Ventilation / Circulation
Air movement in a crop is necessary to create a stable micro climate and a more even climate in the total green house area, in regard to temperature, humidity and CO2. High relative air humidity in general or in pockets due to moist soil or trans-evaporation of plants may cause saturation. In such an environment the plant cannot ‘breath’ anymore. Air currents will take the excess humidity away and the plant can function again, breathing, transpiring and cooling. Ventilation is often the single best method to avoid fungal infections like Botrytis, Downy Mildew, various leaf spots, etc. but we have to be careful about Powdery Mildew. Airflows in the green house shall not be stronger than say, 0.5 to 1.0 meter/second and to be effective it should be faster than 1 meter/second. The density and size of the crop plays a role.
In semi open green houses (fixed top opening, side vents, etc) it is impossible to create a steady airflow across the whole greenhouse. There will be (strong) winds or drafts coming in at the sides and there will be dead corners at other places. In most cases these occurrences of wind or still climate take place mostly in fixed locations. As a result there will be inequalities in the crop and these could be the ‘problem areas’ in the greenhouse. In case of frequent drafts (doors or side vents) initial development of spider mite will start here or powdery mildew spores will originate from these spots. Downy mildew generally starts in areas which lack ventilation. Although these spots can be seen as indicators, it is better to take action and solve the differences with the rest of the green house.
In case forced ventilation is used (extractors or circulation fans), it is good to realize that the intended steady flow is interrupted by turbulences caused by inflows from the top and from the sides. By closing the side vents on the long sides precisely and leaving the short side slightly open a tunnel effect will be created and a more steady flow can be formed. For this the fans have to be put in line, one after the other, at certain intervals, say every 30 meters with a distance of about 15 meter in between lines. The fans are built in tubes to create a turbo effect and thereby speeding up the airflow. Axial fans with a diameter of 10 to 14 inches are used for this purpose, with rpm of 1400 or more. Most efficient are fans that can be operated at different rotation speeds, instead of a total on/off system.
When it comes to management of air-circulation/ventilation in the green house, either forced or through side vents, there are some rules of thumb. Most of the year humidity is high during the night with a peak early morning at sunrise. During midday humidity is lowest, with exception of rainy periods.
The ventilation is often done in accordance with the level of humidity in the green house, maintaining the relative air humidity between 60 and 80%. This means full ventilation when humidity is high and slower air flows when humidity is low, respectively side vents open when high humidity and closed when low. Wind in the crop should always be avoided by closing wind side vents and opening the lee side. Work with hygro- and thermo-meters to make decisions; our feelings are not accurate. The meters have to hang just above the crop.
Automation of the ventilation system, opening and closing of side vents and top openings (by electric motor) can be done by use of a weather station (wind-speed, wind-direction indicators, temperature, radiation and rainfall control) supported by hygro- and thermo sensors in the green house. All this equipment may be linked to a software program, which allows to choose the settings as desired by the grower. The fans can also be operated by this system, measuring air flows. Once the electro motors are in place, these can also be managed with the manual ‘push the button’ instead of computerization.
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!
May you need any further information, please let us know.