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.
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