Relative humidity (RH) is the amount of water vapor in the air compared to the maximum it can hold at that temperature. Excessive RH in a closed growing environment compromises plant health and growth potential. High moisture creates an ideal breeding ground for pathogens like powdery mildew and Botrytis (bud rot). Elevated RH also inhibits transpiration, which is necessary for nutrient uptake and cooling. Controlling atmospheric moisture is paramount for a successful cultivation cycle.
Monitoring and Measurement
Accurate assessment of the grow room atmosphere is required before implementing moisture reduction strategies. The primary tool for this measurement is the hygrometer, which reports the relative humidity percentage. Digital hygrometers offer greater precision and data logging capabilities, while analog versions provide a simple reading. Integrating these sensors with an environmental controller allows for automated adjustments based on predefined humidity set points.
The ideal RH changes depending on the plant’s life stage. Young clones and seedlings thrive in a higher humidity range, typically between 70% and 80%. As plants mature into the vegetative stage, the acceptable range narrows to 40% to 70%. During the flowering phase, when the risk of fungal disease is greatest, maintaining a lower RH of 40% to 50% is standard practice.
Mechanical Moisture Removal
Dehumidifiers represent the most direct and powerful method for removing water vapor from a grow space. These devices work by cooling the air below its dew point, causing water to condense and collect in a reservoir or drain line. Selecting the correct model involves distinguishing between the two primary types: refrigerant (compressor) and desiccant.
Refrigerant dehumidifiers pass humid air over chilled coils, making them highly efficient in warmer environments, particularly above 65°F (18°C). Their performance declines in cooler conditions because coils may frost over, requiring a defrost cycle. Desiccant dehumidifiers use a rotating wheel coated with moisture-absorbing material, which is then heated to release the water. These models are effective at lower temperatures but typically introduce more heat into the grow room than refrigerant types.
Proper sizing is determined by the unit’s capacity, measured in pints or gallons of water removed per day (PPD). Growers should select a unit that handles the room’s volume and the high moisture output from transpiring plants, often requiring a larger unit than residential models. Optimal placement is central within the room, away from drafts or heat sources, to ensure it processes a representative air sample. The dehumidifier should discharge its exhaust air toward the plant canopy to circulate the newly dried air.
Airflow and Ventilation Strategies
While mechanical removal handles the bulk of moisture, ventilation replaces stagnant, humid air with fresh, drier air from the exterior environment. This process relies on a coordinated system of exhaust fans, intake fans, and internal circulation fans. The exhaust fan pulls moisture-laden air out, while the intake system allows controlled entry of new air, which ideally has a lower moisture content.
Establishing negative pressure is a fundamental goal of a ventilation system, meaning the exhaust fans remove air at a slightly higher rate (CFM) than intake fans supply it. This pressure differential ensures air movement is directed outward through the exhaust system, preventing humid air and potential odors from leaking through gaps. Circulation fans move air across the plant canopy and below the leaves, disrupting humid microclimates that form around the stomata.
Adequate air exchange requires calculating the Cubic Feet per Minute (CFM) needed to refresh the entire volume of air within the room every minute. This calculation involves finding the room’s total volume (Length x Width x Height) and applying a multiplier to account for heat from lighting and resistance from carbon filters. A properly sized ventilation system continuously moves air, aiding both temperature regulation and humidity management.
Managing Moisture Sources
Managing moisture sources involves proactively controlling what introduces water into the atmosphere. The most significant source is plant transpiration, followed by evaporation from the growing medium. Adjusting the watering schedule minimizes the duration of peak humidity; watering should occur just as the lights turn on. This timing allows plants to absorb water before the lights raise the room temperature, which accelerates transpiration and evaporation.
Reducing overall plant density lowers the total surface area contributing to transpiration. Standing water in runoff trays or reservoirs is also a source of excess water vapor. Allowing water to sit enables it to evaporate back into the atmosphere, directly counteracting dehumidification efforts. Growers should ensure runoff water is promptly removed or drained away from the immediate grow area to maintain a drier environment.