Relative Humidity (RH) represents the amount of water vapor currently in the air compared to the maximum amount the air can hold at that specific temperature. In the controlled environment of cannabis cultivation, managing this percentage is paramount to plant health and yield. A continuous Relative Humidity of 72% is generally considered too high for most stages of a cannabis plant’s life cycle. While this level might be acceptable for a very young plant, it becomes increasingly detrimental as the plant matures and begins to develop flowers. The optimal humidity level must be precisely matched to the plant’s developmental stage to support its physiological needs without inviting disease.
Humidity Needs Across Growth Stages
The plant’s relationship with atmospheric moisture is best understood through the concept of Vapor Pressure Deficit (VPD). VPD measures the difference between the actual amount of moisture in the air and how much moisture the air could hold when completely saturated. This metric is more accurate than RH alone because it accounts for temperature and dictates the rate at which the plant transpires, which drives nutrient uptake.
During the seedling and clone stage, a high RH is needed because young plants do not yet have robust root systems to absorb water efficiently. The plant relies on absorbing moisture directly through its leaves, making an RH range of 65% to 80% suitable for the first one to two weeks of growth. A 72% RH would be acceptable within this early window, supporting the plant’s moisture needs while it develops a root structure.
Once the plant transitions into the vegetative stage, the humidity requirement drops significantly. The established roots can now draw water from the growing medium, and the RH should be lowered to between 40% and 60% to encourage healthy transpiration. Maintaining 72% RH at this point would slow growth and start to create unfavorable conditions.
The flowering stage demands the lowest humidity levels to protect the developing flowers, with a target range of 40% to 50% in early flower and dropping to 30% to 40% in the final weeks before harvest. A 72% RH during this phase is risky and can compromise the entire crop. Gradual adjustments are always preferred to avoid shocking the plant as it progresses through its life cycle.
Risks of Excessive Moisture
Sustained 72% RH creates an environment where pathogens can thrive, posing a constant threat to the crop. The primary danger comes from fungal diseases such as powdery mildew and Botrytis cinerea (bud rot). These molds require high moisture to germinate and spread. The dense structure of mature cannabis flowers traps moisture, making them vulnerable to internal rot.
Excessive moisture interferes with the plant’s physiological processes, specifically transpiration. When the air is too saturated, the water vapor gradient between the leaf and the air decreases, causing the plant’s stomata to close. This reduced transpiration means the plant cannot effectively move water and nutrients from the roots to the leaves, leading to nutrient deficiencies and slower overall growth.
High humidity causes water to condense on the leaves and flowers, especially during cooler nighttime periods. These persistent wet surfaces are an ideal breeding ground for the proliferation of fungal spores.
Strategies for Controlling High Humidity
When a cultivation space hits 72% RH, the focus must be on actively removing moisture and preventing its accumulation. Effective ventilation and air exchange are the first steps, involving replacing warm, humid air with fresh, drier air from outside the growing area. This constant exchange helps to dilute the moisture released by the plants and maintain a stable environment.
Proper air circulation is necessary to prevent stagnant pockets of air and microclimates. Strategically placed oscillating fans should gently move the air above, below, and through the plant canopy, ensuring no single area retains excess moisture. Growers often remove some inner leaves, a process called defoliation, to further improve airflow within the dense canopy.
A dedicated dehumidifier is required to bring high humidity levels down to the target range, especially during the flowering stage. These machines remove water vapor from the air. Their capacity must be sized for the volume of the grow space and the moisture the plants are releasing. Using a humidistat or environmental controller automates the process, turning the dehumidifier on and off to maintain a precise setting.
Adjusting the watering schedule offers a practical form of humidity control. Watering the plants right after the lights turn on allows the plants to transpire the moisture while the environmental controls are most active and efficient. Avoiding watering right before the lights turn off prevents large amounts of water from evaporating into the air during the dark, cooler period when humidity naturally spikes.