VPD for optimum transpiration: measure vapour pressure deficit in kPa from temperature and humidity — seedling 0.4–0.8, veg 0.8–1.2, flower 1.0–1.5 kPa.
VPD measures the water vapour absorption capacity of the air in kPa. Calculated from temperature and humidity. Optimum VPD: Vegetative 0.4-0.8 kPa, flowering 0.8-1.2 kPa. Too low: Fungal risk, low transpiration. Too high: stress, nutrient problems. Use VPD-Calculator for precise environmental control.
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