Maintaining Consistency With a Greenhouse Ventilation System
Effectively moving air within a greenhouse can be a challenge. Here, Dr. J. Benton Jones, Jr. discusses some design flaws that cause, and some possible solutions to cure, this significant problem.
Anyone who has grown plants in a greenhouse is well aware of the challenges associated with how air is moved into and within the greenhouse structure. Most greenhouses are designed for a specific use so that the spacing and placement of exhaust fans and cooling pads, as well as interior mixing fans, are placed to make atmospheric control within the greenhouse as effective as possible.
Unfortunately, even with the best of designs, growers may be constantly struggling to establish and keep the interior of the greenhouse conforming to the requirements of the plants being grown.
Importance of greenhouse design
With most current greenhouse designs, it is not an easy task to maintain certain parameters within the interior of the greenhouse—as well as the plant canopy—when growing tomatoes. In some greenhouses, I have seen large fans mounted in the greenhouse gable area to push air toward the plants with the hope that there is sufficient force to penetrate and mix the air within the canopy.
In most commonly designed greenhouses, a cooling pad is mounted at one end of the greenhouse and an exhaust fan at the opposite end, usually 4 to 6 ft. off the greenhouse floor. Air is pulled through the cooling pad by the operating exhaust fans, and the cooled air is pulled solely through the top half of the tomato plants, leaving much of the air within the rest of the canopy undisturbed.
However, my best tomato yields were obtained in a polyethylene-covered, 30-ft.-wide greenhouse fitted with a cross-flow ventilation system of the exhaust fans and cooling pads mounted at floor level. This way the air flow was being directed through the plant canopy at the base of the plants.
Moving air in the greenhouse
Having moving air within the plant canopy has several significant advantages: it has a cooling effect, it keeps the plant foliage dry and the carbon dioxide concentration within the canopy air at the ambient level, and it stimulates the evaporation of transpiring water from leaf surfaces.
Maintaining upward movement of water in the transpiration stream within the plant reduces the potential for nutrient element deficiencies—for example, such water movement within the plant can reduce the potential for the occurrence of blossom-end rot in setting and developing fruit.
Keep in mind, however, that ventilation—important as it is—has its flaws. Drawing air into the greenhouse invites the potential for insect invasion; therefore, it is required that all openings be properly screened. However, depending on the mesh size of the screen, air flow can be restricted.
Also, ventilating the greenhouse will remove any extra carbon dioxide that has been generated to enhance photosynthesis—this is a significant, expensive loss that can reduce the effectiveness of the added carbon dioxide on plant growth. Most greenhouses in use today have these design faults. In order to correct some of these inadequacies, some growers place plastic pipes or tubes at the base of each tomato plant row.
This solves the canopy air mixing problem, but other insufficiencies remain. For example, the air being drawn into the greenhouse might not be sufficient to obtain the desired temperature, relative humidity or carbon dioxide content of the interior air.
The ideal ventilation system is to condition air in its own facility before introducing it into the greenhouse through floor vents. The return air is then taken from the greenhouse gable area to be processed through the air handling facility before introducing it once again to the greenhouse.
With such a recirculating system, the air properties can be more easily maintained at the desired temperature, relative humidity and carbon dioxide content. The only required opening into the greenhouse would be a double door entrance for workers; therefore, there are no other openings that can provide the entrance of pests and unconditioned air into the greenhouse.
I visited an experimentally designed greenhouse where air was being conditioned in a separate facility and then introduced into the greenhouse through holed tubes running along the base of the tomato rows.
The return air was drawn from the greenhouse gable and taken back into the air handling facility for reconditioning. The initial report on the yield of tomato fruit produced was a 15% increase as compared to that being obtained in an adjoining greenhouse using the traditional ventilation system of cooling pads and exhaust fans.
Based on this experience, as well as other observations I have made growing greenhouse tomatoes and advising growers, I would design a greenhouse with this type of air circulating system.
So, for growing tomatoes—and possibly for other crop plants—let’s do away with the current greenhouse designs that adversely effect the fruit yield potential of a tomato crop. I wonder who the first greenhouse manufacturer to step forward with such a design will be.