Almost every grower measures temperature and humidity. Almost none understand why these two values alone tell you nothing. A humidity of 65% can be pure poison for your plant — or exactly what it needs. It depends entirely on how warm it is at the same time.
Those who don't understand this fight with deficiency symptoms that aren't deficiencies — and harvest 30% below potential without ever knowing why. VPD — the Vapour Pressure Deficit — is the only number that truly describes how much transpiration capacity your plant has.
What VPD really is — the physics behind it
Plants transpire — they release water vapour through their stomata (leaf openings) into the surrounding air. This process is the engine that transports water and nutrients from the roots to every corner of the plant. No transpiration, no nutrient transport. No nutrient transport, no growth.
Whether and how intensely a plant transpires depends on how much "room" the surrounding air still has for water vapour. This available room is the VPD.
The Saturation Vapour Pressure (SVP) is the maximum amount of water vapour that air can hold at a given temperature. The Actual Vapour Pressure (AVP):
The higher the VPD, the more moisture the air draws from your plant.
The formula in detail
The saturation vapour pressure can be calculated using the Magnus approximation formula:
Concrete calculation example
25 °C and 65% relative humidity:
| Parameter | Calculation | Result |
|---|---|---|
| SVP at 25 °C | 0.6108 × e^(17.27×25/262.3) | ≈ 3.17 kPa |
| AVP | 3.17 × 0.65 | ≈ 2.06 kPa |
| VPD | 3.17 − 2.06 | 1.11 kPa ✓ |
Now just raise the temperature to 30 °C — humidity stays at 65%: VPD jumps to 1.48 kPa. Five degrees difference, 33% more transpiration load — at an identical hygrometer reading.
The factor almost everyone forgets: leaf temperature
What the stomata truly "feel" is not the air temperature in the room — it is the temperature of the leaf itself. Leaves are typically 1–3 °C cooler than the surrounding air due to evaporative cooling. This corresponds to a VPD error of 0.25–0.35 kPa — large enough to misclassify growth phases.
VPD target values by growth phase
| Phase | VPD Target | Temp. | RH |
|---|---|---|---|
| Germination / Cutting | 0.4–0.8 kPa | 23 °C | 70–80% |
| Vegetative (early) | 0.8–1.0 kPa | 24 °C | 65–70% |
| Vegetative (strong) | 1.0–1.2 kPa | 25 °C | 60–65% |
| Early Flower | 1.0–1.3 kPa | 26 °C | 55–60% |
| Flower | 1.2–1.6 kPa | 27 °C | 50–55% |
| Late Flower / Ripening | 1.0–1.3 kPa | 25 °C | 55–60% |
How temperature and humidity interact to determine VPD — and why 65% RH at 20 °C is physiologically completely different from 65% at 30 °C: Temperature & Humidity — Why Two Numbers Are Not Enough.
Which sensors are suitable for VPD
| Sensor | Temp. Accuracy | RH Accuracy | Suitability |
|---|---|---|---|
| SHT40 / SHT41 | ± 0.2 °C | ± 1.8% | Recommended — industrial quality |
| SHT31 | ± 0.3 °C | ± 2.0% | Good |
| DHT22 | ± 0.5 °C | ± 2–5% | Acceptable |
| DHT11 | ± 2.0 °C | ± 5% | Not suitable for VPD |
The Growix Core System uses the SHT4x industrial sensor. An error of ± 2% humidity produces a VPD error of approx. ± 0.06 kPa at 25 °C — that is acceptable. A DHT11 with ± 5% gives ± 0.15 kPa — too much for precise phase control.
VPD in Growix OS — continuous, automatic, reactive
- VPD measurement every 30 seconds via SHT4x industrial sensor
- Real-time calculation and display in the touch UI as a graph
- VPD too high → increase ventilation, optionally activate humidifier
- VPD too low → reduce ventilation, slightly raise temperature
- Target VPD is configurable per growth phase — no manual intervention required