The blackout stage is one of the most misunderstood parts of growing microgreens. Some varieties need only a short covered germination period, while larger seeds such as peas and sunflower benefit from pressure as well as darkness. Other crops may need little more than a simple cover to maintain moisture while germinating.
The most important point is that blackout is not one universal technique with one universal number of days. Broccoli, radish, mustard, peas and sunflower all behave differently. The correct approach depends on seed size, germination speed, root development and how the crop responds beneath the covering tray.
This guide explains the difference between covered germination, weighted germination and blackout, how long common crops in the OzSprout Microgreen Seeds range generally stay covered, and how to tell when it is time to move the crop into light.
If you are new to tray growing, start with our complete guide to growing microgreens at home before using this more detailed blackout guide.
Table of Contents
- What Is Microgreen Blackout?
- Covered Germination vs Weight vs Blackout
- Why Do Growers Use Blackout?
- How Long Should Microgreens Stay in Blackout?
- Broccoli Blackout
- Radish Blackout
- Mustard Blackout
- Pea Shoot Blackout
- Sunflower Blackout
- Which Microgreens Need Weight?
- How Do You Know When to Uncover?
- Watering During Blackout
- What Happens If Blackout Is Too Long?
- Does Blackout Cause Mould?
- What Happens After Blackout?
- Frequently Asked Questions
What Is a Microgreen Blackout Period?
A microgreen blackout period is a stage during germination when the seed or young seedlings are kept beneath an opaque cover and away from direct light.
Growers commonly use:
- A second growing tray placed directly on top of the seed
- An inverted tray positioned over the seedlings
- A purpose-made blackout cover
- Stacked trays during early germination
The OzSprout Microgreen & Wheatgrass Grow Tray can be paired with another suitable tray when a crop requires covered or weighted germination.
Blackout is primarily a germination and crop-management technique. It is not a requirement that every microgreen spend a fixed number of days in complete darkness.
Covered Germination vs Weighted Germination vs Blackout
These terms are often used interchangeably, but they describe slightly different techniques.
1. Covered Germination
A tray or opaque cover is placed over freshly sown seed.
The main purposes are to:
- Reduce moisture loss
- Create a more stable germination environment
- Keep light away from germinating seed
- Help maintain seed-to-medium contact
2. Weighted Germination
A second tray sits directly against the seed and a modest amount of weight is applied.
This is particularly useful for large seeds such as peas and sunflower. As large seeds absorb water and germinate, emerging roots can lift the seed away from the growing surface instead of anchoring strongly into the medium.
Gentle, even pressure can help improve:
- Seed-to-medium contact
- Root anchoring
- Crop uniformity
- Establishment of larger-seeded crops
Pea and sunflower are two of the clearest examples where weighting during early germination can be useful.
3. Post-Germination Blackout
After seedlings have germinated, some growers remove the weight and turn the covering tray upside down so the young shoots have room to grow while remaining in darkness.
This can encourage some additional stem extension before the crop moves into light.
Not every variety needs this additional stage. The goal is not to produce the tallest possible pale seedlings. It is to establish an even crop with useful stem length and a strong root system before normal light growth begins.
Why Do Growers Use a Blackout Period?
Maintaining Moisture
Freshly sown microgreen seeds can dry rapidly, particularly small seeds sitting close to the surface of the growing medium.
A covering tray reduces evaporation and can help maintain more consistent moisture during germination.
Improving Seed-to-Medium Contact
Seeds generally establish more evenly when they remain in contact with a moist growing surface.
This becomes particularly important with larger seeds that can move as they absorb water, swell and germinate.
Helping Roots Establish
Young roots need to anchor into the growing medium before the crop supports a developing canopy.
Covered or weighted germination can help produce more uniform early root establishment.
Managing Stem Length
Seedlings developing without light naturally extend as they search for a light source.
A controlled blackout period can therefore produce useful stem length for harvesting. However, excessive darkness produces unnecessarily stretched seedlings rather than a stronger crop.
How Long Should Microgreens Stay in Blackout?
