Cultivators are like cultivars; no one is identical to another. But in an industry with so much room for differentiation, why are so many cultivators focusing on the concept of Vapor Pressure Deficit (VPD)?
The short answer is this calculation, or relationship, sheds light on every cultivator’s dream: achieving optimal growth conditions. Optimal growth conditions are hard to define because of the variety of cultivation setups and the drastic natural changes that take place over the course of a plant life cycle.
What is Vapor Pressure Deficit?
Vapor pressure deficit is a calculation defined by the difference of moisture in the air and the full saturation point of the air. Unlike relative humidity, vapor pressure deficit does not change with temperature, but instead changes strictly with humidity. Due to this distinction, vapor pressure deficit is a much more accurate way to predict plant transpiration compared to relative humidity.
When applied to cultivation, vapor pressure deficit represents a theoretical amount of pressure of the stomata of the leaf surface. Stomata regulate water loss during environmental changes. Stomata can effectively change the rate of transpiration (transfer of water from plant tissue to the surrounding environment). What is important to understand is that the uptake of water at the root zone is controlled by the rate of transpiration at the canopy. This means that consistent VPD in a growing environment allows for consistent nutrient uptake.
One of the challenges with measuring VPD is that cultivation rooms or areas can vary in equipment and conditions depending on the specific needs of the crop. Cultivators often use a VPD chart (see below) or electronic sensors that calculate VPD based on feedback. Plant temperature is often taken as a leaf surface temperature measurement on the plant canopy; while acknowledging that various areas of the plant may vary.
Different lighting will affect this leaf surface temperature. Specifically, high-pressure sodium (HPS) will generally heat the plant tissues more than Light Emitting Diode (LED) This means LED flower rooms will optimally run at higher temperatures than HPS flower rooms. VPD is measured in a unit of pressure called kilopascal (kPa).
A good rule of thumb is to aim for 1.0 kPa throughout all stages of growth. If possible, achieving a lower kPA (.5kPa-.7kPA) during the vegetative stage via supplemental humidity can accelerate growth rates. A higher VPD (1.3kPa-1.6kPa) in the late stages of flowering is advised to help prevent microbial growth. This can be achieved with the use of properly sized dehumidification systems and sufficient air movement in cultivation areas.