Moisture Management and Cannabis Plant Health

Ask any grower, and they'll tell you that among the most critical factors in any cannabis garden is water. Water plays a role in plant health in many different ways, from the water used to mix nutrient solution to the water found in the air in the form of humidity.
Cannabis plants require proper conditions to maintain their biological processes, such as photosynthesis and nutrient uptake, so it's important for growers to understand the significant role that moisture and moisture management can play in a grow room.

Several elements factor into maintaining proper moisture levels in your garden, such as the environment in which you live, the type of growing medium used, and the type of lighting employed. Other essential factors in indoor cannabis cultivation and moisture management include heating and cooling, proper ventilation, air circulation and filtration.

While it may seem like a lot to manage, it can be broken down and understood fairly simply by defining a few key concepts and understanding the moisture needs for marijuana plant growth.


Humidity vs. Relative Humidity (RH)

Probably the most crucial factor in proper moisture control of your grow room is humidity. Humidity is defined as the amount of water vapor in a unit volume of air and is usually found expressed in kilograms per cubic meter.

Relative humidity (RH), on the other hand, is typically of greater concern to the marijuana horticulturist, as it is a measure of the amount of water vapor in the air compared to how much water vapor the air can hold at a particular temperature. Relative humidity is typically expressed as a percentage; when the RH reaches 100%, the air is no longer able to hold any additional moisture, and precipitation begins.

Relative humidity, as it turns out, has an inverse relationship with temperature, meaning that as temperature increases, RH decreases, and vice versa. This makes it easy to understand why a grow room's lighting can play an integral part in maintaining proper humidity, as many lights tend to put off a large amount of heat.

Maintaining a balance between temperature and humidity tends to be one of the most significant challenges for novice cultivators, as many strategies used to mitigate one affect the other.

Vapor Pressure Deficit (VPD)

Vapor pressure deficit, or VPD, is the difference between the amount of water in the air and the amount of water the air can hold at a particular temperature. In other words, VPD is the measure of how much more room for water vapor there is in the air. VPD is directly related to the opening and closing of stomata, the tiny openings on the bottoms of leaves responsible for transpiration (the movement of water through a plant and its eventual evaporation from leaves).

A low VPD indicates high moisture content in the atmosphere, while a high VPD correlates with a low moisture content. In a high VPD setting, plants will close their stomata to preserve water, while in a low VPD setting, stomata will open widely, though the lack of a pressure deficit will cause plants not to transpire, effectively slowing their metabolic rate to zero.

Moisture Management Throughout the Grow Phases

As your plants mature from seedling or clone into fully-developed plants, their moisture needs will change significantly. Seedlings and clones fare best in high humidity and a low VPD, with most growers preferring to keep their cloning rooms at a humidity level of between 60-80%.

Because young plants (especially clones) have yet to establish a proper root system, all of their water uptake occurs through the stomata openings in their leaves. Increased humidity means more water available in the air for the plant to absorb through its leaves, providing enough water for the plant to establish roots and begin its growth phase.

Once plants transition into the vegetative phase, environmental moisture needs decrease as the plant begins up-taking water through its roots. Many growers prefer to maintain their veg rooms at a humidity level between 60-70% and a higher VPD.

At this phase, stomata begin to play their more specialized role of transpiration, intaking carbon dioxide (CO₂) and expelling water vapor. If humidity is too low, plants will transpire all of the water contained in the growing medium and close their stomata to preserve what water is stored inside the plant, causing it to wilt and eventually die.

In overly humid climates, plants will not feel the need to continue up taking water through roots and thus slow metabolic processes, leading to problems such as nutrient lockout. Often, nutrient lockout can be attributed to moisture issues rather than overfeeding. In addition, overly humid climates are breeding grounds for various pests such as whiteflies and fungus gnats; the development of these pests often results in the unnecessary introduction of pesticides into a grow.

As plants progress through the flowering stage, humidity levels must be controlled even more stringently, as increased moisture during this phase can lead to mold, powdery mildew, Botrytis and other pathogens. Generally, growers reduce the humidity in their flower room to between 40-55%. As buds near completion, humidity is often decreased to below 40% to further decrease the risk of disease as well as draw more resin from the plant, resulting in increased THC and cannabinoid levels, as well as enhanced terpene profiles. A high VPD is maintained in order to avoid excessive humidity build-up.

Moisture Control Solutions

Cannabis growers rely on several solutions to maintain proper moisture control over their grow rooms. Often, humidifiers and dehumidifiers are the weapons of choice in the battle of humidity, but fans, proper ventilation and sufficient airflow play an equally important role. As humidity and temperature are direct factors on one another, proper temperature maintenance plays a critical role in moisture control as well.

At TSRGrow, we've been working tirelessly to create high-quality lighting solutions that minimize temperature fluctuation. Our high-efficiency LED grow lights operate cooler than others, with some options generating practically no heat, as the ballasts have been decoupled from the lights themselves and placed outside of the grow room. With less temperature fluctuation from lighting, growers have increased controllability over their temperature, humidity, and all-round environmental conditions inside their indoor grow room which can also be controlled via environmental monitoring.

Be sure to check out our new podcast, Grow Sessions, a must for all budding cannabis cultivators looking to learn even more about maintaining proper humidity control of their grow room and cannabis plants. Our episode entitled, "Growing Pains - Moisture Management and Cannabis Plant Health," with guest expert Mike Sarro of RX Green Technologies will help answer all of your questions about keeping your cannabis plants healthy. 

If you are interested in learning more about TSRgrow growing solutions, please contact a growing specialist today or visit our website at


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