The thermostat holds 22°C, the hygrometer reads 65% — both parameters "within range." But at night temperature drops to 16°C and the same absolute amount of moisture in the air is now 90% relative humidity. Condensate on leaves, grey mould and powdery mildew in the morning, bacterial spots by the end of the week. Temperature and humidity are not two independent parameters — there is a direct physical relationship between them, and they must be managed together.
Quick Glossary
- Relative humidity (RH) — the percentage of the maximum possible water vapour in air at a given temperature; the same RH value at different temperatures represents different absolute amounts of moisture
- Dew point — the temperature at which air with a given moisture content reaches 100% RH and condensation begins; if a leaf surface is cooler than the dew point, condensate forms on it
- Absolute humidity — the actual amount of water vapour in g/m³ regardless of temperature; as air cools, absolute humidity stays constant while relative humidity rises
Why Night Cooling Is More Dangerous Than Daytime Heat
During the day at 24°C and 60% RH, there is a certain amount of water vapour in the air. At night the temperature drops to 16°C — and the same absolute amount of vapour now represents 85–90% relative humidity. If the leaf surface is slightly cooler than the air, dew condenses on it.
Condensate on a leaf at 16–18°C is an ideal environment for germination of powdery mildew conidia, bacterial necroses, and Botrytis. Most fungal and bacterial problems in a greenhouse begin at night or early morning through exactly this mechanism.
Practical rule: a day-to-night temperature swing greater than 6–8°C combined with typical daytime humidity of 55–65% is a risk zone. Fans that maintain air movement through the night reduce this risk — air movement equalises temperature between leaf and air and reduces condensation.
Temperature and Humidity Interaction: VPD as a Single Indicator
Measuring temperature and relative humidity separately gives an incomplete picture. VPD (vapour pressure deficit) combines both parameters into one indicator that characterises the "dryness" or "humidity" of the air relative to the plant.
At low VPD (high humidity and/or low temperature), the plant does not transpire effectively: calcium and magnesium uptake slows — both elements move with the water stream. Leaves at shoot tips receive fewer microelements and more frequently show deficiency even with correct EC.
At high VPD (dry atmosphere and/or high temperature), the plant transpires excessively: leaf edge burn, tip burn in lettuce and greens, wilting of young leaves even with adequate substrate moisture are all possible.
Working VPD range for most crops in the vegetative phase: 0.8–1.2 kPa. Calculate VPD when adjusting conditions rather than adjusting temperature and humidity separately.
Zone Differences: Plant Canopy vs Upper Zone
Temperature and humidity within the plant canopy — between leaves and stems — always differ from readings taken at walking height. In a dense canopy with poor air movement, temperature can be 2–4°C higher and humidity 10–15% higher. With overhead grow lights the difference is even greater.
Place temperature and humidity sensors in the plant canopy or at mid-plant height — not near the ceiling or adjacent to an air duct. A reading of "all normal" from a sensor above the plants while actual canopy humidity is 80% RH is a typical situation that explains where diseases come from despite "correct" parameters.
Horizontal circulation fans blowing between rows — the primary tool for equalising conditions between canopy and the upper zone.
Reference Ranges for Common Crops
Lettuce and greens: day 18–22°C, night 14–16°C, RH 60–70% during the day and no higher than 75% at night.
Tomato and pepper: day 22–26°C, night 16–18°C, RH 60–70% during the day. During fruit set, humidity should not exceed 65% — high humidity disrupts pollination and reduces pollen viability.
Cucumber: day 24–28°C, night 18–20°C, RH 70–80% — the most moisture-demanding crop. But even in cucumber, RH above 85% for extended periods carries a risk of bacterial disease.
Microgreens: 18–22°C; during germination under cover — humidity 85–95%; after uncovering — 55–65% with air movement.
Three Mistakes That Cost the Most
Setting the night temperature without accounting for daytime humidity. "Dropped night temperature to 14°C to save on heating" with daytime humidity at 65% — and the night runs at 90% RH. Night temperature is determined not only by the crop's agronomic requirements but also by the dew point of the daytime air.
Measuring parameters outside the plant canopy. A sensor near the ceiling or next to a vent reads the air entering the system — not the conditions the plant is actually experiencing. For making decisions — sensor in the canopy.
Adjusting humidity without considering ventilation and CO₂. Opening vents to reduce humidity and losing all the CO₂ that was enriched through the night. Lowering humidity with a humidifier without increasing air movement — producing uniformly humid canopy air instead of localised moisture near the roots. Parameters are managed as a system, not one at a time.
Signs That Temperature and Humidity Are Under Control
- Sensor positioned in the plant canopy
- Morning inspection shows no condensate on leaves
- VPD in the working range of 0.8–1.2 kPa throughout the light period
- Night humidity below 80% regardless of night temperature
- No fungal or bacterial outbreaks starting in the morning