Ideal Greenhouse Temperature: How to Maintain It with Ventilation and Light

Greenhouses are an essential part of gardening, allowing plants to grow in a controlled environment and achieve maximum productivity regardless of external weather conditions. In this article, we will discuss the key aspects of greenhouse temperature, ventilation, heating, and lighting that are necessary to ensure optimal conditions for plants. By understanding the importance and function of each element, you will be able to better plan and implement effective climate management strategies. We will also cover heating methods, which are vital in winter and during cold nights, and highlight the importance of lighting for photosynthesis, as well as the use of modern timers that help regulate light levels, save energy, and extend the lifespan of lighting equipment.

Ideal greenhouse temperature at a glance

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. Typical ranges by 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)

The sections below explain how to stay inside these ranges with ventilation, heating and lighting.

Temperature

The goal in a greenhouse is to maintain optimal temperatures for the plants inside. Monitoring day and night temperatures helps determine which heating methods work best and which types of plants should be grown depending on the greenhouse climate. The three most common greenhouse climate control devices are:

A thermostat automatically regulates the heating system according to the set temperature.

• Mechanical thermostat: although less precise than a digital thermostat, a simple mechanical thermostat can be used to operate heating and ventilation equipment.
• Digital thermostat: multi-stage digital thermostats can control several heating and ventilation devices. Once the desired temperature is set, the system switches cooling or heating equipment on and off to maintain optimal conditions.
• Computer-controlled systems: these can be used with environmental sensors to manage all heating and cooling aspects. They also allow monitoring and adjusting carbon dioxide (CO2) levels in the greenhouse.

Ventilation and cooling

Since the main purpose of a greenhouse is to capture as much solar heat as possible, it can easily overheat in hot weather. Overheating can damage or even kill plants. Sometimes simply opening doors and windows for natural ventilation is enough to control temperature and humidity, though this can also attract pests and diseases.

A HAF-type fan used in an industrial greenhouse.

Electric fans are useful for increasing airflow and cooling the greenhouse. Horizontal airflow fans (HAF) circulate air inside, helping distribute heat evenly and reduce condensation on leaves, which can cause disease. Larger greenhouses may use end-wall fans powerful enough to exchange all the inside air with outside air.

A shade net used to reduce sunlight intensity.

For a non-electric option, shade nets can be used. These woven fabrics cover parts of the greenhouse interior to provide cooling shade. Shade cloths are available in different densities, making them adaptable to greenhouse coverings and sunlight levels.

Heating

Although greenhouse coverings are effective at converting sunlight into heat, they also lose a lot of heat since they are not strong insulators. Depending on your location, this can make it difficult to maintain a stable climate when the sun is not shining and outdoor temperatures are low.

A circulating hydraulic heating system designed to warm the soil.

Greenhouse heating provides plants with the warmth they need during cold periods. Heat can be supplied through electric hot-air heaters with fans (most common) or radiant hydraulic systems based on water circulation. These systems are installed in the soil or on growing beds. A compost bin inside the greenhouse can also help maintain temperatures during colder times.

Water tanks in the greenhouse help maintain higher temperatures at night.

To keep greenhouse temperatures higher, water tanks can be used. Water heats up during the day and releases warmth at night, maintaining a more stable temperature. It is recommended to paint tanks black to absorb maximum heat during the day. GRAAS iRigator continuously measures greenhouse temperature, making it easier to monitor, understand, and regulate the climate with the GRAAS automatic irrigation system.

Lighting

All plants need light for photosynthesis – the process that converts light, oxygen, and water into carbohydrates (energy). Plants require this energy to grow, flower, and produce seeds. Greenhouses are designed to maximize natural light. The more light plants receive, the better the yield. However, plants can suffer if light is too intense, too weak, or too short in duration.

When natural daylight is insufficient, supplemental lighting can provide the right amount of light. Supplemental lighting allows control over light quality and intensity, which is especially useful during shorter days in spring or autumn, or in cloudy weather. As daylight changes over time, plant needs also change.

Full-spectrum LED lamps used for seedlings before transplanting into the greenhouse.

Automatic lighting timers help ensure plants receive the right amount of light. Timers are useful early in the morning and in the evening, saving energy and extending lamp lifespan.

Different types of lamps can be used depending on needs and budget. A detailed overview of commonly used lamp types is available in this article.

Summary

To ensure an ideal greenhouse climate and create optimal growing conditions, consider the following:

Ventilation and cooling: essential for controlling temperature and humidity. Natural ventilation is the simplest method but can bring pests and diseases, so mechanical fans, shade nets, or automated systems are recommended to reduce overheating risks and maintain optimal plant conditions.

Heating: greenhouses lose a lot of heat, so additional heating may be required in cold seasons. Options include electric heaters, hydraulic systems, or compost. Water tanks can also store heat during the day and release it at night.

Lighting: plants need light for photosynthesis, and greenhouses maximize natural light. When sunlight is insufficient, supplemental lighting is necessary. Automatic timers provide consistent and efficient light management.

Together, these measures help optimize the greenhouse environment, ensure plant health and productivity, and make efficient use of resources.