Control airflow, humidity, and temperature to prevent mold environmentally

Environmental control is the single most effective mold prevention strategy. Mold requires three things to grow: moisture, warmth, and stagnant air. Remove any one of these and mold cannot establish.

Temperature

• Optimal range: 18-24C (65-75F) for most varieties
• Below 15C (60F): Germination slows dramatically but mold risk also decreases
• Above 27C (80F): Mold and bacterial risk increases sharply -- this is the danger zone
• Critical threshold: Temperatures above 75F combined with humidity above 65% create ideal mold conditions
• Use a thermometer near your trays; do not guess

Humidity

• During germination/blackout: 50-65% relative humidity is acceptable since trays are covered
• After uncovering: Reduce to 40-50% -- this is the target range for growing
• Below 50%: Significantly inhibits fungal growth
• Above 65%: Mold becomes very difficult to prevent
• Use a hygrometer ($10-15) to monitor -- conditions vary room to room
• In humid climates, a small dehumidifier near the growing area helps

Air Circulation

• Fan type: Small oscillating fan is ideal -- sweeping motion prevents any area from getting continuous direct blast
• Placement: Point fan near trays but not directly at seedlings -- you want a gentle breeze, not a windstorm
• Duration: Run continuously after removing blackout cover; some growers run 24/7, others run during light hours only
• Clip-on fans: Work well mounted above the canopy on shelf racks
• For larger rooms (10x10+): Mount wall fans every 10 feet along perimeter
• Extractor fans: Should exchange room air every 3-5 minutes for optimal conditions
• A USB desk fan ($10-15) is sufficient for 1-4 trays

Blackout Period -- The Highest-Risk Phase

• Covering trays with a solid lid creates a warm, humid microclimate -- ideal for mold
• Use vented blackout domes (Bootstrap Farmer's domes have adjustable vents)
• Prop up one corner of solid covers to allow air exchange
• Limit blackout to the minimum needed: 2-3 days for most varieties (fast growers like kale and broccoli need only 1-2 days)
• Slow varieties (oregano, thyme) need 4-7 days and require extra vigilance
• Check trays daily during blackout for condensation buildup
• Wipe condensation from lids if it accumulates

Light Exposure

• 12-16 hours of light daily after blackout
• Light itself does not kill mold but helps dry the canopy surface
• The transition from blackout to light is when mold is most often first noticed
• UV components in natural sunlight have mild antifungal effect

Tips

• The combination of bottom watering + fan + temperature control eliminates 90% of mold problems
• If growing in a basement or enclosed space, ventilation becomes even more critical
• In summer, increase air circulation and reduce watering frequency
• In winter, heat mats under trays can raise local temperature above 27C -- monitor this
• A fan also strengthens stems by simulating wind, producing sturdier microgreens