Should You Cover Microgreen Seeds? – A Test

Should You Cover Microgreen Seeds?

The general consensus among microgreen growers is to cover newly planted seeds either by placing another tray of microgreens on top or using some sort of cover to darken the planted seeds.

At Home Microgreens, we are all about listening and taking advice from experts. But we aren't going to follow blindly. We like to see the evidence, Home Microgreens is all about testing the internet dogma.

In this article, we present the results of an experiment we did using radish seeds.

We planted the same mass of Champion Red Radish seeds on four trays filled with professional soil mix. All of the trays are prepared and planted the same.

The only difference is the way the seeds will be germinated. Below are the four methods used to grow the radish microgreen seeds.

  1. Covered to eliminate light and with weight added to the cover,
  2. Covered to eliminate light and with no weight added to the cover,
  3. Covered with a low, transparent cover (quarter- to half-inch of space) and exposed to light, and
  4. Covered with a high transparent cover (two and a half to three inches of space) and exposed to light.

These methods are shown in the photo below, respectfully left to right.

Testing the blackout period

Covering Microgreen Seeds

As mentioned previously, most experts cover microgreen seeds during germination. This is known as the blackout period.

The blackout period is more than covering the seeds to exclude light. During the blackout period, weight is always added to the cover pressing the seeds into the soil to help the microgreen roots grow deeper and better support the plants.

We won't go into the details of the blackout period in this article. For more information click the button below.

Learn More About the Blackout Period

Get more information on the Blackout Period in this article.

What is the microgreen blackout period

Explanation of the Experiment

The purpose of this experiment is three-fold.

  1. To see if it's necessary to place a cover or tray right on top of the seeds.
  2. Add weight to the cover or upper tray to press down on the seeds.
  3. To exclude light from newly planted microgreen seeds.

We will grow four trays of radish microgreens using the same size tray, same soil, the same amount of water, and the same mass of seeds. 

Below is a video of how we plant microgreens seeds using the Home Microgreens trays. Also, we explain in more detail, the variables on how each tray is allowed to germinate.

The Control Tray - Covered & Weighted

The first of two trays where we cover microgreen seeds after planting. 

After planting the radish seeds, this tray is covered with a lid tipped upside down, so the lid is in contact with the seeds, a weight is added, and then a tea towel is placed over the tray to keep the seeds in the dark.

control microgreen tray

Tray will be covered with a tea towel.

Seeds Blacked Out Without Weight

This tray will be covered with a tea towel like the control tray. However, the lid will be placed on the tray right-side up, providing an air space between the seeds and the lid.

The purpose is to see if it's necessary to add weight to the seeds. 

no weight added to microgreen seeds

A tea towel will be placed over this tray.

Seeds Under Transparent Cover - Small Air Space

This tray is the same as the last tray, but it will not be covered with a tea towel. 

The purpose is to see how well radish microgreen seeds will germinate and grow when exposed to light. The cover will help retain moisture.

germinate microgreen seeds under light

These radish microgreen seeds will be exposed to light.

Seeds Under Transparent Cover - Large Air Space

This tray is placed in a larger tray (10 by 20) with a high dome transparent cover and the seeds exposed to the light. 

The purpose is to see how well radish microgreen seeds will germinate and grow with lots of air space while being exposed to light. The cover is used to help retain moisture since the seeds are on the surface and not covered with soil.

microgreen seeds germinated under high cover

Radish microgreen seed tray placed under transparent high dome.

Cover Microgreen Seed Test - Results

The reason we used radish seeds is that they germinate and grow quickly. In fact, radish microgreens can be ready in as soon as seven days.

The sequence of photos below is of the radish microgreen trays with the elapsed time shown in the heading.

You can expand the photos by clicking on them to get a better view.  

I believe the photos will show you the results.

Covering Microgreen Seed Test - Day 0

Three of the four seeded radish trays are shown (three fit into the photo better) below. 

radish microgreen seeds

Radish seeds sown on a 37.5 square-inch tray. Notice uniformity of seeding between trays.

Covering Microgreen Seed Test - Day 2

All four trays were left untouched for 48-hours. This is how we handle all freshly planted microgreen seeds. 

On the second day, even if we know the seeds might not germinate until day three or four, we check the seed and soil to make sure the moisture content is okay. 

Radishes, however, usually germinate after only 24-hours. But we still wait until the second day before disturbing them.

The white you see around the seeds is root hairs and not mold or fungus.

All trays are returned to their respective positions after the photos are taken.

Below are photos of each of the four test trays. The captions contain a description of the progress. Click image to expand the viewing area.

Covered & Weight

radish microgreens 2-days after seeding

Great germination on this tray of radishes. Yellow color is because they were covered and received no light.

Covered & No Weight

radish microgreen seeds

Germination is okay, but not as good as the weighted tray. Again, yellow color is from lack of light.

Light & Small Air Space

radish microgreens on day 2

Germination is okay, growth is uneven across the tray, some radishes are growing upwards, others sideways. You can see how these are greener than the previous two trays due to access to light.

Light & Large Air Space

radish microgreens on day 2 left open to air and light

Very little germination and growth. Notice how the soil along the edges is dry. More air circulation removes moisture from soil.

Covering Microgreen Seed Test - Day 3

In the images below, you can see that the trays not exposed to light grew quite a bit in the last 24-hour period. Those under light didn't grow as much.

