Quick answer: Greenhouse cucumbers are a fast, warm, high-transpiration crop: typically about 21–25 °C by day and 18–20 °C at night, with 70–85 % RH (roughly 0.4–0.9 kPa VPD). Their dense canopy and heavy transpiration make humid nights the main risk. Long leaf wetness favours downy mildew, gummy stem blight (Didymella) and Botrytis on stems and fruit. Very dry, hot air stresses the crop. Keep a separate day and night setpoint, keep air moving through the canopy, keep leaves above the dew point at night, and remove moisture when venting would cost too much heat. Follow cultivar and advisor guidance for exact targets.
Quick answer (typical ranges)
- Day about 21–25 °C; night about 18–20 °C.
- RH about 70–85 %; avoid long periods of wet leaves at night.
- Separate day and night setpoints.
Day and night are different problems
| Period | Typical target | Humidity risk | Main actions |
|---|---|---|---|
| Day | 21–25 °C, 70–80 % RH | Heat and dry-air stress in sun; high moisture from transpiration | Vent and cool; humidify only if VPD is too high |
| Dusk | Falling temperature | RH rises fast | Start moisture removal before sunset |
| Night | 18–20 °C, ≤ 85 % RH | Condensation on leaves and fruit; leaf wetness | Heat to keep leaves above dew point; dehumidify; HAF fans |
| Dawn | Rising radiation | Cold fruit and leaves below dew point | Pre-heat before sunrise; keep dew point low |
See the VPD chart guide and greenhouse night-time humidity.
Humidity-related problems in cucumbers
| Problem | Humidity link | What helps |
|---|---|---|
| Downy mildew (Pseudoperonospora cubensis) | Needs free water on leaves; long wet nights | Shorten leaf wetness; dry canopy before night |
| Gummy stem blight (Didymella) | Wet stems, wounds, high humidity | Dry conditions after pruning and harvest; hygiene |
| Botrytis | Wet tissue at night, wounds | Night moisture control; remove debris |
| Powdery mildew | Can develop at moderate humidity on dry leaves | Scouting and control programme; humidity alone doesn't stop it |
| Fruit quality (soft, short shelf life) | Very high humidity, low transpiration | Moderate VPD; steady conditions |
See greenhouse humidity and disease, which covers downy mildew, powdery mildew and leaf wetness duration.
Airflow in a dense canopy
- Run horizontal airflow (HAF) fans at night to mix air and warm leaves.
- Check humidity inside the canopy at fruit height, not only above the crop.
- Leaf removal and canopy management reduce still, wet pockets.
Turn the crop plan into moisture-load cases
| Case | Inputs |
|---|---|
| Mature crop, mild humid night | Night ET, leakage, outdoor g/kg |
| Young crop, cold night | Lower ET, higher heating, drier outdoor air |
| After irrigation or a cloudy day | Short-term peak |
Calculate each with the greenhouse moisture load method; the mild humid night with a mature crop usually sets the size.
Assign control roles
- Climate computer: setpoints, staging, sequence.
- Heating: leaf temperature above dew point.
- Vents: cooling and drying when outdoor air is drier.
- Dehumidifiers: moisture removal when venting can't or costs too much heat.
- HAF fans: mixing in the canopy.
Our greenhouse dehumidifiers work with the climate computer as a humidity stage.
FAQ
Frequently asked questions
What humidity is best for greenhouse cucumbers?
About 70–85 % RH, avoiding long periods of wet leaves at night.
What temperature do greenhouse cucumbers need at night?
About 18–20 °C; follow cultivar guidance.
Why do cucumbers get downy mildew in greenhouses?
The pathogen needs free water on leaves; long wet nights and condensation let it infect.
Sources
- ASHRAE Handbook—Fundamentals, Psychrometrics.
- University extension guidance on greenhouse cucumber diseases.
