yellow or brown spots on leaves
The problem is: your cannabis can’t keep up with the demand from the atmosphere.
On the other side of the spectrum, 0kPa means the atmosphere is 100% saturated, which means your plants can’t effectively transpire, and as a result, it slows their growth.
This is why it’s crucial for you as a grower to find the sweet spot for your cannabis and that value changes as they develop.
Thankfully, the VPD chart below can help.
How to calculate air VPD, aka vapor pressure deficit
- Figure out the SVP
- SVP = 610.78 x e^(T / (T +238.3) x 17.2694))
- T is in degrees Celsius
- The result, SVP, is in pascals (divide by 1000 to get kPa)
- e is a mathematical constant called Euler’s Number, approximately equal to 2.71828.
- SVP = 610.78 x e^(T / (T +238.3) x 17.2694))
- Calculate the VPD
- SVP x (1 – RH/100) = VPD
How to calculate leaf VPD (vapor pressure deficit):
- Figure out the air SVP (ASVP)
- Same formula as getting the SVP for air VPD
- Figure out the leaf SVP (LSVP)
- It is the same formula as ASVP, but you use the leaf temperature in your calculation (typically 1-3 °C or 2-5 °F cooler)
- Leaf VPD = LSVP – (ASVP x RH/100)
Relative Humidity vs Vapor Pressure
Although related to one another, there’s a clear difference that separates relative humidity from vapor pressure.
While relative humidity measures how much water vapor there is currently in the atmosphere of your grow room, vapor pressure measures how much water vapor your grow room can hold.
This is why VPD is a more accurate way to measure water loss in your cannabis as opposed to relative humidity. It’s also why we stressed the importance of VPD and VPD charts.
Because depending on what stage your cannabis is in, they require different levels of VPD just as much as they need different levels of humidity.
And despite separating the two, relative humidity and vapor pressure are both important for cannabis.
Humidity is there to preserve the quality of your cannabis’ trichomes, while vapor pressure helps maintain healthy transpiration in your plants.
However, we made a distinction between these two environmental factors s to help you as a grower understand what you need to do to solve growth problems in your plants.
Because, while humidity and temperature are essential for your cannabis plants, you could run into a problem with vapor pressure that you otherwise would not be aware of.
Problems such as powdery mildew and wilting leaves may look like a nutrient deficiency when high VPD actually caused them.
How to use VPD Charts for your Weed Plants
We’ve stressed enough about the importance of VPD, but how do you apply that to cannabis?
Well, first, you’ll need to understand how to read a VPD chart.
Many experienced growers utilize some kind of marijuana VPD chart whenever they’re checking the conditions of their grow room.
It’s usually part of a chart that shows how relative humidity and temperature interact together to provide an ideal environment for cannabis.
And it’s usually read by looking at where the relative humidity and temperature of your grow room meet. That point is the VPD inside your grow room.
The ideal range of VPD that you need to look for is around 0.8 to 1.1 kPa for cannabis during their vegetative phase.
Different stages of growth require different VPD, and it’s why commercial growers have separate cannabis VPD charts.
Lastly, there are two ways in which VPD charts are shown.
One way is with separated charts for each phase, while the other is a single chart showing the ideal ranges for the different growth phases.
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Below is a single chart.
Cannabis VPD Chart – A Guide for Every Growth Stage
If you remember anything from this article, remember two things.
- VPD is important for healthy transpiration in your cannabis plants.
- Transpiration is a vital function to plant growth.
When you understand these two things, you understand why it is crucial to sustain a certain temperature and relative humidity in your grow room - to maintain an ideal VPD.
Read more about this topic in our support forum: https://support.ilovegrowingmarijuana.com/t/humidity-and-temperature/54555/9
FAQ about VPD Charts
What should VPD be in flower?
Given that you’re using LED grow lights for your indoor setup, the recommended VPD for the early flowering stage is around 0.64 to 1.06. For the late flowering stage, the recommended range is 0.95 to 1.34.
How do I bring my VPD down?
You can bring your VPD down by adjusting the humidity and temperature in your grow room. By increasing humidity and lowering temperature, you would also lower your VPD.
Does VPD increase with temperature?
Yes, it does. In fact, a 3°C increase in temperature can increase VPD by 45%. However, the temperature isn’t the only factor. Humidity also plays an important role in increasing VPD.
Thanks for your comment. Are you sure you are looking at equivalent VPD charts? Keep in mind some show the temperature in Celsius rather than Fahrenheit.
Let us know if that’s not it, as we’re curious to hear about it!
I have been studying the VPD chart you show in this article. The values for a given temperature/humidity relationship show much lower VPD rates than the other charts I have. I am I missing something?