The correct blackout time depends on the crop.
Current OzSprout growing guidance uses the following approximate ranges:
| Microgreen | Pre-soak | Blackout / Covered Period | Pressure |
|---|---|---|---|
| Broccoli | None | 2–4 days | Gentle tray contact; heavy weight unnecessary |
| Daikon Radish | None | 2–3 days | Light pressure only |
| Purple Sango Radish | None | 2–3 days | Light pressure only |
| Green Mustard | None | 2–3 days | Light pressure only |
| Maple Pea Shoot | 6–8 hours | 3–5 days | Useful during early germination |
| Blue Pea Tendril | 6–8 hours | 3–5 days | Useful during early germination |
| Black Sunflower | Approx. 8 hours | 3–4 days | Light-to-moderate weight recommended |
These ranges should be treated as crop-specific starting points rather than timers that automatically determine when the cover comes off.
Temperature, moisture, seed batch, growing medium and germination speed can all affect the actual timing. Check the crop itself as well as the calendar.
How Long Should Broccoli Microgreens Stay in Blackout?
Broccoli is a small-seeded, fast-germinating crop.
The current OzSprout method uses approximately 2–4 days of blackout. Broccoli seed does not require pre-soaking.
After sowing:
- Mist the seed.
- Provide good contact with the moist growing medium.
- Cover the tray.
- Check germination each day.
- Move the crop into light once roots are established and the seedlings are pushing upwards evenly.
Heavy weight is unnecessary for broccoli. Gentle tray contact is generally sufficient for these small seeds.
How Long Should Radish Microgreens Stay in Blackout?
Radish is one of the fastest microgreen crops.
Both current OzSprout Daikon and Purple Sango varieties use approximately 2–3 days of blackout.
Radish should normally be sown dry rather than pre-soaked. Because it germinates vigorously, leaving radish covered for longer than necessary can produce overly stretched stems.
Look for an even stand of seedlings firmly rooted into the growing medium rather than automatically waiting until a particular hour or day.
For the complete crop method, see How to Grow Radish Microgreens at Home.
How Long Should Mustard Microgreens Stay in Blackout?
Green Mustard behaves similarly to many other fast, small-seeded microgreens.
Current OzSprout guidance uses approximately 2–3 days of blackout, with no pre-soaking required.
Once the seed has germinated evenly and roots have established, move the tray into suitable light.
Mustard grows quickly, so there is little benefit in deliberately keeping an already established crop in darkness for several additional days.
How Long Should Pea Shoots Stay in Blackout?
Peas need a longer establishment period than small-seeded broccoli, radish or mustard.
OzSprout Maple Pea Shoot Microgreen Seeds currently use:
- Pre-soak: approximately 6–8 hours
- Blackout: approximately 3–5 days
- Growing temperature: approximately 15–22°C
Large soaked peas benefit from firm seed-to-medium contact during early germination.
Place a second tray directly over the seed and use modest, evenly distributed pressure. As the roots establish and the shoots begin pushing strongly upwards, remove or reduce the weight rather than continuing to compress the developing crop.
Depending on the stage of development, a short additional period beneath an inverted cover can then be used before the tray moves into light.
See How to Grow Pea Shoots at Home for the complete method, including the differences between Maple Pea Shoots and Blue Pea Tendrils.
How Long Should Sunflower Microgreens Stay in Blackout?
Sunflower is another crop where weighted germination is particularly useful.
OzSprout Black Sunflower Microgreen Seeds currently use:
- Pre-soak: approximately 8 hours
- Blackout: approximately 3–4 days
- Weight: light-to-moderate pressure during germination
- Growing temperature: approximately 18–24°C
Sunflower seeds are large and have substantial black hulls. Pressure during germination helps keep the seeds against the growing medium while roots develop and can also help the expanding seedlings loosen some seed hulls.
Remove the weighted cover once the crop is firmly rooted and pushing strongly upwards.
For detailed soaking, hull management and harvest instructions, see How to Grow Sunflower Microgreens at Home.
Which Microgreens Need Weight During Germination?
