Grow science · 02
Leaf vs. air
Your thermometer measures the air. Your plant feels its leaf.
The problem
Under LEDs, leaves usually run 1–3 °C cooler than the air around them – LEDs radiate less heat than HPS lamps, and transpiration cools the leaf.
Same room, two numbers
| 26 °C air · 60 % RH · 24 °C leaf | |
|---|---|
| Air VPD | 1.34 kPa |
| Leaf VPD | 0.97 kPa |
| Difference | 0.37 kPa |
That’s the gap between “flower” and “veg” – just from leaf temperature.
How to measure
- Lights on for at least 1–2 hours.
- Point an infrared thermometer at a top leaf directly under the light, 10–20 cm away.
- Make sure the measuring spot is fully on the leaf.
- Measure 3–5 leaves and take the average. Emissivity 0.95.
A kitchen probe thermometer won’t work – it mostly measures the air and heats up under the lamp.
The formula
Leaf VPD = SVP(leaf) − SVP(air) × RH/100
Or just use the VPD calculator with a leaf offset.