Quick answer: A common general guide for greenhouses is 60–80 % RH (University of Arizona CEAC). The right band depends on the crop, the growth stage and the time of day: lettuce is often grown at 50–70 %, fruiting vegetables at about 60–85 %, and unrooted cuttings much higher. RH alone isn't enough. The same 70 % RH means a different VPD and dew point at 24 °C than at 18 °C. At night the key question is whether leaves and fruit stay above the dew point. Set a crop-specific band with temperature, check it at canopy height, and watch the dusk-to-dawn period.
Quick answer
- General guide: about 60–80 % RH.
- Crop and stage decide the band; pair it with temperature (or use VPD).
- At night, keep surfaces above the dew point; avoid long wet-leaf periods.
Typical ranges by crop
| Crop | Typical RH | Typical temperature (day / night) | Watch out for |
|---|---|---|---|
| Tomato | 60–80 % | Crop-specific | Botrytis, leaf mould on humid nights |
| Cucumber | 70–85 % | 21–25 / 18–20 °C | Downy mildew from leaf wetness |
| Sweet pepper | 60–80 % | 21–25 / 18–20 °C | Botrytis; blossom-end rot with very dry air |
| Lettuce (hydroponic) | 50–70 % | ≤ 24 / about 19 °C | Tipburn; rot on lower leaves |
| Orchids (Phalaenopsis) | 60–80 % | Warm vegetative; cooler for spiking | Crown rot; Botrytis on flowers |
| Cut flowers (e.g. roses) | Avoid constantly very high RH | Crop-specific | Botrytis; short vase life |
| Unrooted cuttings | Very high (low VPD) | Warm | Wilting; then drips and damping-off |
These are typical ranges from extension and crop guides. Your cultivar, crop advisor or propagator sets the actual target. Crop guides: tomato, cucumber, pepper, lettuce, orchids, propagation.
The Cornell CEA Hydroponic Lettuce Handbook lists 24 °C day / 19 °C night and 50–70 % RH for its production system (Cornell CEA). The University of Arizona's Sustainable Greenhouse Systems gives 60–80 % RH as a general guide (University of Arizona CEAC).
Same RH, different meaning
| Air | VPD | Dew point |
|---|---|---|
| 24 °C, 65 % RH | about 1.04 kPa | about 17.0 °C |
| 19 °C, 65 % RH | about 0.77 kPa | about 12.3 °C |
(Calculated air values; leaf VPD uses leaf temperature.) A setpoint in RH should always carry its temperature. See the VPD chart and psychrometric calculation.
Why night matters most
- Temperature falls, RH rises and leaves cool by radiation, often below air temperature.
- If leaf temperature reaches the dew point, leaves get wet. Most humidity-related diseases need free water.
- Keep the dew point a margin below leaf temperature from dusk to dawn; see night-time dew-point control.
Wageningen research on process-based humidity control shows how crop and climate processes can set humidity limits, rather than a fixed RH (WUR).
Turn a range into a target
- Crop, stage, and source of the range.
- Day and night bands with temperature (or VPD).
- Measurement point: canopy height, aspirated sensor.
- Alarm limits separate from control setpoints.
- Test during the risk period: dusk, night, dawn.
How to hold the target
| Too humid | Too dry |
|---|---|
| Heat and vent when outdoor air is drier | Reduce venting; shade |
| Dehumidify when venting costs too much heat or can't dry | Fog or misting |
| Air movement (HAF fans) | Irrigate at the right time |
| Water earlier in the day | Lower temperature where possible |
See ventilation vs dehumidification and our greenhouse dehumidifiers.
FAQ
Frequently asked questions
What is the ideal humidity for a greenhouse?
About 60–80 % RH as a general guide; crop, stage and time of day set the actual band.
Is 90 % humidity too high for a greenhouse?
For most crops at night, yes: long periods near saturation lead to wet leaves and disease. Propagation of cuttings is an exception.
Should greenhouse humidity be lower at night?
The target is often similar, but at night the crop is more likely to reach the dew point, so moisture control matters more.
Sources
- University of Arizona CEAC, Sustainable Greenhouse Systems.
- Cornell CEA, Hydroponic Lettuce Handbook (2013).
- Wageningen University & Research, Process-based humidity control regime for greenhouse crops.
- FAO Irrigation and Drainage Paper 56, Chapter 3.
