Seed density is one of the easiest variables to overdo. The goal is a full canopy with enough room for airflow and healthy stems—not simply the maximum number of seeds that fit on the tray.
Start with seed size
Penn State Extension gives a useful broad reference: Johnny’s Selected Seeds recommends roughly 6–8 large seeds per square inch or 10–12 small seeds per square inch. Larger seeds generally need more space than tiny brassica seed.
A weight-based starting point
In Penn State’s teaching systems, students have had success using about 10–15 grams of seed in a standard 12-by-20-inch 10-row seeding tray, with one tablespoon of brassica seed weighing about 11–12 grams. This is a starting reference—not a universal recipe for every crop or tray design.
Why rates change
- Seed size and germination percentage.
- Crop growth habit.
- Temperature and season.
- Growing medium and moisture behavior.
- Desired harvest size and timing.
- Airflow and disease pressure.
Signs you may be sowing too densely
- Persistent wet pockets at the tray surface.
- Weak, stretched seedlings competing for light.
- Increased mold or stem-collapse pressure.
- Uneven canopy where seed piles overlap.
Use the grow log
Record seed weight, tray size and harvest result. After a few cycles, your own yield and quality data are more useful than copying one density from someone else’s room.
Tool: print the Microgreens Grow Log and compare seed amount against harvest quality.
Source: Penn State Extension, “Growing Microgreens.”
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