Microgreen Blackout Period – What Exactly Is It?

Microgreen Blackout Period for the Home Grower

You may have encountered the term blackout period in articles about growing microgreens or seen the term in seed catalogs or product listings in online stores.

The blackout period is a nebulous term in that it’s talked about but never explained in detail; authors assume the reader understands its purpose and how to go about it.

The microgreen blackout period is when trays of microgreens are stacked or covered to exclude light from the seeds. 

microgreen in a blackout period

These microgreen trays are in a blackout period. The length of the blackout period depends on the variety and temperature. The tray stacking and rock weights are discussed in another article.

Listen to an Audio Version of the Article

We don’t just read the article word for word in the audio version; it’s a stand on its own piece of content that includes more details on the topic. These can include more tips, opinions, details, data, and information on this and related topics. 

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What is the Purpose of the Blackout Period?

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The blackout period mimics placing soil over the top of your microgreen seeds as you’d typically do when planting in an outside garden.

There are several reasons soil (this includes soilless potting mixes) isn’t used to cover microgreen seeds. Below are some of the main reasons.

  • The seeds are sown so tightly that the emerging leaves would “throw” soil everywhere when they sprout—reducing the soil support for the plants.
  • The soil around the young plants would increase the likelihood of disease and fungal growth.
  • Soil would stay on the tightly growing plants making for a dirty crop.
  • After germination, the covered soil would be loose, making harvesting messy and dirty.

The blackout method and equipment we use on newly planted microgreen seeds have additional purposes in the growing process. These purposes will be discussed further below in this article.

Microgreen Blackout Materials

There are many options available to you when it comes to blackout materials.

Here at Home Microgreens, we use the lid that came with the planting tray as a foundation. The lid helps keep all the seeds in contact with the planting medium (we’ll deal with the opacity below). 

covers use during microgreen blackout period

You can also use a spare planting tray if the bottom configuration is the same size as the top of the tray.

We’ve also cut down sheet plastic to the same size as the top of the planting tray to cover the seeds.


Yes, we do need to blackout or darken the microgreen tray. As you can see in the above photo, most tray lids allow light to penetrate or reach the seeds along the edges.

We use tea towels to exclude all the light or cover the tray lids with a cardboard cutout to block the light.

tea towel used as blackout material
tray lid used to blackout microgreen seeds

Using dark plastic planting trays with similar-sized top and bottoms work great. However, most of those trays are large, like 10- by 20-inch trays, that are usually too large for home use.

Smaller trays usually have narrower bottoms than tops, so we use the lid on top of the seeds and soil.

Whatever you use, as long as it covers the majority of the planting area and greatly reduces the light reaching the seeds, it will work just fine.

Is the Blackout Period Necessary?

This is a great question!

So great, in fact, that we’ve tested it, we examined the effects of light and darkness on microgreen seeds and tested the need for weight to be added to the top of the tray of newly planted microgreen seeds.

Below is a teaser photo of the test.

testing microgreen blackout period and weight

The test results, without a doubt, showed that at least radishes need to be covered or blacked out and have weight added to the top of the tray. For more information and to see the test results read Should You Cover Microgreen Seeds?

The short answer is that the microgreen blackout period is beneficial for optimal germination and growth.

We’ve read that some microgreens, such as basil, don’t need a blackout period. We will test that in the future and edit all relevant posts with the outcome. 

Again, use the button above to sign-up for updates and not miss the results of our experiments and tests.

Dual Meaning for Microgreen Blackout Period

In reality, the blackout period is more than keeping light away from the seeds.

As you saw in the pictures above, when we plant more than one microgreen tray, we usually stack them on top of each other and place a weight of some sort on the stack.

adding weights to microgreens trays during the blackout period

We use anything lying around for weights. Whether it’s a 2.5- or 5-pound weight, a rock, or sometimes even a fossil. We haven’t tested if there’s a preferred weight range, but that experiment is in the planning stage.

Microgreen seeds love (well, most do) the weight on top of them for the following reasons.

Besides holding the blackout material on the trays, the weight helps keep the seeds in contact with the moist planting medium.

Also, the weight forces the tiny seedling to anchor itself in the soil. This produces a stronger and better-rooted plant.

Don’t worry; the plants are strong and can handle weight. Think about weeds that can grow up through asphalt.

seedling growing through pavement

Five pounds is nothing for the hundreds of plants growing below the tray cover. 

In fact, if left long enough, the plants will push the weight right off the tray!

microgreens lifting top off blackout cover

How Long is the Blackout Period?

There’s no set answer to this question.

The microgreen blackout period lasts as long as necessary for the seeds to germinate, root deeply, and grow to a point where they’re ready to start photosynthesis. 

There are many variables to consider, such as:

  • the microgreen variety;
  • soil and air temperature; and
  • moisture level, to name a few.

In general, you should check the seeds after 48 hours to see how well they have germinated and their growth stage. 

For some varieties, such as radishes, turnips, mustards, broccoli, and kohlrabi, 48 hours is long enough. The blackout period might last four to six days for other types like cilantro, celery, borage, and others. 

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In the literal sense, the microgreen blackout period mimics covering seeds with soil.

We don’t cover microgreen seeds with soil for the following reasons:

  • It would increase the possibility of fungal diseases;
  • the seeding density would cause all of the soil to be lifted off the surface of the tray when the seeds germinate; and,
  • harvesting the microgreens would be more of a mess.
keeping microgreens under blackout

To cover up or blackout the seeds, we use a stiff material, such as a sheet of plastic, a tray lid, or the bottom of a similar-sized tray.

If the plastic is transparent or opaque, you can use a cardboard cutout or a tea towel to cover and completely darken the tray.

Additional Reasons to Cover Microgreen Trays

In reality, there are other reasons we use a blackout period when we plant microgreens.

A plastic lid or tray bottom keeps the microgreen seeds in contact with the moist planting media.

The plastic lid or tray also helps to keep even pressure over the seeds and tray when weight is added to the top of the tray.

Weight is added to the tray, forcing the seeds to root deeper and grow stronger after germinating.

The weight also keeps the planting surface compacted during and after germination reducing loose soil during harvest.

How Long Are Microgreens in the Blackout Period?

There are too many variables to give a definite answer, but check the seeds after 48 hours and see their condition.

Some varieties will be ready to place under light after 48 hours. For others, it may take several more days in the blackout.

In general, the seedlings should be pushing the top off the tray. Don’t worry if the microgreens appear white or yellowish, and they will green up as soon as they are under the lights.

Have a Question?

If you have any questions about the information in this post or microgreens in general, please comment below or contact me using the Ask a Question page

We’d love to hear your option on how long you leave microgreens during the blackout period.

Frequently Asked Questions – New!

There have been many questions on the blackout period, so I decided to publish a series on the methods I use to grow microgreens.

The articles are linked below.

The How to Germinate Microgreens Series

Part 1: The Weighted Blackout Method
Part 2: The Buried Blackout Method
Part 3: The Domed Blackout Method
Part 4: The Hybrid Blackout Method


  • Todd

    Todd is the founder of Home Microgreens & the Home Microgreens store. He also writes for several other websites, including MyViewFromTheWoods.com. Todd worked at a large farm market, garden & nursery center for 20 years. Somehow he snuck off to become a geologist and professor before coming back to his senses to write & lecture about microgreens and gardening. When not at the computer, he can be found in the garden, trout stream, or mountain trail with his new Springer Spaniel Caden.

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