VPD (Vapor Pressure Deficit)

VPD (vapor pressure deficit) is the most sophisticated environmental control metric in modern indoor cannabis cultivation, replacing the simpler relative humidity (RH) measurement used by less advanced growers. VPD calculates the actual driving force of transpiration — the rate at which plants pull water from soil through their roots, up through their xylem, and out through stomata in the leaves. When VPD is too low (humid environment), plants cannot transpire effectively, slowing nutrient uptake and inviting mold and powdery mildew. When VPD is too high (dry environment), plants close their stomata to prevent water loss, halting CO2 absorption and stunting photosynthesis. The optimal VPD changes throughout the cannabis lifecycle: Seedlings (0.4-0.8 kPa) — high humidity protects underdeveloped root systems; Vegetative (0.8-1.0 kPa) — moderate transpiration drives nutrient uptake for rapid growth; Early flower (1.0-1.2 kPa) — slightly drier conditions promote stretch and bud initiation; Late flower (1.2-1.5 kPa) — drier conditions concentrate terpenes and prevent bud rot in dense colas. VPD is calculated from leaf temperature, air temperature, and relative humidity using the Magnus equation, but most modern grow controllers (TrolMaster, AC Infinity, Pulse Grow) calculate and display VPD automatically. Maintaining target VPD requires coordinated control of temperature (heating/cooling), humidity (humidifier/dehumidifier), and airflow (fans). Commercial cultivators monitor VPD obsessively because deviations of 0.2 kPa from target can reduce yield by 5-10%.