Microgreens: Trays, Seed Rates and Sprout Food Safety

Sprouts, microgreens and baby greens

Stage	What is eaten	How it is grown	Harvest
Sprouts	The whole seedling: shoot, root and often the seed (Penn State)	Soaked seed, often germinated in the dark (Penn State); warm, humid conditions (FDA)	Before the seed leaves (cotyledons) are fully developed (FDA)
Microgreens	Stems, cotyledons and first true leaves, cut above the medium; roots are not eaten (UMD, UT)	Dense sowing in a tray of mix or a mat, under light	2–3 in tall, 7–21 days after sowing (UMD); 1.5–3 in (Penn State); 1–4 in (UT)
Baby greens	Leaves and stems; always past the true-leaf stage (UT)	Thinned to about 2 in apart (UMD)	4–6 in tall, 21–40 days; regrow for 1–2 more cuts (UMD)
Mature greens	Full-size leaves	Garden or container spacing	Over 6 in, 40–60 days (UMD)

Drawing by The More One Sows, based on the cited sources. Not to scale; stages follow the UMD, Penn State and FDA descriptions in the text.

1. Sprout: seed, root and shoot are eaten together; the seed leaves are not yet developed (Penn State, FDA)
2. Sprouts grow in water, often in the dark, in the warm, humid conditions that also suit bacteria (Penn State, FDA)
3. Microgreen: cotyledons plus the first true leaves, about 2–3 in tall, 7–21 days after sowing (UMD)
4. Cut just above the medium with clean scissors; roots stay behind and the tray does not regrow (UMD)
5. Baby green: 4–6 in tall, 21–40 days after sowing (UMD)
6. Cut baby greens regrow for 1–2 more harvests (UMD)

Trays, mix, light and temperature

Trays. Shallow containers make harvest easy: plastic food containers, standard 10 × 20 in greenhouse flats or microgreen trays. UMD describes a drained top tray nested in an undrained bottom tray that holds water. Penn State used 1020 flats and 10- or 20-row seedling trays.
Medium. A lightweight soilless mix, 1 to 1½ in deep and moist (UMD); 1–2 in (UT); about 2 in (UVM). Deeper mix may mean less frequent watering (UT). Do not use garden soil (UMD). Mats of hemp, wood fiber, coir, bamboo or jute replace mix in hydroponic trays (UMD). No fertilizer is needed (UMD); Penn State says little or none, and 50–100 ppm nitrogen if used.
Light. Grow lights generally perform best: about 16 hours a day, 6–12 in above the foliage (UMD); Utah State runs 18 hours on, 6 off. For window light, UT asks for more than 5–6 hours of sun and UVM at least four hours of direct sun. Lettuce, arugula, mustards and Asian greens tolerate weaker light (UMD).
Temperature. 70–75°F (UMD); 65–78°F (UT); Penn State gives 60–75°F for germination and 60–70°F air. Utah State uses 74°F to germinate, 72°F lights on and 68°F lights off.

Seed rates and days by crop

Utah State's table is for a standard 1020 tray (10 × 20 in). Days run from sowing; "covered" is the dark period under a weighted tray.

Crop	Seed per tray (USU)	Presoak	Covered	Days to harvest (USU)	Other sources
Arugula	10 g	No	1–2 days	6–8	UMD: 1 tablespoon per 10 × 20 tray yields about 8–12 cups
Radish	42 g	No	1–2 days	8–12	Penn State: daikon about 122 g harvest per tray
Kale	28 g	No	1–2 days	8–12	UMD photo: cabbage-family tray cut 14 days after sowing
Broccoli	28 g	No	2–4 days	8–12	Penn State (Di Gioia): brassica cycle 7–8 days at most
Pea	150 g	6 h	3 days	8–10	UMD: soak 6–12 h
Sunflower	48 g	12 h	3 days	9–10	UMD: soak 6–12 h
Basil	28 g	No	4–7 days	12–16	UT photo: 25 days at true leaf; UMD calls it slow
Cilantro	28 g	No	7–14 days	21–28	UMD: soak 6–12 h; slow

Other sources sow far less. UMD broadcasts about 1 tablespoon (10–15 g) per 10 × 20 flat, 1,500–4,500 seeds depending on size, and Penn State's class used 10–15 g per 12 × 20 in tray. Penn State sows lighter in summer than in winter and lighter for large seed. By area, Di Gioia gives 2 seeds per square inch for large seed such as pea and sunflower up to 12 for small brassica seed.

