Vapor Pressure Deficit (VPD) and Cannabis Cultivation

A deep dive into Vapor Pressure Deficit (VPD) and cannabis plant growth.

Nadia Sabeh​​, PhD, PE, LEED AP
​Owner, Founder of Dr. Greenhouse, Inc.

According to Dr. Sabeh, vapor pressure deficit (VPD) is the primary driver of evapotranspiration inside the indoor plant environment. VPD is
​a measure of the amount of water in the air versus the maximum about of water the air can hold at a given temperature (the saturated condition). VPD is measured in units of pressure, typically in kilopascals (kPa), but may also be reported as bars, milibars, inches of Hg, or other pressure units.

In general, a low VPD means the air is "humid" and a high VPD indicates the air is "dry." VPD is directly responsible for the stomatal opening of plant leaves. If VPD is too high ("dry" air), stomata will close to conserve water. If the VPD is too low ("humid" air), stomata may be fully open, but evapotranspiration will be slow since the difference in water vapor in the air versus at the leaf is very small. Both conditions can cause leaf wilting, leaf tip burn, and other crop maladies, including susceptibility to pests and molds. On the other hand, when VPD is managed correctly, plants will transpire freely, moving nutrients readily to cells and maximizing CO2 uptake and photosynthesis.
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The correct VPD level depends on the type of crop, as well as the stage of crop for fruiting and flowering plants.

Dr. Nadia (A.K.A. “Dr. Greenhouse”) is President and Founder of Dr. Greenhouse, Inc., an agricultural and mechanical engineering firm that specializes in the design of HVAC systems for indoor plant environments. Dr. Sabeh first became interested in controlled environment agriculture (CEA) as an undergraduate, while working on a small shiitake and oyster mushroom farm in southern Idaho. She was inspired by the farmer’s way of life – the daily rhythm, the appreciation for the land, the hard work necessary to grow a crop, the friendly competition, and the respect and camaraderie between growers, helping each other to troubleshoot problems, fill orders, and make money. It was then that she decided she wanted to use her engineering know-how to help farmers control their environments. More than 20 years later, she is still on that path, helping farmers grow crops indoors, in greenhouses, and in locations that would otherwise make it impossible or impractical to do so.

This conversation was filmed at the Resource Innovation Institute (RII) Indoor Agriculture Energy Solutions conference, which brought state regulators together with light, plant, HVAC, green house, water, vertical farming experts and cannabis cultivators to talk energy and cultivation best practices. The hope is that when regulators get to learn from experts and the farmers they regulate, they will create sensible agriculture regulatory policy. Farmers (indoor, greenhouse and outdoor), on their part, will hopefully move towards ever increasing energy efficiency and reduced carbon footprints.

Learn more about Resource Innovation Institute

https://resourceinnovation.org/