Use hydrogen peroxide spray to treat active mold on microgreens

Food-grade hydrogen peroxide is the safest and most widely recommended treatment for active mold on microgreens. It kills mold on contact, breaks down into water and oxygen, and leaves no harmful residue on edible plants.

Standard Spray Solution

• Mix ratio: 10-15 ml of 3% food-grade hydrogen peroxide per 1 liter of water (approximately 1 tablespoon per 16 oz)
• Alternative ratio: 1 teaspoon of 3% H2O2 per 500 ml (16 oz) of water for lighter preventative misting
• Application: Spray affected areas once daily until mold is gone
• Coverage: Mist lightly -- you want to wet the mold, not saturate the growing medium

Using 35% Food-Grade Hydrogen Peroxide

• Always dilute to 3% first: Mix 1 part 35% H2O2 with approximately 11 parts water
• Then use the diluted 3% solution at the ratios above
• Handle 35% concentrate with extreme care -- it causes chemical burns on contact with skin
• Wear gloves and eye protection when handling concentrate

When to Spray vs. When to Discard

• Spray and save: Mold covering less than 25% of the tray, caught early
• Discard the tray: Mold covering more than 25% of the tray, or mold that returns after treatment
• Always discard: If mold has a musty smell, is dark colored (green, black, blue), or if the growing medium is slimy
• Small patches of white surface mold can be cut away -- the greens above the mold line are usually safe to eat after washing

Vinegar Alternative

• Dilute 5% white distilled vinegar: 1 tablespoon per 16 oz spray bottle
• Less effective than hydrogen peroxide but useful if H2O2 is unavailable
• Can also use 1 teaspoon of organic apple cider vinegar per liter of water
• Never combine vinegar and hydrogen peroxide in the same container -- creates peracetic acid, which is toxic
• You can use them sequentially (spray one, let dry, spray the other)

Baking Soda Solution

• Mix 1 teaspoon of baking soda per 1 liter of water, add 2-3 drops of liquid soap as a surfactant
• Raises surface pH which inhibits fungal growth
• Potassium bicarbonate is 25-35% more effective than sodium bicarbonate (baking soda) and less likely to damage plants
• MilStop (potassium bicarbonate fungicide) is the commercial OMRI-listed version
• Limitation: Repeated use can cause bicarbonate buildup affecting soil nutrients

Tips

• Spray early in the day so foliage dries under lights
• Increase air circulation immediately after spraying
• Reduce watering for 1-2 days after treating mold
• If mold recurs in the same spot each cycle, the issue is environmental, not just surface mold
• 3% hydrogen peroxide is cheap ($1-3 for 32 oz at any pharmacy) -- the "food grade" distinction matters mainly for the 35% concentrate