Relative Humidity (RH) is the amount of water vapor in the air compared to the maximum amount the air can hold at that temperature, expressed as a percentage. For cannabis cultivation, controlling RH is crucial, similar to managing light intensity and nutrient delivery. The plant’s environmental requirements change significantly throughout its life cycle, demanding a dynamic approach to humidity management. Maintaining the correct RH ensures the plant performs its biological functions efficiently, directly impacting health, yield, and quality.
The Mechanism: How Humidity Affects Plant Function
Humidity must be controlled because it directly influences transpiration—the movement of water through the plant and its evaporation from the leaves. This process is driven by the difference in water vapor pressure between the inside of the leaf and the surrounding air, known as Vapor Pressure Deficit (VPD). VPD measures the “drying power” of the air and is the most accurate way to determine the rate at which a plant loses water.
When humidity is low, VPD is high, creating a strong pull that forces the plant to transpire rapidly. Excessive water loss causes the plant to close its stomata (tiny pores on the leaves) to conserve moisture. Closing the stomata limits the intake of carbon dioxide, which slows photosynthesis and restricts growth.
Conversely, when humidity is too high, VPD is low, and the air around the leaves is nearly saturated. This low VPD dramatically slows or stalls transpiration because the air cannot accept more water vapor. Since transpiration pulls water and dissolved nutrients from the roots, a stalled rate can lead to nutrient deficiencies. Furthermore, high humidity fosters an environment where fungal pathogens, such as mold and mildew, thrive.
Optimal Relative Humidity Across Growth Stages
The optimal Relative Humidity level for cannabis cultivation gradually decreases as the plant matures, aligning with its changing physiological needs. This progressive reduction ensures maximum growth while mitigating the risk of disease in the canopy.
Cloning and Seedling Phase
During the initial phase, clones and young seedlings have a limited or non-existent root system. To compensate, the plant must absorb moisture directly through its leaves, necessitating a high humidity level. Growers should aim for a Relative Humidity range between 65% and 75%. This high moisture content supports the delicate tissues, prevents the young plant from drying out, and allows it to focus energy on establishing a root structure.
Vegetative Phase
As the plant develops a robust root system and enters the vegetative phase, the Relative Humidity should be lowered to promote efficient transpiration. The ideal range during this period of rapid growth is between 40% and 70%. Maintaining moderate humidity ensures the plant can move nutrients effectively and build strong, leafy structures. Many growers gradually reduce the Relative Humidity by about 5% each week to prepare the plant for the next stage.
Early Flowering/Transition
The transition to the flowering phase requires a further drop in atmospheric moisture to protect developing flower sites. High humidity at this point poses a significant risk for fungal infections, such as bud rot and powdery mildew. The recommended Relative Humidity should be maintained between 40% and 50% during the first few weeks of bloom. This reduction encourages the plant to focus energy on flower production and resin development.
Late Flowering/Ripening
In the final weeks leading up to harvest, the dense, fully formed flowers are highly susceptible to moisture damage. The Relative Humidity must be at its lowest point to prevent mold from developing deep within the buds. Growers target a range of 35% to 45% during this ripening stage to ensure maximum density and quality. This dry environment also aids in preserving cannabinoids and terpenes, which contribute to the final product’s potency and aroma.
Post-Harvest Drying and Curing
Environmental control continues after harvest, as the post-harvest drying and curing process is important for quality. For the initial drying period, the Relative Humidity should be held between 55% and 62%. This range ensures the buds dry slowly and evenly over several days, preserving aromatic compounds and chlorophyll for a smooth final product.
Tools and Techniques for Environmental Control
Accurate monitoring is the first step in effective humidity management, requiring the use of reliable thermo-hygrometers to track both temperature and Relative Humidity within the cultivation space. Placing these sensors near the plant canopy provides the most representative data for the plant’s immediate environment. Digital models offer greater accuracy and are often easier to read.
To increase Relative Humidity, especially during the seedling and vegetative stages, a humidifier is the most direct solution. For smaller, enclosed spaces, simple passive techniques can be employed, such as placing wet towels or open containers of water near the plants. Clones and seedlings often benefit from a humidity dome, which creates a highly saturated microclimate around the young plants.
When the goal is to decrease Relative Humidity, typically necessary during the flowering phase, a dehumidifier is the standard equipment. These devices actively pull moisture from the air, often equipped with internal hygrometers to maintain a specific setpoint automatically. Consistent air circulation and ventilation are also powerful tools for reducing humidity and preventing localized moisture pockets.
Using oscillating fans to move air across the plant canopy helps the air exchange process and discourages the formation of a stagnant, humid layer around the leaves. Growers should be mindful of sudden humidity spikes, which commonly occur immediately after watering the plants. Scheduling irrigation to occur just as the lights turn on allows the room’s ventilation system to manage the excess moisture during the light cycle.