When someone says their grow runs at "24 degrees and 60 percent", that sounds like a complete climate description. It is not. Without context, these two numbers say nothing about whether the plant is currently operating at an optimal vapour pressure deficit — or whether it is transpiring under stress.
The problem: temperature and relative humidity are almost always treated separately in growing. Everyone knows tables with "Veg: 20–26°C, 50–70% RH". What these tables do not show: the same humidity at different temperatures is physiologically completely different.
Why Two Numbers Are Not Enough
Relative humidity (RH) is a relative quantity — it describes how much water vapour the air contains relative to its maximum capacity. This maximum capacity rises exponentially with temperature. This means: 60% RH at 20°C and 60% RH at 30°C contain completely different absolute amounts of water.
Vapour pressure deficit (VPD) describes the difference between the actual water vapour content of the air and the maximum possible. It is the physical pull that draws water out of the plant. This pull determines transpiration rate, nutrient transport and gas exchange — not the individual values of temperature or RH.
VPD — The Real Climate Variable
VPD is simplified as: VPD = SVP(T) × (1 − RH/100), where SVP(T) is the saturation vapour pressure at temperature T. SVP rises from approx. 2.34 kPa at 20°C to 3.60 kPa at 27°C and 4.24 kPa at 30°C. This exponential rise explains why the same RH at higher temperature produces a drastically higher VPD.
Three Scenarios — One RH, Three Climate Situations
| Scenario | Temperature | RH | VPD | Climate assessment |
|---|---|---|---|---|
| Scenario 1 | 20°C | 65% | 0.70 kPa | Good for early veg — low transpiration pull |
| Scenario 2 | 26°C | 65% | 1.12 kPa | Upper veg limit — acceptable but monitor |
| Scenario 3 | 32°C | 65% | 1.70 kPa | Too high — stomata close, growth reduced |
Target Values Per Growth Phase
| Phase | Temp target | RH target | VPD target | Rationale |
|---|---|---|---|---|
| Seedling / clone | 22–24°C | 70–80% | 0.4–0.6 kPa | Weak root system — low transpiration pull needed |
| Veg early | 22–26°C | 60–70% | 0.6–0.9 kPa | Build phase — moderate growth, low stress tolerance |
| Veg late | 24–28°C | 50–65% | 0.9–1.2 kPa | Higher VPD promotes nutrient transport and stem thickness |
| Flower early | 22–26°C | 45–55% | 1.0–1.3 kPa | Terpene development begins — moderate conditions |
| Flower late | 20–24°C | 40–50% | 1.1–1.5 kPa | Mould prevention — lower RH, slightly reduce temp |
- Sensor on the wall: Measures wall temperature, not air temperature in the canopy zone. Deviation of 2–5°C possible.
- Measuring in direct airflow: Fan airflow cools the sensor — measured temperature too low, RH too high.
- Single sensor for entire space: Temperature gradient from floor to lamp can be 8–12°C. One reading is not a climate.
- No leaf temperature correction: Leaf temperature is 2–4°C below air temperature. VPD calculation without correction overestimates evaporation rate.
Growix Climate Control
The Growix Core uses three separate fan circuits: filtered intake air, internal circulation for canopy homogeneity, and negative pressure exhaust with active carbon filter. The SHT4x sensor (±1.5% RH, ±0.2°C) delivers readings every 10 seconds to the Growix OS, which calculates VPD in real time and adjusts fan speeds via PWM automatically.