Choose the right growing medium to minimize mold risk

The growing medium directly affects moisture retention, drainage, and pathogen load. Different media have very different mold profiles, and switching medium can solve chronic mold problems.

Growing Media Comparison for Mold Risk

Coconut Coir -- Low mold risk - Naturally sterile, no weed seeds or pathogens - Excellent moisture-to-air ratio prevents waterlogging - No sterilization needed out of the bag - Best all-around choice for mold-prone growers

Terrafibre Hemp Mats -- Low mold risk - 100% natural industrial hemp fiber, needle-punched (no chemicals) - Hemp is naturally resistant to mold, mildew, and microbes - Holds 1050% its weight in water while remaining breathable - Available in 300gsm (standard), 500gsm, and 750gsm thickness - pH neutral, biodegradable, compostable - Price: approximately $0.85-1.50 per 10x20 mat - Best for clean, soilless operations; requires nutrient supplement for longer-growing varieties

BioStrate Felt Mats -- Low-moderate mold risk - Synthetic biopolymer felt, specifically engineered for microgreens - Highest germination rates and yields in comparative tests - Dries out quickly -- requires careful moisture monitoring - Requires nutrient supplementation - More expensive than hemp: approximately $1.00-1.50 per 10x20 mat in bulk - Best overall performance but less forgiving of watering mistakes

Sure to Grow -- Low mold risk - Polyester-based hydroponic mat - Good drainage and airflow - Reusable, though harder to clean than disposable mats

Seed-Starting Soil Mix -- Higher mold risk - Contains organic matter that can harbor spores - Higher yields than mats due to nutrient content - Requires careful moisture management - Can be sterilized before use (see sterilization methods below)

Wood Fiber Mats -- Highest mold risk - Hold water too well during germination - Multiple grower reports of entire trays lost to mold - Avoid for mold-prone varieties

Soil Sterilization Methods

Oven method: Preheat to 180F (82C), fill pan with 4 inches of moist soil, cover with foil, bake for 30 minutes after reaching 180F. More aggressive: 320F (160C) for 2+ hours until soil is dry.

Microwave method: Place 2 lbs of damp soil in an open zip-top bag, microwave on high until center reaches 180-200F (approximately 2-5 minutes per kg). Screen soil for metal objects first.

Tips

• Mats generally produce less mold than soil because they lack the organic matter mold feeds on
• Soil produces substantially higher yields than mats -- weigh the trade-off based on your mold situation
• Never reuse growing medium between cycles -- always start fresh
• If using soil, buy sterile seed-starting mix rather than garden soil
• For mold-prone varieties (sunflower, pea, wheatgrass), hemp mats are the safest choice