Use proper seeding density and bottom watering to prevent mold

Overcrowding and top-watering are two of the most common causes of mold in microgreen trays. Correct seeding density allows airflow between plants, and bottom watering keeps the leaf canopy dry.

Seeding Density Guide (per standard 10x20 / 1020 tray)

Small seeds: - Radish: 30-35 grams (1 oz) - Broccoli: 15-20 grams - Kale: 15-20 grams - Amaranth: 15 grams - Arugula: 20 grams - Basil: 3 grams (mucilaginous -- do not overseed) - Sorrel: 3 grams - Fennel: 20-30 grams

Large seeds: - Sunflower: 125-250 grams (varies by hull size) - Pea shoots: 250-280 grams (9.8 oz)

General rule: If seeds are touching but not overlapping, density is correct. If seeds are piled 2-3 deep, you have too many.

Why Density Matters for Mold

Overcrowded trays trap moisture between seedlings
Reduced airflow within the canopy creates humid microclimates
Roots compete for space, weakening plants and making them more susceptible to damping-off
Dead or ungerminated seeds in overcrowded conditions become mold food sources

Bottom Watering Technique

Setup: Use a dual-tray system -- one tray with drainage holes (growing tray) sitting inside one solid tray (water reservoir)

During germination (days 1-3): Mist from the top with a spray bottle to keep seeds moist. This is the only time top-watering is appropriate.

After cotyledon leaves appear: Switch to bottom watering exclusively.

Process: 1. Lift the growing tray 2. Pour 1.5-2 cups of water into the bottom solid tray 3. Set the growing tray back down 4. Let it absorb for 10-15 minutes 5. Drain any remaining standing water -- never leave water sitting 6. Repeat once or twice daily depending on how fast the medium dries

How to check moisture: Lift the growing tray -- a light tray needs water, a heavy tray does not. You can also touch the medium surface; it should be moist but not wet.

Mesh Trays for Enhanced Drainage

1020 mesh trays have small holes throughout the entire bottom
Superior drainage compared to standard trays with fewer, larger holes
Particularly useful for large-seeded crops: pea shoots, wheatgrass, sunflower, popcorn shoots
Small enough holes to hold growing medium in place

Tips

When in doubt, use fewer seeds rather than more -- overcrowding causes more problems than thin coverage
Bottom-watered microgreens have stronger stems because roots grow downward seeking water
Overwatering is the number one killer of microgreens -- soggy medium breeds mold
After watering, the medium should be like a wrung-out sponge: moist throughout but not dripping
If you see water pooling on the surface of your growing medium, you are overwatering