Vapor Pressure Deficit sounds like a super complicated growing term at first for most growers. But we’ll make it easy to understand what VPD means and why it’s so important to growing healthy plants. Hint: It’s extremely important! Before we even get into what VPD means, we want you to know that we’re going to skip the formulas in this article. We want to keep the explanation as straight forward and practical, so you can take this information and apply it immediately. And you will. Trust me!
What is the Vapor Pressure Deficit or VPD of a Plant?
Vapor Pressure Deficit or VPD is very simply the measurement of how much drying power the air has. This effect’s the plant directly. The greater this force, the faster the plants dry out. The lower this force, the more moisture stays within the plant. If you remember from biology, in order to survive, plants need to transpire (like we sweat) through their stomata. In order to transpire effectively, the air needs to evaporate from their leaves within a certain range, not too fast and not too slow. Vapor Pressure Deficit is a numerical representation of this force. We can control VPD mainly by controlling temperature and humidity.
Why is VPD so Important!?
Keeping VPD in the target range is crucial for healthy plant growth. It is probably the most straightforward way to understand the effect of temperature and humidity on your plants.
When VPD is too high your plants will dry out rapidly. This can cause a range of issues. Some look like a nutrient deficiency, but mostly growth is retarded. Like really messed up. Stretchy plants. Crispy leaves. It looks bad, and it is bad.
On the other hand, when VPD is too low, moisture builds up on the surface of the plant’s leaves and growth is stalled. This happens because if the moisture can’t evaporate from the surface of the leaves, the plants can’t transpire. And that means they aren’t pulling up nutrients into their roots! Aaaand that means they aren’t growing. Period.
What’s worse, low VPD usually creates the ideal environment to breed pathogens like Powdery Mildew or other fungi!!!! Enter sinister music and turn off the lights.
Finding the VPD of Your Plants
Okay, so now it’s time to learn how to actually figure out your VPD. All you need is the VPD Chart below, the temperature of the room, the temperature of your plant’s leaves, and you need the relative humidity of your grow room.
We recommend the following equipment:
a good thermometer for the room temperature
an infrared thermometer for the leaf temperature* (around $30 on Amazon)
a good hygrometer
And then reference the chart we provide! We make it easy because we love you.
*If you don’t can’t afford an infrared thermometer, you can also estimate leaf temperature by subtracting 5° Fahrenheit (about 3° Celsius) from your ambient room temperature. Keep in mind, an IR thermometer is the most accurate measurement.
If you use the IR thermometer, use this VPD Calculator. It calculates both room and leaf VPD for you.
Once you have that information you can very easily calculate your VPD using a VPD chart like the one below.
Now that you know how to find your VPD level, you need to know the ideal range to keep it in. Basically, the ideal range to keep the plants in depends on the growth stage they are in. Smaller and younger plants are more delicate, and require a lower Vapor Pressure Deficit. You’ll steadily raise the VPD as the plants mature and grow in size. This list should give you a good idea where to start:
Propagation into Early Vegetative Stage: Leaf VPDs between 0.8 kPa and 1.0 kPa
Late Vegetative into Early Flowering Stage: Leaf VPDs between 1.0 kPa and 1.2 kPa
Mid into Late Flowering Stage: Leaf VPD Values between 1.2 kPa and 1.6 kPa
Just use the chart and the value guidelines above to adjust the humidity and temperatures to create the ideal conditions for your plant. With a little trial and error you’ll start to learn how to control your room’s VPD values as necessary for the best plant growth.
Automatic VPD Measurements and Logging
It can become tedious to track Vapor Pressure Deficit manually. Even with the chart. It’s really tough to take care of when the lights are out!
So when you’re ready to get something that automatically takes care of this for you, consider getting the a VPD package by DimLux. All you need is the MaxiController Datalogger, Thermometer, Humidity meter, and Plant Camera (Infrared Thermometer). This will automatically log the exact levels of VPD for you onto a USB thumb drive.
Then, using the DimLux software, you can view the history of your room at your convenience. Maybe things are getting out of hand when you’re not there, and this will illuminate that for you.
Another thing to mention, is you don’t need to use DimLux lights to take advantage of the VPD package by DimLux. This can work independently. But when you do add some DimLux lights, it is capable of controlling them to help with room temperature as well!
I hope this has helped you to take your first few steps towards better plant growth. Let me know in the comments if you have any other questions for me!