Effective environmental controls key in managing grow house VPD, transpiration

Temperature. Relative Humidity. Airflow. While these are indisputably the core variables in the HVAC industry, managing them through effective environmental controls takes on a heightened role in the booming U.S. cannabis indoor grow market.

And booming it is. Currently in the U.S. marijuana is fully legal in 19 states and the District of Columbia, and 27 states have a mixed-status of medical, recreational, and/or CBD.

Phillip Daugherty

Group Product Manager, North America


Nearly half of the U.S. population lives in states where adult-use cannabis is legal or will be this year (2022). Legal sales hit $24 billion in 2021 and are expected to grow to $47.6 billion by 2026. As the legal cannabis market in the U.S. continues to experience exponential growth and attracts many newcomers hoping to cash in, more and more facilities are coming online.

Environmental Control is crucial to a healthy harvest

While the allure to join the legal cannabis fray is enticing, there are many components and variables that must be in synch to achieve success. Any successful cannabis grow house requires proper plant genetics, lighting, the right mix of nutrients and water called fertigation, and most importantly, environmental controls.

Environmental control requires specific HVAC and related equipment in the grow space to maintain proper temperature and relative humidity – the amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature. The balance of temperature and relative humidity creates the Vapor Pressure Deficit, VPD, which is the difference between vapor pressure within a plant’s leaves and the vapor pressure of the surrounding air.

Ideal VPD slowly draws water vapor from the leaves’ pores, a process known as transpiration, thus allowing the additional drawing of water and nutrients up through the roots. The constant intake of water allows the plant to absorb nutrients and exhale by-products such as O2 and water vapor. Up to 95% of all water supplied to a plant is transpired into the air surrounding the plant.

If the pressure deficit is too low the plants cannot exhale water vapor, which stops the flow of water and nutrients through the plant. If the pressure deficit is too high the plants react by closing the stomata to conserve water. However, this also prevents the intake of CO2, which stresses the plant and restricts growth. To further complicate matters, the ideal VPD changes throughout the grow cycle, highlighting the importance of selecting the proper equipment and controls to regulate the VPD in the grow space.

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Effective airflow helps plants grow

Temperature and relative humidity maintenance is the job of HVAC units. Airflow is the key element to ensure that the HVAC equipment can do its job. The best HVAC system in the world is useless without air distribution. Effective air distribution ensures an even mix of conditioned air throughout the grow space. This prevents hot and cold spots and humidity spikes in isolated pockets called microclimates.

Microclimates are most prevalent in the dense canopy present at peak flower when transpiration is highest. This is also where microclimates are most destructive. Humidity and heat can create a ripe breeding ground for powdery mold, mildew, and numerous other pests, effectively destroying the value of entire crops.

Thorough mixing and distribution of conditioned air throughout the canopy and grow space are essential in allowing the HVAC equipment to do its job. After all, the system cannot remove the excess heat and humidity from the air if it remains trapped in the plant canopy.

Many growers have turned to fabric ducts for air distribution in their grow space. The fabric duct is custom engineered and makes use of linear laser-cut perforations to deliver the conditioned air precisely and evenly to the entire canopy. This distribution is critical to remove humidity from the plant canopies before it has a chance to form microclimates. Coupled with the ease of installation and simple maintenance, fabric duct is the “go-to” for growers “in the know.”

Growers must take the time to identify VPD and transpiration rates. It’s critical to have these ideal rates in mind when designing a new facility or updating an existing one to verify that ventilation specifications are on target. As you can see, effective management of VPD and transpiration plays a critical role in ensuring happy plants, cultivators, and investors.


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