Vapor pressure deficit explained in 5-ish minutes. What is VPD? How do you measure VPD in your grow room? What VPD range is good for propagation, vegetative and flowering? How are relative humidity, leaf temperature and air temperature combined into VPD and which units of pressure are used?

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Humidity 102: Vapor Pressure Deficit — VPD Chart

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  • Glen Johnson

    I really like your videos in general but you have several concepts in this video completely wrong. VPD doesn't measure anything inside a plant. VPD is very similar to Relative Humidity in that it is dependent on Temperature. It tells the difference between the water vapor in the air at the current temp and the potential amount of water vapor the air could possibly hold at that temp (what you are calling the saturation point).
    The internal chambers inside the plant are not always at 100% humidity because that would just be liquid water.
    VPD does NOT tell you what’s going on inside the plant. VPD tells you about the POTENTIAL for water to evaporate from the leaves.
    Closing the stomata does not stop photosynthesis.
    Cute video but…. Many concepts wrong here.

  • Frode Tyholdt

    Great video! As these parameters have an interdependent relationship one can also look at it the other way and adjust the leaf temperature by changing the humidity. At e.g. 25 oC there are many combinations of leaf temperature and humidity that will give you the same VPD. But, they will not be equal in terms of growth. I guess a delta T of around -2 oC is nice to aim for?
    Also, I am curious on how stable the humidity needs to be controlled. Some humidifiers are really hysteretic and will probably lead to a kind of stomatical yo-yo behavior.

  • tebindyeh black

    Hello Everest and whoever reads this comment can you please explain how you get the vpsat of 4.5993545385 from 0.6626968882 from The (4.5 number) I can work the rest out and I can workout the first part too (0.66 number) but don't get how you go from the 0.66 to 4.5 someone please help me get my head around this

  • delosombres

    Btw, lower than air leaf temp isn't better if you want to maximize growth especially with high CO2. Cannabis plants can benefit from up to 36C leaf temp to most effectively assimilate 1500ppm of CO2. However, you need high RH (70-80% in bloom) to keep VPD under 1.5 kPa which is risky. Good air movement, little day/night temp/RH difference and vital plants (plenty of calcium, potassium and silica) is required to prevent mold.