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Black out seeds

Seed germination – blackout period or not?

I am first time grower, sorry for the maybe obvious question, but google isn’t an help here.

I have a newbie question: why do microgreens germinate in the dark, but all other youtube guides about germination they place the tray near a window or a growing light, why?

In my understanding microgreens are just normal germinating plants harvested after 12-14 days. But why they blackout period is not used to germinate any plant you intend to re-pot later on? If place seed in a paper towel, why should be under the light and not having a blackout period?

Fundamentally, I don’t get why there are two different methods for what seems to ME the same process.

The blackout period stimulates vertical growth. The plants stretch out looking for light. You generally don’t do this for plants you are growing to maturity because compared to the rest of the growth over the lifetime of the plant it’s pretty minimal, and its usually detrimental in the long run.

It can lead to the plants being too ‘leggy’ to support their own weight if they get too top-heavy before they have a chance to harden off and thicken. But you generally don’t want them to harden much at all for microgreens. You want them to grow as much as possible and remain as tender as possible.

The only reason to expose them to light in the end is that it makes them look more appealing, and the stress can help them to develop a better flavor. They don’t really ‘need’ the light at all, since before true leaves emerge (and to some extent afterward) they are getting all they need from water and the seed endosperm.

Microgreens and When to to Use a Blackout Dome

Nick & Nathan from On the Acre in Houston, TX discuss the topic.

When to Use Blackout Domes?

We do not use blackout domes unless we have selected that product to grow in blackout specifically. It’s not a regular part of our germinating or planting process. When they’re stacked, the trays are mostly blacked out, and beyond that, we haven’t found it necessary or beneficial to use that method.

What are the Drawbacks to Using Blackout Domes?

I worry, if you’re trying to germinate seeds in a blackout dome, you know there’s no light, and there’s no air movement, and that moisture is just sitting there not going anywhere, so you run the risk of mold or fungus issues that you wouldn’t encounter otherwise.

Why do you think that that’s such a prevalent problem saying that people are always told to do that blackout? Where did that idea come from? I have no idea; my best guess using domes would be best for people trying to create additional mass; they would use a blackout dome to lengthen the stems. The lack of light causes the stems to stretch toward the light. When there’s no light present, they’ll continue to grow and grow, and then they can cut at the very bottom, you know, with these long stems and little leaves on top, and they would be able to get more money because they’re selling by mass rather than volume.

Stacking During Germination Vs. Black Out Dome

Using blackout domes would use extra space. Our domes take up more space than when we stack the trays. We do two-three times as many in the same space without domes, so if we use that regularly, we would need so much more room to do what we’re doing, and for us, space is a premium. I talked to one of our students a couple of weeks ago, and they’re asking how to grow a new variety. They said that they needed like 25 trays a week or something, and they were planning on using blackout domes, and I was like do you know how much space that’s going to take? I advised them to use a stacking system because arranging 25 trays a week under blackout domes will use a lot of space.

Using Blackout Domes to Create New Products

Just as an option and the products you get when you use a blackout dome, it’s noticeably different like Nathan said, the stems are longer; they have a different color; they have a different taste. We do peas, corn, and sunflower currently in blackout domes.

We have been experimenting with other stuff, but amaranth is next. It gives us another option, and we like to present our chefs with options. It’s just another product that we can offer from something that we’re already growing in non-black out domes. But the point here is, we’re growing it in its entirety in a blackout dome. We’re not growing it for just the germination period, then moving into the light. We are creating completely different products with the use of blackout domes.

Microgreens Blackout Period: Why is it important?

You must have come across the term blackout period while reading articles about growing microgreens. Perhaps you have read the term in seed catalogs or product listings in online stores.

Blackout period is a vague term in that it’s talked about in passing but never fully explained. Many authors assume their readers are well-versed about it, not elaborating on it.

The microgreen blackout period is a phase in which trays of microgreens are either loaded or enclosed to prohibit light from reaching the seeds.

