The blackout period is the first few days after sowing, when microgreen trays are kept covered and in the dark. It is one of the simplest techniques in microgreens growing and one of the most effective: it gives faster, more even germination, taller and straighter stems, and helps large seeds shed their hulls.
In short
- Cover trays immediately after sowing.
- Most crops need 3–4 days of blackout; slow herbs need 5–7.
- Small seeds just need a cover; peas and sunflower benefit from 5–10 lb (2–4.5 kg) of weight on top.
- Uncover when seedlings are about 1 inch tall and lifting the cover.
Why blackout works
Two things happen under the cover.
- Humidity stays high at the surface. Seeds need constant moisture to germinate. A cover stops the top layer of the medium from drying out between mistings, so all the seeds start together instead of in waves.
- Seedlings stretch towards light. In darkness, young stems elongate quickly as they look for light. A short blackout uses this natural behavior to produce taller microgreens with more harvestable stem, which means more yield and cleaner cutting above the soil.
When the cover comes off, light stops the stretching and the seedlings turn green within a day.
How long to keep microgreens in blackout
| Crop | Blackout length | Weight on top? |
|---|---|---|
| Radish | 3 days | Optional, light |
| Broccoli, kale, cabbage, kohlrabi | 3–4 days | Optional, light |
| Mustard, arugula | 3 days | No |
| Pea shoots | 3–5 days | Yes, 5–10 lb |
| Sunflower | 3–4 days | Yes, 5–10 lb |
| Wheatgrass | 2–3 days | Optional |
| Amaranth | 3–4 days | No |
| Beet, Swiss chard | 4–6 days | Light weight helps seat the seeds |
| Basil | 4–5 days | No |
| Cilantro | 5–7 days | Light weight optional |
Times are counted from sowing. These are typical starting points; follow what the seedlings are doing rather than the calendar.
Method 1: Cover tray (for small seeds)
- Sow the seed on moist medium and mist well.
- Place an empty tray upside down over the top, or a second tray right side up resting on the seeds.
- Put the trays somewhere at 65–75°F (18–24°C). Light does not matter much as long as the cover blocks it.
- Lift the cover once a day. Mist if the surface looks dry, and look for mold.
A simple humidity dome or even a sheet of cardboard will do. The goal is darkness and humidity, not an airtight seal.
Method 2: Stack and weight (for large seeds)
Large seeds such as peas and sunflower germinate better with weight on top. The pressure keeps the seeds in firm contact with the medium so the roots anchor quickly, and the seedlings grow stronger stems by pushing up against resistance. For sunflower it also helps scrape off the hulls.
- Sow soaked seeds on moist medium.
- Place an empty tray directly on top of the seeds (bottom of the empty tray touching them).
- Add weight to the top tray: a bag of soil, a brick wrapped in plastic, or water jugs. Aim for 5–10 lb (2–4.5 kg) on a 10×20 tray.
- Check daily. When the seedlings start to lift the weight, that is a good sign.
- After 2–3 days, remove the weight and keep just a cover on for the last day or so if the stand is uneven.
Many growers stack trays of the same crop on top of each other so each tray weights the one below, with a weighted empty tray on top.
When to uncover
Uncover when:
- most seedlings are about 1 inch (2–3 cm) tall, and
- they are pushing up the cover or weight.
Do not wait until they are very tall and yellow. Seedlings left too long in darkness become weak, pale and floppy, and they may never fully recover. If a few slow seeds have not germinated, uncover anyway — they will not catch up.
What to expect after blackout
Freshly uncovered seedlings are pale yellow or white. This is normal: they have not yet made chlorophyll. Under good light they turn green within 12–24 hours. If they are still yellow after two days, the light is too weak — see why microgreens turn yellow.
From this point on, switch to bottom watering and give the tray 12–16 hours of light. Our watering guide and grow light guide cover both.
Blackout mistakes to avoid
| Mistake | Result | Fix |
|---|---|---|
| Medium too wet under the cover | Mold in 2–3 days | Water so it feels like a wrung-out sponge, not soaking |
| No airflow at all for many days | Mold and a sour smell | Lift the cover daily; run a small fan nearby |
| Blackout too long | Weak, yellow, collapsing stems | Uncover at about 1 inch |
| Too much weight on small seeds | Crushed, crooked seedlings | Use only a light cover for brassicas |
| Forgetting to mist | Patchy germination | Check daily; mist the surface if it looks dry |
Is blackout really necessary?
No. Microgreens will grow without it, especially on a moist mat in a humid room. But skipping blackout usually gives shorter, uneven stands that are harder to harvest cleanly. For the cost of an extra tray as a cover, blackout is worth doing.
Frequently asked questions
Can I keep microgreens in a cupboard during blackout?
Yes, as long as the cupboard is not cold. A kitchen cupboard away from the oven is fine. Remember to check them daily.
Do I water during blackout?
Mist the surface daily if it looks dry. Most trays that were well moistened at sowing need little else until they are uncovered.
My seedlings are white with tiny fuzz on the roots — is it mold?
Usually that fuzz is root hairs, which are completely normal during blackout. Learn the difference in mold vs root hairs.
