The problem of humidity in the Greenhouse
Introduction: Humidity and Its Interrelation with Energy in Greenhouses
Humidity and energy in the greenhouse are closely interrelated. Heat is extracted or released in the form of latent heat through the processes of evaporation and condensation. Additionally, changes in temperature and air movement within the greenhouse—caused by radiation and convection—significantly influence water's physical states and processes.
Solar radiation, by increasing the temperature of the leaf's surface, plays a key role in regulating the rate of transpiration (the evaporation of water from the leaves).
During the day, the production of water vapor in the greenhouse is higher because the evapotranspiration rate is much greater than at night. However, relative humidity is lower during the day due to higher air temperatures and the operation of ventilation systems. Conversely, at night - particularly just before sunrise, when temperatures are at their lowest - the relative humidity is very high, often reaching saturation, even though absolute humidity is lower.
The Importance of Relative Humidity for Plant Growth and Productivity
Maintaining an appropriate relative humidity in the greenhouse is essential for the normal growth and productivity of plants. The effects of relative humidity on plants are primarily due to the following:
a) High relative humidity: This promotes moisture condensation on the foliage, which facilitates the extensive growth of pathogenic microorganisms. It also significantly reduces the transpiration rate, often leading to nutritional issues in the plant.
b) Low relative humidity: This significantly increases the rate of transpiration, which, in some cases, can prevent the plant from maintaining its water balance. Additionally, it creates a favorable environment for the growth and proliferation of various insects and mites.
Challenges of Managing Humidity in Unheated Greenhouses
In unheated greenhouses, very high relative humidity is common at night, often reaching saturation. This leads to condensation on surfaces such as the greenhouse cover and plant leaves, creating highly unfavorable conditions for cultivating horticultural crops and most cut flowers.
Appropriate regulation of relative humidity not only improves plant health but also reduces the need for chemical plant protection measures, contributing to a more sustainable and productive growing environment.
Fig.1 The condensation of water vapor or the fall of water from the greenhouse cover at night on the surface of the leaves facilitates the germination of fungal spores, favoring plant diseases.
Causes of High Relative Humidity in Greenhouses
The conditions that favor excessive relative humidity at night inside the greenhouse are:
- mainly the drop in temperature and
- the confined space in the greenhouse.
At night, the air inside the greenhouse cools through convection from the walls and radiation from the cover, plants, and soil. With the windows closed and minimal ventilation (which would otherwise allow moisture to escape), the relative humidity of the air steadily increases. This occurs despite the minimal transpiration through the stomata during the night.
Effective Ways to Avoid High Relative Humidity in Greenhouses
- Use tall greenhouses: Taller greenhouses experience less abrupt temperature and relative humidity fluctuations. Additionally, water vapor, being lighter than the rest of the air, tends to rise and accumulate farther from the plants.
- Minimize large slopes in greenhouses: In large greenhouses, colder air tends to accumulate in lower areas, leading to higher relative humidity in those zones.
- Control water usage in the afternoon during winter: Avoid overwatering in the late afternoon to prevent excessive moisture on the soil or plant surfaces when the windows are closed.
- Avoid excessive evening spraying in winter: Especially in unheated greenhouses, spraying large volumes of water in the evening can result in excessive humidity levels at night.
- Prune and defoliate appropriately: Proper pruning and defoliation enhance air circulation around the plants, reducing the likelihood of excessive humidity.
Fig.2 Remove lower leaves from tomato plants to improve air circulation.
- Install fans high up: Fans circulate air inside the greenhouse, enhance air circulation around the plants, and greatly improve the environment in its space.
Fig.3 Fans at the top for air circulation inside the greenhouse
Fig. 4 In ornamental plants that are sensitive to high humidity, the perforated surface of the growing table facilitates air circulation through the plants and prevents condensation on their foliage.
Provide limited ventilation at night when relative humidity is very high: In heated greenhouses, even during cold weather, it is important to slightly open the windows for a short period when the relative humidity inside is excessive. This allows the humid air inside to escape and be replaced by outside air, which, due to its lower temperature, has a lower absolute humidity. When the cooler outside air mixes with the warmer air inside, its capacity to hold water vapor increases, reducing the overall relative humidity. Although this process results in some heat loss, it can be compensated by the greenhouse's heating system.
Energy-efficient ventilation options: To save energy, specialized heat exchangers can be used. These devices allow the outgoing internal air to transfer its heat to the incoming external air. As a result, the temperature of the incoming air is only slightly lower than that of the outgoing air, maintaining a more stable internal environment with reduced energy loss.
To minimize the risk of plant disease, in most horticultural and cut flower crops, it is necessary to regulate the relative humidity in the greenhouse at levels that track the difference between plant and air temperature to move the dew point away from the plant surface. This approach helps prevent condensation on plant surfaces.
A common point of confusion arises when the relative humidity outside the greenhouse is significantly higher and the outside temperature is much lower than the temperature inside. In such cases, growers often wonder whether ventilating the greenhouse would increase or decrease indoor humidity.
Although the relative humidity inside may initially be lower than outside, the greenhouse's absolute humidity (measured as grams of water vapor per kilogram of air) is often higher—ventilation in such situations results in replacing the humid indoor air with drier, colder outdoor air. For every kilogram of air exchanged, more water vapor is expelled from the greenhouse than is brought in. Consequently, the absolute humidity of the greenhouse air decreases. If the heating system is operational and maintains the initial indoor temperature, the relative humidity will also drop significantly. This is because relative humidity depends on the amount of water vapor present and the air temperature. Warmer air has a higher capacity to hold water vapor, so maintaining the temperature further reduces the relative humidity percentage.
If the heating system is not operating, the air temperature inside the greenhouse will drop. Although the absolute humidity inside the greenhouse will decrease, the relative humidity is unlikely to decrease with ventilation, as the lowering of the air temperature counteracts this effect.
Opening windows at night in unheated greenhouses, especially those with a cover permeable to long-wave radiation, can have beneficial effects when the temperature of the plants is lower than that of the outside air. This situation often occurs during winter and spring nights with clear skies, as the plants experience a negative long-wave radiation balance.
Fig. 5 High heating pipes create a positive balance of long-wavelength radiation for the flowers and upper leaves, ensuring they maintain a higher temperature than the greenhouse air and preventing water condensation on them.
Fig. 6 Heaters high up help to avoid condensation on flowers and foliage.
Fig. 7. The heating pipes (of relatively low temperature) between the plants, along with the radiation and air movement, prevent water vapor from condensing on the foliage.
In greenhouses equipped with heating systems and thermal curtains for nighttime energy savings, the use of thermal curtains - especially on clear nights - not only conserves energy but also significantly reduces the drop in plant temperature, preventing it from falling below the greenhouse air temperature. However, it requires careful attention because if adequate ventilation is not maintained, there is a risk of excessive humidity buildup within the greenhouse while using the thermal curtains.
In theory, using a thermal curtain is generally not recommended when a cloudy night is anticipated with low wind speeds and moderate temperatures. This is because the curtain can reduce air leaks, which may result in excessively high humidity in the growing environment. Additionally, the potential energy savings in such environmental conditions are minimal.
Way to Avoid Low Relative Humidity
The low relative humidity, which primarily occurs during the day, can be effectively managed by spraying water as very fine droplets, allowing them to evaporate before reaching the plant level.
Further reading
Integrated Pest & Disease Management in Greenhouses: A Comprehensive Guide
Protected Cultivation: Boosting Yields and Sustainability in Modern Agriculture
How Controlled Environment Agriculture is Transforming Farming in the MENA Region?