The high relative humidity (RH) within a grow tent presents a common challenge for indoor cultivators, directly impacting plant health and yield. Plants naturally release substantial water vapor through transpiration, which rapidly saturates the air in a small space. Excessive moisture creates an environment highly susceptible to the growth of destructive pathogens like mold, mildew, and bud rot, especially during the flowering stage. High humidity also inhibits the plant’s ability to transpire and properly uptake nutrients. This guide provides actionable methods to effectively reduce and maintain optimal humidity levels in a grow tent.
Establishing Control: Monitoring and Target Humidity Levels
Effective humidity control begins with accurate measurement and a clear target, requiring a digital thermo-hygrometer placed at the canopy level to monitor temperature and RH. The necessary humidity level changes significantly throughout a plant’s life cycle. Seedlings and clones require a high RH, often between 60% and 85%, to prevent desiccation while their root systems are developing.
As plants mature into the vegetative stage, the RH target should gradually be lowered to a range of 40% to 70%. The flowering stage demands the lowest humidity, typically 40% to 50%, with some growers aiming for 30% to 40% in late flower to prevent fungal issues. The ultimate metric for precise environmental control is Vapor Pressure Deficit (VPD). VPD represents the difference between the actual amount of moisture in the air and the maximum amount the air can hold at a given temperature. This provides a more accurate measure of the plant’s transpiration rate, allowing growers to adjust temperature and humidity in tandem to optimize water and nutrient uptake.
Optimizing Air Exchange and Movement
The most fundamental method for reducing humidity is continuously replacing the warm, moisture-saturated air inside the tent with cooler, drier air from the surrounding environment. This is accomplished using an exhaust fan system that actively pulls air out of the tent through ducting. The performance factor for an exhaust fan is its Cubic Feet per Minute (CFM) rating, which must be calculated to ensure a complete air exchange occurs frequently, ideally every one to three minutes.
The basic CFM requirement is determined by multiplying the tent’s length, width, and height in feet, which gives the volume of the space. This calculated volume must be adjusted upward to account for resistance factors that decrease fan efficiency. Factors like the use of a carbon filter, which can reduce airflow by 25% or more, and the resistance created by ducting and 90-degree bends necessitate a fan with a higher rated CFM.
The exhaust system creates negative pressure inside the tent, causing fresh air to be passively drawn in through lower intake vents. Increasing the speed or frequency of the exhaust fan directly lowers the RH by venting humid air outside. Internal air movement, achieved with oscillating fans, is also necessary to prevent pockets of stagnant, humid air around the plant canopy. These circulation fans help strengthen plant stems and ensure the air being exhausted is representative of the entire environment.
Reducing Environmental Moisture Sources
Controlling the sources of water vapor within the tent is a preventative strategy that complements mechanical dehumidification and air exchange. The schedule and method of watering significantly contribute to the humidity level, as any water not immediately absorbed by the roots will evaporate. Watering plants only when the top one to two inches of the growing medium are dry prevents excess moisture from evaporating.
Scheduling watering sessions to occur just as the lights turn on allows the plants to transpire the moisture while the temperature is warmer, promoting rapid evaporation and removal by the exhaust system. Growers should ensure that all planting containers have proper drainage to prevent water from pooling at the base, which increases the surface area for evaporation. The growing medium itself can be a factor, with certain choices retaining less water and promoting quicker dry-back cycles.
It is also important to remove any standing water, such as runoff from watering trays or uncovered hydroponic reservoirs, immediately after the plants have finished drinking. Overcrowding the tent restricts air movement and leads to excessive collective transpiration from the foliage. Regular pruning to remove dense lower leaves increases airflow through the canopy and reduces the total leaf surface area available for releasing moisture vapor.
Mechanical Dehumidification Equipment
When ventilation and source control are insufficient to maintain the target RH, mechanical dehumidifiers provide the most direct means of removing moisture from the air. Dehumidifiers are categorized as either compressor-based or desiccant-based, and the choice depends on the operating temperature of the grow space. Compressor (refrigerant) dehumidifiers operate similarly to air conditioners by drawing air over a cold coil to condense the moisture. This makes them efficient in warmer environments, typically above 60°F (15°C).
Desiccant dehumidifiers use a chemical material to absorb moisture and are more effective in colder temperatures, as their performance is not dependent on a cooling coil. However, desiccant models often consume more energy and release warmer air, which can increase the ambient temperature of the grow tent. For most horticultural applications where temperatures are maintained near room temperature, compressor models are the more efficient and preferred choice.
Proper sizing is necessary, as an undersized unit will be unable to keep up with the constant moisture load from plant transpiration. Dehumidifiers are rated by the pints of water they can remove in a 24-hour period. Small to medium grow tents often require units rated for 20 to 50 pints or more. The dehumidifier should be placed to allow for adequate air circulation, and continuous drainage via a hose is recommended over relying on the internal collection bucket to prevent operational downtime.
In some setups, an air conditioning (AC) unit may be used to manage both temperature and humidity simultaneously. AC units cool the air, and in doing so, they dehumidify it as a byproduct by condensing water vapor onto their cold coils. Integrating an AC unit can reduce the need for a separate dehumidifier, particularly where cooling is required to manage the heat produced by grow lights.