What is VPD? Vapor Pressure Deficit, VPD, is an indicator of the evaporation potential of water into the air. It’s a measurement that’s defined as the difference, or deficit, between pressure exerted by the moisture present in the air currently and the pressure at saturation. It’s one of several different methods that you can use to evaluate crop stress or water stress within a plant.
Commercial cannabis growers should care because VPD is a superior way of evaluating potential water stress. It’s not a direct measurement of water flow or water loss, but it’s an indicator of potential stress. Typically people only measure relative humidity (RH) as an approximation for water stress, but RH is just the moisture content in the air at a specific, given temperature.
Vapor Pressure Deficit is superior because it accounts for the relationship of temperature and humidity. Water’s ability to be present as a vapor within the air is dependent on the temperature, such that a warmer mass of air will have the ability to hold more water while a colder mass of air can hold less water. Relative humidity only gives you a percentage of water vapor saturation in the air, but doesn’t tell you much about temperature fluctuations.
Unless you’re at a perfectly uniform environment, like cuttings or the seedling stage, you’re going to have temperature swings like day and night-time temperatures. Temperature can even vary along different parts of the plant, between separate plants, and between plants and the structure. How close the plants are to the heating and cooling equipment also matters. This adds up to make RH less useful as a measuring tool.
The example I give is that RH is like knowing that your car can get 20 MPG. That value doesn’t take into account how much I push on the pedal, if two or five of us are riding in the car, if there’s wind going against us, or if I’m in stop and go traffic. If you were to add the additional capability of measuring all the other factors, you could more accurately determine what your real mileage is. That’s what VPD does. It’s more precise.
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