Excessive heat can stunt plant growth and requires more frequent watering to keep plants healthy. The “ideal” house temperature will somewhat vary based on the crop. The majority of plants require daytime temperatures between 70 and 80 degrees Fahrenheit and slightly lower nighttime temperatures. Other crops grow better with cooler overnight lows. In general, as the internal temperature of a greenhouse exceeds 90 degrees Fahrenheit, plants begin to shrivel and die.
“It’s important that you find out what the recommended growing conditions are for the crop you’re raising, said Daniel Ciolkosz, P.E., research associate and assistant professor, academic program coordinator, at the Penn State Extension.
“If you’re growing multiple crops in the same house, you’ll have to find a happy medium that will work well for all of the plants,” he added.
Shade screens or shade tarps deflect the sun’s rays away from greenhouse crops, but screens aren’t enough to raise healthy crops in high temperatures. Ventilation, either natural or mechanical, is key to maintaining ideal indoor growing conditions even during the peak of summer.
“Temperature isn’t the only concern for greenhouse growers,” he said.
Humidity is equally important. “When the humidity is too high it can lead to fungus and other plant diseases,” he added.
Effective climate control is an important step toward producing plants and or produce that meets consumer demands. A carefully planned ventilation system allows a grower to effectively and efficiently manage both temperature and humidity.
As the name implies, natural ventilation relies on the movement of external air into and through the greenhouse structure. Originally, natural ventilation systems used roof vents and roll-up sides to draw fresh air into the greenhouse and circulate it throughout the structure. The exchange of air is less consistent compared with other ventilation systems and only works in facilities with ample space in between each structure.
Open roof ventilation, another natural ventilation strategy, uses a mechanical system to open the greenhouse roof, exposing plants to the open air. In this set-up temperatures can be more effectively managed with low energy costs.
Read more: Year-Round Greenhouse Growing
“Some growers prefer natural ventilation. They say it is more cost effective because there aren’t energy costs like there is with fan systems,” Ciolkosz said.
However, a well-designed fan system can be energy efficient and offer more precise control of internal environmental conditions. In an article written for eXtension.org, John Bartok, Jr., an agricultural engineer said, “Fan systems can provide well-controlled air movement through the greenhouse under all weather conditions. As the fans exhaust the heated air, a slight vacuum is created that draws in cooler outside air through louvers, open doors and cracks. The main advantage of fan systems over natural ventilation is that they provide precise airflow that operates regardless of the outdoor wind speed.”
Fan sizing, location and maintenance is critical to the system’s effectiveness. Bartok’s eXtension article, Greenhouse Ventilation, offers detailed recommendations for designing and installing an effective fan system.
Ciolkosz cautions that with a fan system there is a potential for pulling harmful insects into the greenhouse along with cooling air. “It may be necessary to install screening on the inlets to exclude insects from the growing area,” he said.
Even the most well-designed fan ventilation system is unable to cool a greenhouse during extreme heat. In extreme situations, the temperature inside the greenhouse can exceed the external temperature by as much as 10 to 20 degrees. “Evaporative cooling can help drop the internal temperature more than a fan ventilation system alone, often by several degrees,” Ciolkosz said.
In “Greenhouse Ventilation,” Bartok explained, “The heat in the air is used to evaporate water off plant leaves and other moist surfaces. Evaporative cooling works best when the humidity in the outside air is low. These conditions are most common in the dry Southwest, but even in the more humid northern sections of the United States there are many days in the summer when significant cooling can be obtained.”
The fan and pad system is the most commonly used design in evaporative cooling. “Aspen or cellulose pads are mounted on an end wall or a sidewall,” Ciolkosz said. “A water supply wets the pads. The fans pull air through the wet pads and cool the greenhouse.”
The damp pads can encourage algae growth, so adding an algaecide to the water can help prevent buildup.
The University of Massachusetts-Amherst Greenhouse Crops and Floriculture website provides detailed information on how to design, install and maintain an effective evaporative cooling system.
Another evaporative cooling alternative system uses fog or mist. “Similar to the misting systems you may see at large sporting events or other packed entertainment venues, a fog or a fine mist is injected into the inlet of the air stream and pushed out by the fans,” Ciolkosz said.
Commercial greenhouse supply companies sell pre-manufactured systems for easy installation. Do-it-yourself minded growers can assemble their own system using fog nozzles and a high-pressure piston pump.
Regardless of which ventilation system you choose, close monitoring is necessary. Multiple thermostats and humidity sensors should be installed throughout the greenhouse. To get the most accurate temperature control, thermostats should be located near the center of the greenhouse at plant height.
Without sensors, it can be difficult to visibly tell how stressed the plants are until it is too late. Once signs show up in the plant growth it may be too late to reverse the issues and the crop could be a total loss.
“Use more than one sensor to make sure the conditions are good throughout the greenhouse,” Ciolkosz said. “Computerized monitoring systems are available and send instantaneous information about the conditions in the greenhouse direct to you.”
You may be surprised to learn that temperatures that are too high can be more deadly to plants than temperatures that are too low. Heat can stunt plant growth, and cause plants to stop growing and even die.
Without proper ventilation, greenhouses and the crops they house become susceptible to a wide range of problems. An effective ventilation system regulates temperature and infuses fresh air, which plants need for photosynthesis. Ventilation also encourages pollination.
Each approach to ventilation has its advantages and drawbacks. Finding the system that best suits your operation is key. There is an abundance of information available to help you plan for good ventilation in your greenhouse. “For practical advice, go to your local extension office,” Ciolkosz said.
Additional detailed information is available online here and the UMass Greenhouse Crops and Floriculture program.