Heat and ventilation for greenhouses

Greenhouses offer year-round opportunities to expand gardening efforts. With an environment under glass or some type of plastic, the conditions can become either quite hot or quite cold, depending on outside conditions. Balancing the environment for optimal growing conditions can take some trial and error, and requirements will depend on the type of greenhouse you have, how it is outfitted, and whether the plants you are growing need a cold, cool, temperate, or warm environment. 

Does your greenhouse need heat and ventilation?

Heating and ventilating a greenhouse go hand-in-hand. Heat loss results by air infiltration so if louvers and fan shutters do not close tightly, for example, the efficiency of the heating system is affected. On the other hand, even when it may be very cold outside, some ventilation in the greenhouse is important. As Florida Extension points out, fresh, outside air ventilated into the greenhouse to remove warm, moisture-laden air is necessary. Otherwise, an environment high in humidity and condensation can result in the risk of fungal diseases. 

The temperature in your greenhouse makes a difference. Plants grow faster and flower earlier at the optimal temperature. Purdue Extension shares that slow growth occurs at 50 to 68 degrees F while optimal growth occurs at 68 to 78 degrees F. Plants will quit growing when temperatures dip below 50 degrees and excessive growth occurs at 78 to 85 degrees while 90 degrees and more can cause heat damage. Heating and ventilation for both cooling and better air circulation are important. 

Greenhouse heating considerations

Heating your greenhouse is important for year-round gardening. There are several options when it comes to the type of heat. Among them are electricity, wood, natural gas, LP gas, or fuel oil. The cost and availability vary depending on the region. Georgia Extension says that convenience, investment, and operating costs are other considerations. For example, saving in labor may justify a more expensive heating system with automatic controls. 

4 types of greenhouse heating systems

Oklahoma State Extension shares these four types of heating systems and considerations for each:

• In small hobby greenhouses, low-cost heating options are space heaters. Pair them with fans to help improve circulation. If using gas, oil, or coal-fired heaters, make sure you have a fresh air supply and exhaust venting to avoid carbon monoxide and ethylene accumulation. 
• A forced-air heater with a duct or plastic tube system to distribute heat and a thermostat to control the temperature is a good option for small greenhouses. 
• Hot-water or steam heaters with a circulator or a steam system linked with automatic ventilation provides more control over temperature fluctuations than forced-air systems.
• Electric heating system with overhead infrared heating equipment combined with soil cable heat provides a localized plant environment that allows plants to thrive. 

Ventilation systems for your greenhouse

Good ventilation is essential year-round, particularly during both summer and winter months. In the summer, the hot sun bearing down on glass will increase the temperature. Ventilation through either a fan ventilation system or by natural means will help remove the inside air and replace it with outside air and carbon dioxide, which plants need for photosynthesis. In the winter, ventilation helps keep the relative humidity levels and prevents any fumes or water vapor buildup due to certain heating systems, such as kerosene or natural gas. Proper ventilation helps eliminate any condensation, which is important in helping reduce any chances of diseases that often thrive in moist conditions.

Popular types of ventilation

There are several types of ventilation systems. Among them:

• Natural or wind ventilation optimizes air movement when ventilators are placed on the sides and roof of the greenhouse. 
• Louver ventilators are positioned lower in the greenhouse and, according to the American Horticultural Society, are more useful in controlling the flow of air throughout the greenhouse in winter, when roof ventilators may allow too much heat to escape. In other seasons, a combination of roof vents and lower ventilators, sometimes referred to as a chimney ventilation system, is effective in removing hot air as it rises and replacing it with fresh air. 
• Hinged ventilators are typically fitted to the roof of the greenhouse and open wide to allow for airflow. 
• Fan and duct ventilation suspend plastic ducts that distributes air flow from a fan-heater-louver system.

Automatic ventilation

Oklahoma State Extension recommends automatic ventilation that uses a thermostat and electric motor to open and close the vents as the best way to cool a greenhouse. It cuts the manual work that is needed with hand-operated roof vents.  

Temperature sensitive hydraulic pistons can be used to open and close small side and roof vents. This is attractive since proper ventilation can be maintained in case of an electrical outage. In addition, Oklahoma Extension recommends circulating and exhaust fans. They recommend placing the exhaust fans high enough to prevent direct drafts on the plants near the intake.

Planning heat and ventilation for a greenhouse

As you consider installing your own greenhouse, explore the various heating and ventilation systems that best meet the size of your greenhouse and the needs of your plants along with your budget requirements and what best optimizes your time and efforts. In addition, the type of greenhouse, its location, shading cloths to help lower temperatures, and greenhouse supplies are among the other considerations needed as you look to take advantage of year-round gardening.