Vapor Pressure Deficit (VPD) measures how thirsty the air is. Too dry and the plant transpires too fast. Too humid and stomata close, growth stalls. Get VPD right and every other variable gets easier.

Seedling VPD
0.4–0.8 kPa
warm, humid
Veg VPD
0.8–1.2 kPa
moderate
Flower VPD
1.2–1.5 kPa
drier

Practical targets

  • Seedling/clone: 75–80°F at 65–70% RH
  • Veg: 75–82°F at 55–65% RH
  • Early flower: 75–80°F at 50–55% RH
  • Late flower: 65–75°F at 40–50% RH (mold prevention)