The environment inside a grow tent is a closed system where plants continuously release moisture through transpiration. This moisture accumulates, raising the relative humidity (RH). Maintaining appropriate RH, typically between 40% and 60% during the vegetative stage and 40% to 50% during flowering, is necessary for healthy plant development. High RH saturates the air, slowing the plant’s ability to transpire and absorb nutrients, which can lead to stunted growth and nutrient deficiencies. Excess moisture also provides an ideal breeding ground for pathogens, such as powdery mildew and Botrytis (grey mold), that can rapidly compromise an entire crop.
Optimizing Airflow and Ventilation
The primary strategy for controlling humidity involves continuously replacing the moist, stale air inside with drier air from the surrounding room. This requires a properly sized exhaust fan system to pull the humid air out of the tent. To size the fan correctly, calculate the tent’s volume (length × width × height). Select a fan rated for a cubic feet per minute (CFM) output that can exchange the air at least once every one to three minutes.
Air replacement uses either passive or active intake methods. Passive intake relies on the negative pressure created by the exhaust fan, pulling air through open vents or ports at the bottom of the tent. For a more controlled exchange, an active intake fan can push air into the tent. This fan should be rated for a slightly lower CFM than the exhaust fan to maintain slight negative pressure, which prevents odors from escaping.
Beyond moving air in and out, internal circulation prevents pockets of stagnant, humid air around the plant canopy. Oscillating clip-on fans should be placed inside the tent to gently move air across the leaves and stems. This constant air movement breaks up the boundary layer of saturated air around the leaves, promoting healthy transpiration and nutrient uptake.
The ducting setup also determines ventilation efficiency. Sharp bends or long runs of flexible ducting significantly reduce the fan’s effective CFM rating. Minimizing 90-degree turns and using smooth, rigid ducting helps the fan operate closer to its rated capacity. This maximizes the system’s ability to extract humid air.
Active Moisture Removal Systems
When the ambient air outside the grow tent is high in humidity, ventilation alone is insufficient, requiring specialized equipment to physically remove moisture. Dehumidifiers draw air over a cold coil, cooling the air below its dew point. This causes water vapor to condense into liquid water that is collected in a reservoir.
For larger tents, a compressor-based dehumidifier is the standard choice, offering high efficiency and capacity. These units are robust but generate a small amount of heat, which can be beneficial in cooler environments. Smaller tents may utilize a thermoelectric (Peltier) dehumidifier, which is quieter and produces less heat, though its capacity is lower and better suited for managing localized humidity spikes.
Proper placement is important for maximum effect, typically near the exhaust fan intake or in the main airstream. Many growers opt for a continuous drainage setup, routing the collected water directly to a drain. This eliminates the need to manually empty the reservoir and ensures uninterrupted operation.
Integrating a humidity controller, or humidistat, automates the process by allowing the grower to set a target RH level. The dehumidifier turns on only when the humidity exceeds this set point and shuts off once the target is reached. This maintains stable conditions and prevents the air from becoming too dry.
Controlling Water Sources and Canopy Density
Growers can proactively manage humidity by addressing moisture sources, primarily water evaporating from the medium and water transpired by the plants. The timing of watering affects humidity spikes. Watering right before the lights turn on allows the ventilation system and heat to remove the immediate spike caused by fresh water. Conversely, watering before the lights turn off can lead to prolonged periods of high RH in the dark, cool environment, increasing the risk of fungal growth.
The density of the plant canopy plays a large role in the overall moisture load. An overly dense canopy restricts airflow and creates a humid microclimate within the foliage. Strategic pruning and defoliation reduce the total surface area available for transpiration. This practice lowers water vapor release and improves the penetration of circulating air.
Managing the growing medium and controlling runoff minimizes non-transpired water evaporation. Ensure that saucers or trays beneath the pots do not contain standing water, as this evaporates directly into the tent air. Warmer air holds more water vapor than cooler air. Understanding this relationship explains why relative humidity spikes when the lights turn off and the temperature suddenly drops.