The clearest candidates are larger seeds such as:
- Peas
- Sunflower
These crops physically move more as they absorb water and germinate. Weight helps maintain contact with the growing medium while roots establish.
Small seeds such as broccoli, mustard and radish generally need much less pressure. A second tray resting on the seed can provide sufficient contact without adding a heavy load.
How Much Weight Should You Use?
There is no single kilogram value that should be applied to every microgreen tray.
The appropriate amount depends on:
- Seed size
- Crop variety
- Tray dimensions
- Growing medium
- Seeding density
- Stage of germination
The aim is firm, even contact—not crushing pressure.
Once established seedlings are strongly lifting the upper tray, reassess whether the weight is still serving a useful purpose.
How Do You Know When to Remove the Blackout Cover?
The seedlings are usually a better guide than the calendar.
Look for several signs together:
- Most viable seeds have germinated.
- Roots are visibly anchoring into the growing medium.
- The crop is developing evenly across the tray.
- Seedlings are beginning to push the covering tray upwards.
- Cotyledons are beginning to separate.
If large areas have not germinated, investigate moisture, temperature, seed-to-medium contact and seed performance rather than automatically extending blackout for the entire crop.
Why Are Microgreens Yellow When Blackout Ends?
This is normal.
Seedlings developing in darkness have not yet produced their full green or coloured appearance because normal chlorophyll development requires light.
After the cover is removed and the tray is moved into suitable light, green varieties generally colour rapidly. Purple and red varieties also begin developing their characteristic pigmentation.
Should You Water Microgreens During Blackout?
Yes—but only when the growing medium actually needs water.
A covered tray loses moisture more slowly than an uncovered tray.
Before adding more water:
- Lift or inspect the cover.
- Check the growing medium.
- Check whether the seed layer is drying.
- Add only enough water to restore even moisture.
The medium should remain moist, not saturated.
A common beginner mistake is assuming a tray must be misted several times every day simply because it is in blackout. If the medium is already sufficiently moist, adding more water provides no benefit.
What About Condensation?
Condensation beneath the blackout tray tells you that the covered environment is humid. It does not automatically mean the root zone needs more water.
Check the growing medium itself before watering.
For the full method from sowing through established growth, see How to Water Microgreens: Misting vs Bottom Watering.
What Happens If Microgreens Stay in Blackout Too Long?
Excessive blackout can produce:
- Very elongated stems
- Weak or leaning growth
- Seedlings tangled beneath the cover
- Excess humidity
- Reduced airflow
- Greater opportunity for fungal problems
The purpose of blackout is controlled establishment—not maximum stem stretching.
If seedlings have germinated strongly and are pushing upwards, they generally need to progress to the next growing stage rather than spend another arbitrary day in darkness.
What Happens If You Remove the Cover Too Early?
Removing the cover early is not automatically disastrous, but it can make germination less uniform.
Possible effects include:
- The surface drying faster
- Some seeds germinating later than others
- Less uniform seed-to-medium contact
- Shorter stems
- Patchier establishment
If the crop is otherwise healthy, continue managing moisture and light rather than assuming the tray has failed.
Does Blackout Cause Mould?
Blackout does not automatically cause mould, but a covered tray creates a humid environment. Excess moisture, poor drainage or leaving an established crop covered unnecessarily can therefore make fungal problems more likely.
Factors that can contribute include:
- Excessive watering
- Overly dense sowing
- Poor drainage
- Very warm conditions
- Contaminated equipment or growing medium
- Poor airflow after the crop has established
- Leaving the crop covered longer than necessary
Fine white hairs attached directly to young roots are often normal root hairs rather than mould.
Irregular web-like growth spreading between plants is more concerning, particularly when accompanied by slime, seedling collapse, deterioration or sour or musty odours.
If you are unsure what you are seeing, use our dedicated guide to Mould vs Root Hairs on Sprouts & Microgreens.
Hygiene During Covered Germination
Good hygiene is particularly important when crops are growing in warm, moist conditions.
- Start with clean trays and covers.
