Guide

How many microgreen seeds per tray, crop by crop

In Johnny's Selected Seeds' yield trial, a standard 10 by 20 inch tray took about 10 grams of seed for most brassicas, 13 grams of broccoli, 22 to 28 grams of radish and 5 to 6.5 grams of basil, all cut at the first true leaf. Large seeds need more weight per tray because each one is heavier, and fewer of them fit.

By Moses Emmanuel

Microgreen seeds per tray: the chart

These are the averages from the 2017 trial at Johnny's research farm, for a 1020 flat (the common 10 by 20 inch tray). Staff sowed each variety by eye at least three times between mid-August and late October and weighed the seed packet before and after, so each figure is a weighed amount. Flats with poor germination were left out, and the figures are rounded to the nearest half.

The seed column falls into two groups. The radishes, beet, chard and cilantro all sit at one tray per ounce of seed. Most brassicas, from kale and arugula to mizuna and the pac chois, land between 7.5 and 14 grams, or two to three and a half trays to the ounce. Johnny's suggests using that kind of family resemblance to estimate a variety the trial did not include.

Seeding by the spoon or by eye

Few home growers own a scale that reads to the gram. The University of Illinois Extension handout gives volume instead, for the same 10 by 20 inch flat of 200 square inches:

Illinois broadcasts the seed evenly over leveled, lightly tamped mix and says to pay attention to the edges, where coverage tends to thin out. The handout treats the small-seed range as a starting point to experiment within, which matches how Johnny's presents its own numbers.

Microgreen seed calculator for any tray

Illinois scales the rate to other containers by area: measure the inside of the tray, divide its square inches by 200, and multiply. The calculator does that with Johnny's figure for the crop you pick. Yield scales the same way, so treat it as a rough guide.

Penn State's own rate is a useful cross-check. Its microgreens article uses 10 to 15 grams of seed for a 12 by 20 inch, 10-row seeding tray, which is right in line with the brassica rows above once the larger tray is allowed for.

How much does a 1020 tray of microgreens yield?

From about 5.5 to 15 ounces in Johnny's trial. Tokyo Bekana gave the most at 15 ounces; broccoli, Hong Vit radish and the mild mix 11.5; the slow herbs and flowers (Red Rubin basil, red dandelion, marigold) gave 5.5 to 6. Penn State's course trays averaged about 0.30 pound, or 136 grams, per 10-row flat across the types grown, with Red Rain mustard reaching 201 grams.

Johnny's calls its yield figures imprecise and says technique, growing system, conditions, harvest stage and seed lot all move them widely. Use the column to compare crops with each other, and to plan how many trays a week you need.

Harvest stage changes the seed rate

Every rate on this page assumes a cut at the first true leaf, the stage after the seed leaves when the plant starts to look like the vegetable it would become. Johnny's says that if you want bigger microgreens you should sow fewer seeds per flat, to leave more air between them and make the water and fertility last longer. Illinois makes the same point one stage earlier, lowering the density for greens grown to the first true leaf instead of cut at the seed leaves.

Penn State harvests at about 1.5 to 3 inches tall, anywhere from seven to 21 days after seeding. Illinois cuts with a knife or scissors about a quarter to half an inch above the soil line. Radish is the quick one: Johnny's cut Red Arrow at 8 days. Grown on for the root, the same crop takes 22 to 30 days, as the days to maturity chart shows.

Growing a tray indoors in winter

Illinois uses a container 1 to 2 inches deep with drainage holes, nested inside a second one without holes to catch the water, and a plastic lid until the seeds sprout. Its ideal soil temperature for germination is around 75°F; Penn State gives 60 to 75°F for the soil and 60 to 70°F for the air. Cover the seed with a damp paper towel or a second tray. Mix spread on top sticks to the leaves, Illinois warns. Once the roots take hold and lift the towel, take it off.

Light needs are modest. Penn State classes microgreens as a low-light crop, below a daily light integral of 20 mol/m²/day. Illinois grows them in a sunny window and turns the tray as the seedlings lean toward the glass. If your winter window is dim, the grow light distances and hours in our herb sunlight chart are a reasonable place to start with a tray.

Water from below. Both sources prefer filling the bottom tray, which keeps leaves and stems dry. Damping-off was the only disease in Penn State's course, and warm, humid conditions favor it. Penn State also uses new growing media for every cycle and recommends organic or untreated seed, since the seed coat can stay on the seedling and get eaten. That makes a microgreen tray the one container where we do not reuse potting soil; Illinois composts the stems and mix after the last cut.

Penn State stores the harvest at 41°F in the dark, which stretches shelf life to 10 to 14 days against 2 to 4 at room temperature, and advises washing the greens just before eating. Microgreens are a different crop from the baby lettuce and greens cut from a bed a few weeks later, which are spaced by the square instead of by the gram.

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