Humidity in Greenhouses

Maintaining the humidity in your house is important if you want to avoid mold, pests, and potential health issues. Similarly, it’s also important to maintain the humidity in a greenhouse. While plants do need some humidity to survive, humidity levels should be carefully monitored, along with the temperature of the greenhouse. Both temperature and humidity can shift dramatically within a day.

Relative humidity describes the amount of water vapor in the air. When the temperature changes in a greenhouse, it also affects the relative humidity. Greenhouses always have some moisture that evaporates from plant leaves and soil. This moisture then becomes suspended in the air as water vapor. A wide variance in temperature can lead to a problematic moisture buildup. At high temperatures, the air can hold more water, leading to higher relative humidity. As the air swings to colder temperatures, it loses water vapor in the form of condensation, which can be harmful to plants.

One of the other issues with too much humidity is the increased risk of disease. When moisture sits on leaves from high humidity, it can cause the plants to be unhealthy. This is often times a fungal or mildew issue, such as botrytis, which is powdery mildew on the leaves of plants. In addition, too much humidity combined with high temperatures could force plants to conserve water. In this situation, the stomata of the plant will close to save water. This action restricts the flow of O2 and CO2, which could suffocate the plant if it goes on too long. Once a disease starts, it is common for the disease to spread throughout the entire greenhouse.

The thermal environment of a building is comprised of temperature, relative humidity, and airflow. Thermal environment plays a huge role in the health and happiness of employees. Often times, simply regulating the temperature or humidity so it’s more comfortable for the workers can erase many of their symptoms. A large portion of thermal environment is determined by the HVAC system but it is also affected by amount of sunlight, position of air vents, and amount of activity by employees.

Even without high humidity, pests are more dangerous in a greenhouse where there are no predators or harsh elements compared to their natural environment. Now add in high humidity which attracts pests, and the plants are very susceptible. Common pests found in a greenhouse may include aphids, thrips, and whiteflies, all of which can spread diseases to plants.

There are steps that can be taken to help maintain the humidity in a greenhouse and ensure the health of plants. One example is to make sure the plants receive the proper amount of watering versus over watering them. Additionally, make sure the plants have adequate spacing and that the floors are well drained. Another important method for maintaining the humidity is ensuring there is proper air flow and ventilation. Monitoring the humidity, in addition to following these steps, can ensure high humidity doesn’t become a problem in the greenhouse.

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Bartok, John. “Reducing Humidity in the Greenhouse.” Center for Agriculture, Food and the Environment, University of Massachusetts Amherst, 30 Mar. 2017,
Moore, Sarah. “What Is the Ideal Humidity Level for a Greenhouse?” Home Guides | SF Gate, Hearst Newspapers, 21 Nov. 2017,
Nichols, Robin. “Promoting Greenhouse Ventilation and Dehumidification.” Garden & Greenhouse, NUGL Media Group, Nov. 2017,