Leaf vs Air Temp & How Environment Drives Feeding
Why leaf temp runs cooler than air, how that changes real VPD, and the crop-steering logic that ties VPD × leaf temp × light to nutrient uptake — with an SOP for a daily VPD check.
9 min read
VPD is a leaf-surface phenomenon — it depends on the temperature of the leaf, not the air. And a transpiring cannabis leaf under LED does not sit at air temperature.
- Transpiring leaves run ~1–3 °C (≈2–4 °F) cooler than air under LED; under intense or specific spectra the offset can reach ~–3 °C.
- The offset grows with light intensity and airflow — more light means more radiant load, but a well-transpiring leaf sheds it as evaporative cooling.
- Computing VPD from air temperature alone overstates it. The gap between an apparent 1.4 kPa and a real 1.2 kPa is exactly the difference between healthy transpiration and closing stomata.
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