Microgreens at home: trays, seeds, days to harvest and eating them safely

Microgreens are vegetable seedlings grown in a shallow tray until the first true leaves appear, then cut above the growing mix and eaten. Most are ready 7-21 days after sowing, and a bright windowsill is enough space. This lesson covers choosing seed, trays and mix, how much seed a tray needs, telling mould from root hairs, and the food-safety points where microgreens and sprouts are not the same thing.

How microgreens differ from sprouts

The two get mixed up all the time, but they are grown and eaten differently, and that difference is why their risk is not the same.

	Sprouts	Microgreens
How they grow	in water or damp conditions, dark and warm, no growing mix	sown on a growing mix, with light and airflow
When harvested	only the first seed leaves, or none yet	once the first true leaves appear
What you eat	seed, root and shoot	only the stem and leaves above the mix; roots stay behind
Time	a few days	about 7-21 days

Choosing seed

• Use untreated seed. Some seed sold for garden planting is coated with fungicide, and on some microgreens the seed coat is still attached when you cut them. Read every label; choose seed marked untreated, organic, or sold for sprouting and microgreens.
• Soak large seed first. Peas, other beans and lentils benefit from soaking overnight before sowing; they come up more evenly.
• Buy from a source you trust. Dirty seed and growing material are one of the ways pathogens get into a tray.

Trays, mix and sowing density

Use a shallow tray with drainage holes and fill it with a soilless mix, for example peat with perlite and vermiculite, 2.5-5 cm (1-2 inches) deep and just barely damp before sowing. Avoid garden soil, which readily brings mould, and start each round with fresh mix to limit disease.

Scatter the seed evenly, cover it with a thin layer of mix or vermiculite, and put a lid or plastic over the tray to hold moisture. Take it off as soon as you see the seeds sprouting.

Worked example: how much seed for a 20 x 30 cm tray

The figures come from Penn State Extension’s guide, which uses a standard 12 x 20 inch tray (about 30 x 50 cm).

1. A standard 12 x 20 inch tray = 240 square inches, about 1,550 cm², sown with 10-15 g of seed.
2. A home tray of 20 x 30 cm = 600 cm², which is 600 / 1,550 ≈ 0.39 of the standard tray.
3. Seed needed ≈ 0.39 x 10-15 g ≈ 4-6 g. A tablespoon of brassica seed weighs about 11-12 g, so that is roughly half a tablespoon.
4. The guide’s average yield was about 136 g per standard tray, so the home tray gives about 0.39 x 136 ≈ 50 g per round.

In hot weather, sow less than this, and sow large seed more thinly than small seed. Sowing too thickly is the main reason trays go mouldy.

Light, water and air in a Thai home

Microgreens need little light; a bright window is enough. With too little, the stems stretch and fall over.

Water from below if you can: set the tray in a shallow dish of water until the mix is evenly moist, then lift it out. Do not leave it sitting for more than 20 minutes. This keeps the leaves dry and lowers the chance of disease.

Guides written for cooler countries suggest air at about 16-21°C. Thai homes are warmer than that for most of the year, and warm, humid conditions favour damping off. Make up for it by sowing more thinly, uncovering the tray as soon as droplets form on the lid, and keeping air moving; a gentle fan helps a lot.

Mould or root hairs?

Around days 2-4, many growers panic at a white fuzz around the roots and assume it is mould. Usually it is root hairs, a normal part of the root that takes up water.

	Root hairs	Mould
Where	around each root, evenly	in patches on seeds, mix, stem bases or leaves
Look	fine, soft white fuzz	web-like strands, sometimes grey or dark
After misting	collapse for a while	stay as they were
Smell	fresh	musty, off

If you see white webbing spreading over the seeds, or stems that turn black at the base and fall over, that is fungal disease. Tip the whole tray into the compost, wash the tray, and start again with fresh mix. Do not cut the healthy-looking parts to eat.

White fuzz around the roots that vanishes when misted is root hairs. Don’t stop watering because you think it is mould; the seedlings will simply dry out.

Harvesting and storing

Cut when the plants have their first true leaves and stand at least 2.5 cm tall, usually somewhere around 4-8 cm. Use sharp scissors just above the mix and keep mix out of the harvest. Both the seed leaves and the true leaves are edible.

Rinse lightly just before eating and pat dry. To keep them, wrap loosely in paper towel inside a bag in the crisper drawer. Penn State notes that unwashed microgreens kept dark and cold at about 5°C last 10-14 days, against only 2-4 days at room temperature, and that washing before storage shortens their life, so wash when you are about to eat. Washing produce properly is covered in washing vegetables.

Food safety: sprouts vs microgreens

The U.S. Food and Drug Administration (FDA) explains that sprouts are grown from seed in warm, humid conditions that also suit bacteria such as Salmonella, Listeria and E. coli. If a few bacteria are on the seed, they can multiply to high levels during sprouting, even when you sprout them at home under clean conditions. The FDA therefore advises children, older adults, pregnant women and people with weakened immune systems to avoid raw or lightly cooked sprouts of any kind.

Microgreens grow with more light and airflow, and the roots are left behind. A University of Arkansas food-safety researcher said in 2022 that microgreens had not yet been linked to an outbreak, but that there had been roughly six to eight recalls in the previous three years over possible Salmonella or Listeria contamination, and the topic is still little studied.

What you can do at home: get seed and mix from reliable sources, wash your hands and the tray every round, don’t sow too thickly, never eat from a mouldy tray, and if someone in the house is in a vulnerable group, ask a doctor before eating them raw. Cooking reduces bacteria.

Microgreens and sprouts are grown differently, so their risk differs, but neither is guaranteed free of bacteria.

Common mistakes

• Sowing too thickly. Crowded seedlings block airflow and go mouldy, especially in the hot season.
• Using garden soil instead of a mix. It can carry fungi and other contamination.
• Leaving the lid on too long. Trapped humidity causes rot; uncover as soon as seeds sprout.
• Using treated seed. Always read the label.
• Reusing old mix. Disease from the last round comes along with it.

The next lesson tries another way to grow food without buying seed: regrowing from kitchen scraps.

Sources

• Selecting and Serving Produce Safely — U.S. Food and Drug Administration
• Growing Microgreens — Penn State Extension
• Microgreens — University of Saskatchewan Gardening
• Microgreens’ vulnerability to pathogens characterized by researchers — University of Arkansas via Phys.org