What Purpose Does the Blackout Period Serve?

The blackout period can be likened to placing soil over the top of your microgreen seeds the way you’d normally do when planting in a garden outside. There are a number of reasons soil, including soilless potting mixes, isn’t used as a covering for microgreen seeds. You can find the major reasons listed below:

  • The seeds are sown so compactly that the developing leaves would “throw” soil everywhere when they germinate. This results in a reduction of soil support for the plants.
  • The soil surrounding the young plants would increase the probability of fungal growth and disease.
  • Soil would remain on the compactly growing plants, leading to a dirty crop.
  • Following germination, the cover soil would be loose. Harvesting would become messy and dirty as a result.

The blackout equipment and methods that we use on newly planted microgreen seeds have supplementary purposes during the growing process. These purposes will be elaborated upon in the remainder of this article.

Materials for the Microgreen Blackout Period

When it comes to blackout materials, there are a lot of options available for you.

You can make use of the lid that came with the planting tray as the foundation. What this does is that is assists in keeping all of the seeds in contact with the planting medium.

If the configuration at the bottom is the same size as the top of the tray, you can also utilize a spare planting tray.


Yes, we do need to allow a period to keep the microgreen tray in the dark. Most tray lids allow light to penetrate or reach the seeds along the edges. To solve this, you can use tea towels to exclude all the light. Alternatively, you can cover the tray lids with a cardboard cutout to block out the light.

Using a dark plastic planting tray with similar sized top and bottoms work great. However, many of those types of trays are large, like ten by twenty inches, that is usually too large for use at homes. A smaller tray typically has a narrower bottom than the top. It is for this reason you should use the lid on top of the seeds and soil.

Is the Blackout Period Necessary?

This question is one that many microgreen growers have in their minds. The blackout period is extremely beneficial for optimal growth and germination, especially when considering radishes. You must have read that some microgreens, for instance, basil, don’t need a blackout period.

Dual Meaning for Microgreen Blackout Period

It is little-known that the blackout period is a bit more than keeping the seeds away from light. When planting more than one microgreen tray, you should stack them on top of each other, placing a weight of some kind on the stack.

Most microgreen seeds love the weight on top of them for the following reasons:

Apart from holding the blackout material on the trays, the weight also assists in keeping the seeds in constant contact with the moist planting media.

Moreover, the weight forces the tiny seedling to better anchor itself in the soil, resulting in a stronger and better-rooted plant.

If you are worried that the plants are not strong enough to deal with the weight, they can easily do it. There are weeds that may grow up through the asphalt. The plants may even shift the weight if left long enough.

The Length of the Blackout Period

There isn’t one correct answer to this. The microgreen blackout period can last as long as it is required for the seeds to germinate, root deeply, and mature to the point that they’re ready to commence photosynthesis. The variables you need to consider include:

  • the variety of microgreens;
  • air and soil temperature;
  • and moisture level

Generally, you should check the seeds after forty-eight hours to observe how well they have germinated, and to gauge their stage of growth.

For some varieties, including radishes, turnips, mustards, broccoli, and kohlrabi, forty-eight is sufficient. For some other types such as borage, cilantro, celery, the blackout period may continue for four to six days.

Further Reasons for Covering Microgreen Trays

There are several reasons we use a blackout period when planting microgreens.

A plastic lid or tray bottom is utilized to keep the microgreen seeds in constant contact with the moist planting media.

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

When the weight is added to the tray, it forces the seeds to grow stronger and root deeper after they germinate.

The weight also ensures that the planting surface is compacted during and after germination; this process reduces loose soil during the harvest.

The Timestamps of the Microgreen Blackout Period

There are far too many variables to give a definite answer, but remember to check the seeds after forty-eight-hour to observe their condition.

You can place some varieties in the light following forty-eight hours. Other varieties may take a few more days in the blackout.

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

Now that you are equipped with the necessary tools and knowledge regarding the blackout period be mindful of this guide while growing microgreens.