How Much Seed Should I Use for Microgreens?

Sowing density is one of the most impactful variables in microgreens growing and one of the least discussed by beginners. Too few seeds and you get a sparse, unproductive tray that dries out unevenly. Too many seeds and you create the humid, airless conditions at the surface that lead to mould, damping off and a tangled, hard-to-harvest mess. Getting the density right for each variety is the difference between a reliable harvest and a frustrating one.

Why Density Varies by Seed Size

Seed size is the primary driver of how much to sow per tray. A large-seeded crop like sunflower or peas needs to be sown at much lower weight per tray than a small-seeded crop like broccoli or kale to achieve the same visual density on the medium surface. The goal in all cases is the same: a single, even layer of seeds that covers the medium completely but does not pile up or overlap significantly. Achieving this with small seeds requires precision spreading; with large seeds it is easier to achieve by eye.

Density Guidelines by Category

Small-seeded brassicas (broccoli, kale, cabbage, mustard, kohlrabi): aim for 10 to 15 grams per 10×20-inch tray. These seeds are tiny and the target coverage is approximately 2 seeds per square centimetre. Use a shaker bottle or pinch-and-scatter method to distribute them evenly. At the correct density the medium surface should look nearly covered but with individual seeds just visible.

Medium-seeded crops (radish, amaranth, chard, beet): aim for 25 to 40 grams per 10×20-inch tray. Radish in particular benefits from slightly higher density because it grows fast and the canopy fills in quickly. Too sparse and gaps remain visible at harvest; too dense and mould at the stem base becomes a problem.

Large-seeded crops (sunflower, peas, wheatgrass, corn): aim for 150 to 250 grams per 10×20-inch tray. These seeds are large enough that the correct density — one tight, single layer touching but not stacking — is easy to judge visually. After pre-soaking and rinsing, spread seeds so they touch neighbours but do not climb over each other.

Adjusting for Germination Rate

Older seeds or seeds stored in warm, damp conditions may have lower germination rates. If you know your seed is old or was stored poorly, sow about 20 to 30% more than the standard rate to compensate for gaps where seeds fail to sprout. Most fresh microgreens seed from reputable suppliers has germination rates above 90%, so standard density targets apply without adjustment.

What Happens If You Sow Too Dense

Over-sowing is a more common mistake than under-sowing, particularly with small seeds. The symptoms are: patchy germination (seeds in the lower layer do not get enough oxygen), mould outbreak at the surface or stem base (trapped moisture between tightly packed seeds), tangled stems that are difficult to cut cleanly, and a sour or fermented smell before harvest. If you see these issues, reduce your sowing rate by 20 to 30% on the next tray and ensure you are using the bottom-watering method rather than top-watering.