Formula (Penn State): seeds per tray = tray area (sq in) × seeds per sq in ÷ germination rate. Di Gioia's example: a 5 × 6 in tray of pea at 2 seeds per sq in needs 60 seeds, or about 67 at 90% germination, about 10 g at 3,000 seeds per pound. Penn State's free seed density calculator does the same sum.

Growing steps

Buy suitable seed. Untreated seed, not coated or pelleted (UT, Penn State), ideally sold for sprouts and microgreens (Penn State). Do not grow tomato, pepper, eggplant or potato seedlings, which are not edible (Penn State).
Fill the tray with moist mix to the depth above.
Soak large seed 6–12 hours: pea, sunflower, corn, chickpea, beet, Swiss chard, cilantro, mung bean, buckwheat (UMD).
Sow densely and press seed onto the mix (UMD). Leave small seeds on the surface; cover large ones with about ¼ in of mix (UT). Sow one crop per tray, since growth rates differ (UMD).
Cover to hold warmth and humidity, venting so moisture does not encourage mold (UMD). A few dark days lengthen stems (UMD).
Uncover under light once seedlings are up.
Water by misting for the first days, then from below or above, keeping the mix moist but not saturated (UMD). Bottom watering keeps leaves dry (Penn State). A gentle fan keeps air moving (UMD, Utah State); Penn State saw damping-off in warm, humid conditions.
Harvest with clean, sharp scissors just above the mix once true leaves expand (UMD), or at first true leaves or sooner (Utah State).
Store after a rinse and pat dry, refrigerated up to 10 days (UMD). In Penn State's trials, dark storage at 41°F extended shelf life to 10–14 days, against 2–4 days at room conditions.

Fast crops are covered on the radishes and kale pages; for full-size herbs see basil.

Food safety: sprouts and microgreens

FDA says sprouts are grown from seeds and beans under warm, humid conditions that are also ideal for Salmonella, Listeria and E. coli. A few bacteria on the seed can grow to high levels during sprouting, even when sprouts are grown under sanitary conditions at home. FDA counts 52 U.S. outbreaks linked to sprouts from 1996 through 2020. In one CDC investigation, alfalfa sprouts were linked to 63 illnesses and 10 hospitalizations in eight states (December 2022).

Official advice on sprouts. Children, older adults, pregnant women and people with weakened immune systems should avoid raw or lightly cooked sprouts of any kind, including alfalfa, clover, radish, onion and mung bean (FDA). Cook sprouts thoroughly to reduce the risk of illness (CDC).

FDA's rule draws the line by growth stage. A crop harvested before the seed leaves develop is a sprout; one harvested with true leaves is not a sprout and may be a microgreen. Sprouts grown in soil or substrate and cut above it, without roots, are also outside the sprout provisions. Penn State notes that microgreens are produce eaten raw under the Produce Safety Rule, and their risk is slightly lower than sprouts' because light can reduce bacteria. The same warm, moist conditions still favor bacteria. No federal consumer advice specific to home-grown microgreens was found; FDA's general produce advice applies.

Food-safety checklist

Use untreated seed sold for sprouting or microgreens (Penn State, UT).
Buy composted or heat-treated mix or mats from reputable suppliers; avoid mixes with raw manure (Penn State).
Clean trays, scissors and containers before reuse (UT, Penn State).
Keep mix out of the harvested greens (Penn State).
Rinse under running water before eating; no soap or produce wash. Washing reduces bacteria but does not remove them all (FDA).
Refrigerate promptly (UMD, UT).

Where sources disagree

Question	Positions
Seed per 10 × 20 tray	About 10–15 g (UMD; Penn State class) vs 28 g for most crops, 42 g radish, 150 g pea (Utah State)
Days to harvest	7–21 after sowing (UMD) or after germination (Penn State); 10–14 (UT); one to two weeks (UVM)
Light	16 h (UMD) vs 18 h (Utah State) under lights; window sun 5–6+ h (UT) vs 4+ h (UVM)
Reusing mix	Screen and refresh 50:50 with new mix (UMD) vs new mix each cycle to limit disease (Penn State)

Sources

All sources were opened and checked in October 2026. Rules and figures change; confirm with the original before acting.

Microgreens: trays, seed rates and food safety