Air Ventilation and Circulation in Greenhouses
Creating a better climate for the crop
Introduction
Maintaining optimal air movement within a greenhouse is crucial for establishing a stable microclimate and ensuring a uniform environment across the entire growing area. This uniformity comprises temperature, humidity, and carbon dioxide (CO2) levels, all vital for healthy crop development. In protected cultivation, we have more tools to manage the air movement, but in open field crops, it is also valid to have cross ventilation, for the same reasons.
High relative humidity hinders a plant’s ability to release water vapor, a process essential for internal water transport and self-cooling. Transpiration leads to a build-up of humidity, particularly around the plants themselves. Air currents play a key role in dispersing this excess moisture, allowing the plants to resume their vital functions: respiration, transpiration, cooling, and ultimately, growth.
Air movement in a crop is necessary to create a stable microclimate and a more even climate in the total greenhouse area, in regard to temperature, humidity and CO2. High relative air humidity does not allow the plant to release water vapour, necessary for the internal water flow and cooling itself. Because of the trans-evaporation, the highest humidity is found around the plants. Air currents will take the excess humidity away and the plant can function again, breathing, transpiring, cooling and growing. Ventilation (exchange of greenhouse air) or circulation of the greenhouse air is often the single best method to avoid fungal infections like Botrytis, Downy Mildew, various leaf spots, etc. However, we have to be careful about Powdery Mildew, as airflows will transfer spores throughout the crop. The flow just over the crop shall not be stronger than say, 0.5 to 1.0 meter/second. The density and size of the crop plays a role.
In semi-open greenhouses, characterised by fixed top openings and moveable side vents, achieving a consistent airflow across the entire structure proves challenging. Incoming air through the side vents can create strong winds or drafts in certain areas, while other zones may experience stagnant air. These variations in airflow often occur in predictable locations, leading to inconsistencies in crop development and the potential for “problem areas.” Frequent drafts, particularly near doors or side vents, can encourage the initial development of spider mite infestations or, as mentioned, serve as starting points for powdery mildew outbreaks. Conversely, areas with poor ventilation are often susceptible to downy mildew. While these localised issues can act as indicators of ventilation problems, proactive measures to equalise the environment throughout the greenhouse are essential.
Temperature- and humidity distribution in a greenhouse
Managing humidity levels
When employing forced ventilation systems, such as extractor or circulation fans, it’s important to recognise that the intended steady airflow can be disrupted by turbulence arising from air entering through top and side vents. To mitigate this, precisely close the side vents on the longer sides of the greenhouse while leaving the shorter sides slightly open to create a “tunnel effect,” promoting more uniform airflow. For this strategy to be effective, fans should be positioned in a series, typically spaced at 30-meter intervals with approximately 15 meters between parallel lines of fans. Enclosing the fans in a housing can enhance their efficiency by creating a “turbo effect,” thereby increasing the airspeed. Axial fans with diameters of about 20 inches and operating at speeds of 1400 revolutions per minute or higher are commonly used for this purpose. Fans with variable rotation speed offer greater efficiency compared to simple on/off systems. A general guideline suggests approximately 10 fans per acre, though this can vary depending on the specific crop.
Effective management of air circulation and ventilation, whether achieved through forced systems or natural flows, follows several key principles. Typically, humidity levels are highest during the night, peaking in the early morning around sunrise. Conversely, humidity is generally lowest during midday, except during periods of rain.
Ventilation is often adjusted based on the greenhouse’s humidity levels, aiming to maintain a relative humidity of say 60%. This translates to maximise ventilation when humidity is high and reducing airflow when humidity is low, achieved by opening side vents during high humidity and lowering them when humidity is low. Direct wind on the crop should always be avoided by closing windward side vents and opening leeward vents. Relying on accurate hygro- and thermometers positioned just above the crop is crucial for making informed decisions, as our subjective perception can be unreliable.
Ventilating solely based on temperature can have negative consequences. Opening vents during the hottest periods often coincides with extremely low humidity. While many crops can tolerate higher temperatures, low humidity can cause stomatal closure, hindering growth.
Automation
Automating the ventilation system, including the opening and closing of side vents and top openings using electric motors, can be achieved through integration with a weather station. This station typically provides data on wind speed, wind direction, temperature, radiation, and rainfall. This external data is complemented by readings from hygro- and thermo-sensors within the greenhouse. All this data can be fed into a software program, allowing growers to customise ventilation settings according to their specific needs. Fans can also be integrated into this automated system, with air flow measurements providing further control. Even with automated systems in place, manual override options for the electric motors offer flexibility.
For automation and semi-automation, the equipment has to be installed and maintained precisely and in good condition. A rolling pipe out of place can do a lot of damage when operated by an electromotor!
Conclusion
Because of dead corners in the greenhouse climate factors, like temperature, humidity and CO2 level may differ from mainstream areas. Especially in and around the crop, there may be buildup of temperature and humidity. By creating strategic airflows these differences will be evened out. With ventilation and circulation, we are able to create a more conducive climate for the plants.