Mold represents a significant threat to controlled environment agriculture, capable of destroying an entire crop quickly. Fungal pathogens like Botrytis (bud rot) and powdery mildew thrive in specific environmental conditions within grow rooms. The presence of mold spores leads to substantial losses in both final yield and product quality. Mitigating this risk requires actively managing the physical environment and implementing strict operational protocols.
Managing Temperature and Humidity
Controlling the moisture content in the air is the most direct way to prevent mold spores from germinating. Plants naturally release water vapor through transpiration, which quickly elevates the relative humidity (RH) in a sealed space. During the vegetative growth stage, a higher RH (50% to 70%) supports rapid leaf development. This range should be reduced to 40% to 50% during the flowering stage to prevent moisture from becoming trapped within developing flower structures.
Maintaining a stable RH is important because of the relationship between temperature and the dew point. The dew point is the temperature at which the air becomes saturated and water vapor condenses into liquid water. If the temperature of any surface, including leaves or dense flower clusters, drops below the dew point, liquid water forms, creating an ideal environment for mold growth. A dew point above 55°F (13°C) significantly increases the risk of condensation and fungal infection.
Dehumidifiers are necessary to remove the substantial amount of water vapor released by the plants. Proper sizing is determined by the total volume of water introduced into the room daily, primarily through irrigation. If plants receive 20 gallons of water, the dehumidifier must be capable of removing nearly that much, with capacity measured in pints of water removed per day. Strategic placement of these units, often centrally or suspended, ensures the treated, drier air is distributed efficiently across the entire canopy.
Hygrometers and environmental controllers are necessary for continuous monitoring of temperature and humidity levels. Automating these systems allows for precise adjustments, especially during the transition from the light cycle to the dark cycle, when temperatures drop and RH naturally spikes. This active control prevents the formation of humid microclimates that are frequently overlooked.
Ensuring Adequate Air Circulation
Air circulation is a complementary factor to humidity control, addressing the movement of air both within and out of the growing space. Stagnant air pockets, particularly within the dense plant canopy or room corners, allow localized humidity to build up. Oscillating fans should be positioned to create a gentle, continuous breeze that moves the leaves and breaks up the layer of humid air surrounding the plant surface, known as the boundary layer.
The air within the room must be regularly exchanged with fresh air from outside to replenish carbon dioxide and exhaust humid, stale air. Exhaust fan capacity is measured in Cubic Feet per Minute (CFM) and must be calculated based on the room’s volume and the required Air Changes per Hour (ACH). This calculation involves multiplying the room’s volume by the desired ACH and dividing by sixty minutes.
The resulting CFM value must be increased to account for the resistance caused by ducting and air purification components like carbon filters. An additional 25% reserve capacity is generally added for ducting and another 25% for a carbon filter to ensure the fan can move the required volume of air under operational load. To prevent the introduction of mold spores, dust, and pests from the outside environment, all intake air should pass through a filtration system. High-Efficiency Particulate Air (HEPA) filters are effective at capturing microscopic mold spores before they enter the grow space.
Sanitation and Cleaning Protocols
Physical cleanliness is a preventative barrier that removes the food source for mold and eliminates places where spores can accumulate. Plant debris, such as fallen leaves, trimmings, or dead foliage, must be removed from the floor and canopy immediately. Any organic material left to decay on the floor or in drip trays becomes a breeding ground for mold and attracts pests.
A regular cleaning schedule should include a full room sterilization between crop cycles and a weekly wipe-down of surfaces and equipment. Tools, pots, reservoirs, and non-porous surfaces should be sterilized using specific solutions to kill lingering pathogens. A diluted bleach solution (two tablespoons of household bleach per quart of water) or a 3% hydrogen peroxide solution can be used to sanitize walls and equipment. Ensure all electrical components are protected and allow surfaces to fully dry following any wet cleaning.
Plant Structure and Watering Techniques
Structural management of the plants can significantly reduce the risk of localized mold outbreaks. Techniques such as lollipopping involve removing all lower-level growth and small, non-producing branches, redirecting the plant’s energy to the top canopy. This practice improves yield but also reduces the dense undergrowth where air movement is weakest and humidity is highest.
Strategic defoliation, or the removal of excess large fan leaves, allows light penetration and air circulation deeper into the canopy. Creating space between branches and buds prevents the formation of stagnant, high-humidity microclimates within the plant structure. Avoiding plant overcrowding is necessary, as sufficient spacing ensures that air can flow freely around all sides of every plant.
Watering practices require careful attention to reduce the amount of time foliage remains wet. Watering should be timed to coincide with the beginning of the light cycle when the air temperature is rising and the plants are actively transpiring. This timing maximizes the rate of evaporation, ensuring that plant surfaces and the top layer of the growing medium are dry before the lights turn off. Never water late in the day or during the dark period, as the subsequent drop in temperature will promote fungal growth on wet surfaces.