Vapor Pressure Deficit (VPD) is extremely important for Cannabis plants. VPD is the difference between the amount of water vapour that the atmosphere is able to retain (which depends on temperature) and the amount of water vapour contained in it (relative humidity). This is usually measured in kilopascals (kPa).
VPD helps you identify the correct range of temperature and humidity to aim for in your grow space. With VPD you can achieve the best results while avoiding pest and environmental problems. VPD also controls plant transpiration rates, stomata opening, CO2 uptake, nutrient uptake, and plant stress.
When Vapor Pressure Deficit is too high, the plants might not be able to keep up with the environmental demand upon them. The air is drying them out too fast! And this will cause them to exhibit symptoms that look just like nutrient deficiencies, and the plant will grow poorly.
On the other end of the spectrum, if Vapour Pressure Deficit is too low the plants can’t transpire. Moisture may build up on the leaves, and the plant will grow much more slowly. If this goes unchecked for too long, the plants might get attacked by moulds and fungi, like powdery mildew.
Providing our plants with a correct relationship between temperature and relative moisture will keep us on the right VPD parameters, increasing the plant's activity and thus improving its growth and showing its full potential during the flowering phase. The ideal would be to adjust both parameters (temperature and humidity) to get the best possible outcome
If you master VPD, you master your environment and become a better grower.