The even distribution of air throughout a greenhouse is essential for consistent temperatures and uniform plant growth. Also, greenhouse air should move slightly as often as possible to minimize mildew and other diseases of plants that proliferate under stagnant air conditions. Ventilation of greenhouses is accomplished by using:
• roof and side ventilators
• exhaust fans
• fan and convection tube systems
• a retractable roof
Use of roof and side vents for air exchange is based on the principle that heated air rises. Hot air exits through the roof vent, while fresh, cooler air enters through the side vents. When ventilators run the length of the roof and sides, air exchange is consistent throughout the house. Use of ventilators alone is the oldest method of ventilating greenhouses (Figure 19-11).
Exhaust fans can be used in warm weather to draw old air out of the house and pull fresh air in. Fans can supplement roof and side ventilators or be used in place of them as in small, plastic-covered quonset houses (Figure 19-12).
When the outside air is so cold that its direct entry into the greenhouse would cause plant injury, a plastic convection tube can be used to permit warming and even distribution of the air before it enters the greenhouse atmosphere. A polyethylene sleeve with holes, the tube inflates and pulls outside air into the house when fan jets operate. The convection tube system may be paired with exhaust fans as well as heaters or cooling systems to provide temperature-adjusted fresh air for the greenhouse (Figure 19-13).
figure 19-13. Use of a convection tube ensures even distribution of heated air through the greenhouse. (Delmar/Cengage Learning. Photo by Steven Newman.)
Cooling the greenhouse is necessary many times during the year and is vital if the greenhouse is to be used during the summer months. Three basic approaches are taken depending on the time of year and geographic location:
1. Shading the glass to reduce light intensity.
2. Ventilating to allow cooler outside air to replace warm air inside.
3. Promoting heat exchange through water evaporation.
Greenhouse shading can be obtained by spraying a compound onto the glass externally. It is a temporary shading that gradually washes away with the rain. In the South, a more permanent shading compound may be used. The compounds are available in both liquid and powdered forms. Those containing binders persist longer (Figure 19-14).
Shading can also be applied internally as cotton, polyester, or saran cloth sun screens suspended above a crop to reduce the light intensity (Figure 19-15). The saran is available in different weaves that determine the amount of light reduction. The screens can be automated to be pulled across the greenhouse in response to signals from photocells. This gives the cloth sun screens an advantage over exterior shading sprays. The sun screens only reduce the light intensity when it is needed. Sprayed shading reduces the light intensity all day long, even on cloudy days when the reduced light is reduced even more, perhaps to a detrimentally low level. An additional advantage of some sun screens is that they can also be drawn over crops to retain heat during cold weather.
As described earlier, greenhouse ventilation systems bring fresh air into the greenhouse to replace the warm internal air. As long as the outside air is cooler than the temperature desired for the greenhouse,
ventilation will cool the greenhouse. During especially hot weather, just opening the vents may not be enough. Some greenhouses have been structurally modified to improve their ventilation and cooling capability. While the modifications vary, the structures are collectively termed retractable roof greenhouses. They permit the roof of the production facility to be pulled back when desired, thereby providing a more natural environment for growth of the plants when conditions are suitable. When the outside weather is not good, perhaps too cold, too hot, too wet, too arid, or even too bright, the roof can be restored and the desired environment recreated within the closed greenhouse.