Ideal Greenhouse Temperature: How to Maintain It with Ventilation and Light
The short answer: for most greenhouse crops the ideal temperature is 18–24 °C (64–75 °F) during the day and 13–18 °C (55–64 °F) at night. Above 32 °C (90 °F) most vegetables begin to suffer heat stress, so ventilation or shading is needed. This guide covers how to stay in range with ventilation, heating and lighting.
What is the ideal greenhouse temperature per crop?
| Crop | Day temperature | Night temperature |
|---|---|---|
| Most vegetables (general) | 18–24 °C (64–75 °F) | 13–18 °C (55–64 °F) |
| Tomatoes | 21–27 °C (70–80 °F) | 16–18 °C (61–64 °F) |
| Cucumbers | 24–28 °C (75–82 °F) | 18–21 °C (64–70 °F) |
| Peppers | 21–26 °C (70–79 °F) | 16–18 °C (61–64 °F) |
| Lettuce & leafy greens | 15–21 °C (59–70 °F) | 10–13 °C (50–55 °F) |
| Seedlings | 20–24 °C (68–75 °F) | 16–18 °C (61–64 °F) |
How do you control greenhouse temperature?
A thermostat automatically regulates the heating system according to the set temperature.
Mechanical thermostat — less precise than digital, but enough to operate heating and ventilation equipment.
Digital thermostat — multi-stage models control several heating and ventilation devices, switching them on and off around your set temperature.
Computer-controlled systems — paired with environmental sensors they manage all heating and cooling, and can monitor CO₂ levels too.
Ventilation and cooling
A greenhouse is built to capture solar heat — which means it overheats easily, and overheating can damage or kill plants. Sometimes opening doors and windows is enough, though natural ventilation can invite pests and diseases.
A HAF-type fan in an industrial greenhouse.
Electric fans increase airflow: horizontal airflow (HAF) fans circulate inside air, spreading heat evenly and reducing leaf condensation that leads to disease. Larger greenhouses use end-wall fans that exchange the entire air volume with the outside.
A shade net used to reduce sunlight intensity.
The non-electric option: shade nets — woven fabrics in various densities that cool parts of the greenhouse and adapt to your covering and sunlight levels.
Heating
Greenhouse coverings convert sunlight to heat well but insulate poorly, so cold nights and sunless days drain warmth fast.
A circulating hydraulic system that warms the soil.
Common options: electric hot-air heaters with fans (most common), radiant hydraulic systems circulating warm water through soil or beds, and even a compost bin inside the greenhouse, which releases heat as it works.
Water tanks help maintain higher temperatures at night.
Water tanks store daytime heat and release it at night — paint them black for maximum absorption. The GRAAS iRigator continuously measures greenhouse temperature, making the climate easy to monitor and regulate alongside automatic watering.
Lighting
All plants need light for photosynthesis — converting light, oxygen and water into energy for growth, flowering and seed production. Greenhouses maximise natural light, but plants suffer when light is too intense, too weak or too brief.
Full-spectrum LED lamps used for seedlings before transplanting.
When daylight falls short — spring, autumn, cloudy spells — supplemental lighting controls quality and intensity, and automatic timers deliver consistent light hours while saving energy and lamp lifespan. See our lamp comparison guide for choosing between LED, fluorescent and HID.
Frequently Asked Questions
What temperature is too hot for a greenhouse?
Above 32 °C (90 °F) most vegetables suffer heat stress — ventilate or shade before you reach it.
What is the cheapest way to keep a greenhouse warm at night?
Black-painted water tanks: they absorb heat all day and release it after dark, no electricity needed. A compost bin adds a few degrees too.
Does temperature affect watering needs?
Directly — warm days speed up evaporation and plant water use. The GRAAS iRigator tracks both temperature and soil moisture, so watering follows the actual climate.
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