Growing cannabis has some similarities with growing other plants, like lettuce. Cultivators can use the same principles to nurture both plants, although, obviously, the complexities with cannabis will be greater. Vapor Pressure Deficit (VPD) is a measurement concept used in agriculture. The term is trickling through the cannabis world as aficionados and hobby growers share their experiences.
What is VPD?
VPD measures the pressure difference inside and outside of a leaf. To fully understand the concept, we need to know how humidity and transpiration work. Humidity refers to the water vapor content in the air. Transpiration is essentially how a plant “sweats.” Excess water is released from the plant leaves and transformed into vapor. As the grow room gets hotter, the plant will transpire more, increasing the humidity. This creates an environment where mold and fungi can thrive.
As the water vapor in the air increases, it puts pressure on the plant, making the transpiration process more difficult. VPD measures the difference between the actual water vapor pressure being exerted on the plant and the pressure that could be exerted at 100% relative humidity. It may be reported in pounds per square inch (psi) or kilopascal (kPa); higher values indicate relatively unsaturated air, and a value of “zero” indicates full saturation.
Why Does it Matter?
Growers are interested in the sweet spot, the ideal VPD for cannabis. Knowing this value allows them to increase their crop’s water and nutrient intake.
If VPD is too low, plants struggle to transpire. If it’s too high, they’ll transpire too much. According to Chris Vaughn of Oregon’s Higher Minds Horticulture: “If you control your VPD, you will start to see a plant’s full potential… you can be the greatest grower in the world, but without getting VPD to where it needs to be, you’ll never grow the best plants.”
To manipulate VPD, you need a sophisticated grow operation capable of measuring humidity and temperature. Both metrics need to be tightly controlled.
Image source: Surna