Cannabis

VPD (vapor pressure deficit) is the single most important environmental variable for cannabis cultivation

VPD combines temperature and humidity into one number that directly predicts transpiration rate. Optimal VPD in veg: 0.8-1.2 kPa. In flower: 1.0-1.5 kPa. Running outside these ranges causes either stomatal closure (too high VPD, plant stops transpiring) or pathogen risk (too low VPD, excess moisture). A $30 hygrometer and a VPD chart outperform $5,000 in nutrients for yield optimization.