Issue 119: Importance of ventilation in commercial greenhouses

External view of pad and fan system.

(Image courtesy Elly Nederhoff)

If you have a greenhouse you definitely need to think about ventilation. Effective greenhouse ventilation is a very important function to produce a consistent quantity of high-quality produce.

Article by Wouter Claassens

Modern consumers demand produce that is consistently uniform with respect to aspects such as colour, taste, appearance, shape, shelf life and so forth. The closer the produce meets consumer expectations, the higher the quality of the produce and the higher the price the consumer is willing to pay. For the long-term financial survival of a commercial greenhouse operation, it is therefore of paramount importance to produce a consistent quantity of high quality produce.

Effective climate control is a very important step towards producing produce that meet consumer demands. The design of a climate control system would need to be able to modify the outside climate in order to create the ideal inside climate. Ventilation of a greenhouse is the factor that has the single biggest influence on effective climate control within a greenhouse. To fully understand this point, it is necessary to investigate the role of airflow around a plant.

The first role of airflow is to ensure a constant supply of carbon dioxide and oxygen to the plant. The rate of the two physiological processes, photosynthesis and respiration, can be limited if there is insufficient quantities of carbon dioxide and oxygen respectively. The atmosphere consists of 20.9% oxygen and 0.035% (or 350 parts per million, ppm) carbon dioxide. There is 597 times more oxygen than carbon dioxide in the atmosphere, making carbon dioxide one of the biggest limiting factors for plant growth. Airflow around a plant will help to maintain the level of carbon dioxide to around 350ppm. If there is no airflow, the concentration will drop and plant growth will be affected negatively.

The second role is to help regulate the air temperature around the plant. As the radiation energy from the sun heats the surface of the earth, the surface in return heats the air closest to it. As the air heats up, it rises into the atmosphere and gets replaced by colder air from higher in the atmosphere. Thus the whole volume of the atmosphere is utilised to absorb the radiation energy. This process is called natural convection. In a greenhouse, the volume of air available to absorb the radiation heat is limited by the height of the house, meaning the temperature of the air inside the greenhouse will rise faster than the temperature of the air outside. This is enclosed convection. (Another excellent example of this is the rate at which a car’s interior becomes hot when the windows are closed and the car is standing in the sun on a hot summer’s day.) In essence it can be said that a greenhouse is very efficient at capturing the heat radiated from the sun and making it available at plant level.

The third role is to regulate the water vapour (humidity) content of the air around the plant. During the plant’s physiological processes, water vapour is produced, which creates an air exchange between the plant and atmosphere. The rate of this air exchange is determined by the difference in the pressure of water vapour inside the plant and pressure of the water vapour outside the plant (Water Pressure Difference – VPD). As the physiological processes of the plant progress, the concentration of water vapour around the plant will increase (RH increase, VPD decrease). This will start to slow the physiological processes of the plant (low VPD), and increase the risk of a disease outbreak (high RH). Therefore, airflow is required to remove the excess water vapour from the plant. On the other hand, if the air is dry or the airflow rate around the plant is too high, the water vapour concentration around the plant will be low (high VPD, low RH). This will lead to excessive transpiration from the plant (high VPD) and increase the risk of disease outbreak (low RH).

When there is no ventilation of a greenhouse, the air condition inside will become different from the outside atmospheric air condition, primarily with respect to temperature, RH and carbon dioxide concentration. Once the greenhouse is ventilated, some of the inside air is replaced with outside air, which means the air condition inside will start to change towards the outside air condition. Effective climate control will therefore be striking a balance between providing plants the required airflow and maintaining the ideal climate inside.

It is a very well-known fact that plant production is very much influenced by temperature through complex physiological processes. For each plant species there are specific day and night temperatures that will result in optimum production (the production of a high quantity of high quality produce). While the plant is exposed to temperatures within these limits, life is easy and there is a lot of energy and nutrients available to the fruits. Under such conditions the profitability of the operations should be at its highest. Once the plant is exposed to temperatures several degrees outside the ideal band, production will be decreased. Life for the plant will not be so easy anymore. Some of its energy and nutrients will now be withheld from the fruit while the plant starts moving into day-to-day survival mode. Under these conditions the profitability of the operation will be compromised because the input costs will basically be the same as with optimum production, but production (quality and quantity) will be lower. If the plant is exposed to temperatures even further away from the ideal band, production will decrease dramatically and eventually seize. Life will now be very difficult for the plant and all its energy and nutrient will be spent on day-to-day survival, leaving none for the fruits. Under such conditions the operation will become unprofitable. This principle is illustrated in Figure 1.

Figure 1: When plants are exposed to narrow temperature bands around their ideal, a high quantity of high quality produce can be produced. As the temperature band becomes larger, the quantity and quality of the produce produced will decrease.

A greenhouse is essentially a poorly insulated structure that creates an artificial climate on its inside by isolating the air from the outside. Since the structure is so poorly insulated, the inside temperature is very responsive to changes in the outside temperature. Therefore, an effective climate control system would need to be able to control four variables namely temperature, RH, carbon dioxide levels and solar radiation intensity. There are various interrelationships between the four variables, which constantly leads to situations where trade-offs have to be made. How these trade-offs are approached is usually dependent on how temperature control is approached.

Optimum plant growth occurs within the broad boundaries specified in Table 1.

Table 1: Broad limits for optimum plant growth.

Plant growth is increased as light intensity is increased. However some plants are injured if they are exposed to intense light. The maximum intensity of light a plant can tolerate is very much species dependent and there is no broad guidelines.

Upon studying Figure 1, three possible scenarios for controlling temperature come to mind.

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