Airflow management

Cannabis growers are increasingly moving into greenhouses, but they need to be designed to mitigate the risks of humidity and condensation

Greenhouses are a cost-effective method for cannabis growers looking to improve crop quality and profitability. By combining natural sunlight with the control allowed in an indoor grow, greenhouses can create the optimal growing environment at a reduced cost, while still allowing year-round production. However, one of the primary concerns for greenhouse growers is the development of mold and mildew.

While growing cannabis in a greenhouse is increasing in popularity, the risk of mold and mildew growth is just as relevant as it is with other growing techniques. To prevent mold and mildew, growers should first focus their efforts on controlling humidity. For further risk reduction, there are a few supplemental measures growers can take.

Humidity and condensation are the biggest contributors to the growth of mold and mildew. Depending on the stage of growth and grower’s preferences, relative humidity levels should stay between 30% and 70%. Higher humidity levels, along with stagnant air and improper ventilation, create the perfect conditions for mold and mildew.

Growers will find the highest humidity levels within the plant canopy. This is also the area most likely to hold stagnant air created by a lack of airflow. Improper ventilation and poor plant spacing contribute to increased stagnant air underneath the plant canopy, leading to lower CO2 and oxygen levels, which can stunt plant growth and decrease yield. The more stagnant air and higher humidity within the plant canopy, the more likely mold and mildew growth will happen.

Fortunately, there are plenty of options available to control a greenhouse’s humidity range. The most effective method to lower humidity levels is ventilation. There are two forms of ventilation in greenhouses: passive and active.

Passive ventilation is the more affordable option, as it relies on the wind effect and thermal buoyancy to promote air exchange within the structure. The wind effect can help replace stagnant air with fresh air from outside. Thermal buoyancy refers to hot air’s tendency to rise, so when cooler air enters the greenhouse, stale, humid air is pushed up and out.

Growers can take advantage of this relationship by installing strategically placed ventilation equipment, such as roof and wall vents. Greenhouses require a wind speed of about 2-3 mph for proper passive air exchange. Passive ventilation can be very effective in reducing humidity when outside conditions are just right. Unfortunately, that won’t be the case many days of the year, so growers will also need to utilize active ventilation.

Active ventilation is the use of mechanical equipment to force air circulation throughout the structure. Active ventilation is the best way to create a positive air environment, meaning that when there is an opening in the structure, air is forced out instead of being taken in. Creating a positive air environment is one of the best ways to reduce humidity and decrease risk of mold and mildew.

The main tools for active ventilation are fans. In addition to hanging circulation fans to facilitate air exchange, growers should also use exhaust fans to draw cooler air in through vents, louvers and doors. Greenhouse fan systems should be properly sized to provide one full-volume air exchange per minute. Fan systems also work best when the draw distance is less than 150 feet, so growers should take care when designing their system.

Without ventilation, trapped heat will mix with carbon dioxide produced by photosynthesis to create a hot and humid atmosphere, the perfect scenario for mold and mildew to develop. Growers looking to prevent the spread of molds or mildew can combine passive and active ventilation for effective and affordable humidity control.

Planning the greenhouse design and layout to inhibit the growth and development of mold and mildew adds another layer of protection against contaminants. A south-facing greenhouse will intake more sunlight and help prevent the build-up of moisture, for example. Growers should also organize the layout of their greenhouse to provide adequate plant spacing. When plants are grouped together too closely, they can trap heat beneath the canopy and create a lack of airflow. Since plants transpire moisture out of their leaves, these conditions can increase the risk of mold. Proper spacing will improve air circulation and prevent heat buildup and increased humidity.

Cleaning the greenhouse thoroughly between crops is another way to inhibit mold and mildew. Growers should also take care to complete tasks that can contaminate the crop outside of the grow area. Washing or sanitizing benches, tools and other equipment should done in a room separated from the plants to minimize the risk of contamination.

Mold and mildew can affect any cannabis grow with little to no warning. Luckily, with proper humidity control and use of preventative measures, greenhouse growers can impede the growth of these dangerous contaminants. Using these preventative methods can save time, money and loads of stress.


Nicholas St. Pierre is a junior copywriter for GrowSpan (, which specializes in greenhouses and growing solutions.


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