- Use clean drinking-quality water.
- Use clean growing medium suitable for edible crops.
- Avoid leaving trays waterlogged.
- Inspect covered crops regularly.
- Maintain drainage and airflow as the crop develops.
- Discard crops showing obvious mould, slime, decay or unpleasant odours.
Visible cleanliness does not guarantee microbiological safety, so clean handling practices should be maintained throughout growing and harvesting.
What Happens After Blackout?
Once the crop is ready to uncover:
- Remove the cover and any weight.
- Inspect the crop.
- Move it into bright indirect natural light or beneath suitable grow lights.
- Allow the cotyledons to open and develop colour.
- Begin bottom watering once the roots are well established.
- Maintain good airflow.
- Continue growing until the crop reaches its variety-specific harvest stage.
Do not move a newly uncovered tray directly into extreme hot afternoon sun through glass. During Australian summer conditions, shallow trays can overheat rapidly.
Does Every Microgreen Need the Same Blackout Method?
No—and this is one of the most important lessons when learning to grow microgreens.
Microgreens differ enormously in seed size and growing behaviour.
Compare:
- A tiny mustard seed
- A vigorous radish seed
- A large swollen pea
- A black sunflower seed with a substantial hull
There is no horticultural reason to expect all four to respond optimally to exactly the same pressure and number of days beneath a tray.
Use crop-specific guidance as your starting point, then observe germination, root development and how strongly the seedlings are pushing against the cover.
Frequently Asked Questions About Microgreen Blackout
Do all microgreens need blackout?
No. Many crops benefit from covered germination, but the duration, pressure and need for additional blackout vary between species.
Is blackout the same as weighted germination?
No. Blackout refers to excluding light. Weighted germination applies pressure to the seed while roots establish. The two techniques are often used together, particularly with peas and sunflower.
How long should broccoli microgreens stay in blackout?
Current OzSprout guidance is approximately 2–4 days.
How long should radish microgreens stay in blackout?
Current OzSprout guidance for Daikon and Purple Sango Radish is approximately 2–3 days.
How long should mustard microgreens stay in blackout?
Current OzSprout Green Mustard guidance is approximately 2–3 days.
How long should pea shoots stay in blackout?
Current OzSprout guidance for Maple Pea Shoots and Blue Pea Tendrils is approximately 3–5 days.
How long should sunflower microgreens stay in blackout?
Current OzSprout Black Sunflower guidance is approximately 3–4 days with light-to-moderate pressure during germination.
Why are microgreens yellow after blackout?
Pale yellow seedlings are normal immediately after developing in darkness. They generally develop their normal colour once exposed to suitable light.
Should the blackout tray be airtight?
No. Blackout means excluding light, not sealing the crop inside an airtight container.
Should I mist every day during blackout?
Only if the seed or growing medium needs additional moisture. Check the tray before watering rather than following an automatic schedule.
Can I leave microgreens in blackout longer to make them taller?
Additional darkness can elongate stems, but excessive blackout can produce weak, stretched seedlings and unnecessarily humid growing conditions. Use the crop's development rather than maximum height as your guide.
When should I remove weight from pea or sunflower microgreens?
Remove or reduce the weight once roots are established and the developing crop is pushing the upper tray strongly upwards. Do not continue compressing an established canopy simply because the planned blackout period has not ended.
Microgreen Blackout: The Bottom Line
Blackout is best understood as a crop-specific germination tool, not a rule that every microgreen must follow identically.
For reliable results:
- Start with evenly moist growing medium.
- Use a cover to maintain suitable germination conditions.
- Use additional pressure mainly where the crop benefits from it.
- Give peas and sunflower stronger seed-to-medium contact than small-seeded crops.
- Check moisture rather than watering automatically.
- Watch the seedlings instead of relying only on the calendar.
- Remove the cover once germination and root establishment are strong.
- Move the crop into suitable light before excessive stretching occurs.
Once you understand the difference between covered germination, weighted germination and post-germination blackout, the process becomes much easier to adjust for each microgreen variety you grow.