After the photos, all of the trays are exposed to light. The lights are on timers set for 14-hours.

The trays are also bottom watered at this point. Bottom watering reduces the likelihood of disease issues. Water is added into a larger bottom watering tray, and the planted trays are allowed to soak for an hour or two.

The water is removed from the bottom tray (or the planting tray is placed into a new dry tray), and the planting trays are allowed to drain if they are saturated.

This is the advantage of using a coco coir based planting mix. Overwatering is almost impossible if they're able to drain.

Differences Are Becoming Obvious

The last 24-hour period has amplified the differences between the four methods. Again, the white is root hairs and not a fungus.

Covered with Weight Added

Even though these seeds were grown in the dark and with a heavy weight pushing down on them, they have the best germination and growth of the four methods.

The yellow color is due to being grown in the dark.

Covered with No Weight Added

Not quite as good germination or growth as the weighted tray. Notice that more of the root hairs are growing above the soil line.

Without the weight, the roots have no incentive to grow down to anchor the plants. They have enough energy in the cotyledons and moisture on top of the soil. They have no need to anchor themselves and push up through the weight.

Grown in Light with Small Air Space

This tray is very similar to the tray grown under the blackout without weight added. The exception is that the plants are green and full of chloroplasts.

The plants are also growing on or near the surface of the soil. They're not well anchored at all and are growing along the surface.

Grown in Light with Large Air Space

This tray has poor germination. Most likely because the larger air space allowed the seeds to dry out. Maybe a bowl of water added to the tray would help, but probably not enough to make a significant difference.

All four trays are now placed under LED lights for 14-hours per day.

Below are photos of each of the four test trays. The captions contain a description of the progress. Click image to expand the viewing area.

Covered & Weight

radish microgreens day 3

Even though these radish microgreens have been covered with a heavy weight on them they have the best germination and growth of the four methods.

Covered & No Weight

radish microgreens grown without weight

Less growth here than the covered and weighted tray. Also, the plants aren't well anchored into the soil. Many are growing just above or in the loose top surface of the soil.

Light & Small Air Space

radish microgreens day 3 without blackout period

Grown in light, these radish microgreens are green compared to those in the blackout. However, without weight on top of them, they are very loosely rooted in the soil.

Light & Large Air Space

radish microgreens day 3 grown uncovered

Grown basically in the open, these seeds are well behind the other three trays. Lack of moisture is most likely the cause for poor germination.

In the photo below, you can see how the roots of tray 3 are growing mostly out of the soil. Tray 2 is about the same, except the plants aren't as green.

radish microgreens

Without weight on top of the seeds, there is little incentive for the roots to anchor into the soil.

Covering Microgreen Seed Test - Day 5

We've seen all we need to see to make a choice on which method is best. It's evident in the photo below that the radish microgreens grown under a full blackout period (covered & weighted) have grown the best.

The full blackout period means the seeds are grown in the dark with weight on top of the seeds. In the case of these radishes, the blackout period was three days. 

The last tray didn't have good germination because the large air space did not keep moisture around the seeds.

The two middle trays are very similar, both had reasonably good germination, but because the roots did not anchor into the soil, many plants didn't get enough moisture from bottom watering and died due to lack of water.

Now, they could be watered from the top, but then you increase the risk of fungus growth.  

Below are photos of each of the four test trays. The trays are in order for how they are explained in the article from left to right. 

radish microgreens day 5

From left to right: Covered with weight; covered no weight; grown in light with small air space; grown in light with large air space. You can see that poor rooting in trays 2 and 3 have cause some microgreens to die because of lack of water from below.

Covered & Weighted

radish microgreens day 5 covered and weighted

These radish microgreens were placed in a full blackout period, excluding light and adding a heavy weight on top of the seeds. By far the best tray of microgreens.

Covered & No Weight

radish microgreens grown in blackout but no weight

These radishes were covered to exclude light, but no weight was added to the top. The roots did not anchor well into the soil. You can see many roots above the soil surface.

Place Microgreen Seeds into a Full Blackout Period

As you can see, it's best to cover microgreen seeds to exclude light and add weight to the top of the seeds so they're pressed down into the soil. 

As you saw in the photo series, microgreen seeds grown in the dark germinate the best.

Weight added to the top of the tray forces the roots to grow into the soil and anchor the plant. 

The weight also helps retain or trap moisture between the cover the and soil helping the seeds to germinate. This may be the reason why there is more consistent germination using this method.

The pictures tell the story, at least for radish microgreen seeds.

However, after growing almost 30 different varieties of microgreens, it appears that germination in darkness and under weight is preferred for most microgreen seeds.

No internet dogma here, the microgreen growers are correct when they recommend you to stack up several microgreen trays. The darkness and weight produces better quality microgreens.

What is Your Experience?

How do you start your microgreens? Do you use another method?

Leave a comment below, we'd love to hear about your success!

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

Leaving a comment or using the Ask a Question page does not add your email to any mailing or marketing list. 

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Author of this Article is Todd

Todd is the founder of Home Microgreens & the Home Microgreens store. He also writes for several other websites, including

His microgreens have appeared in Better Homes & Garden magazine and other websites.
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. He will be in the garden, trout stream, or on a mountain trail with his Springer Spaniel Caden when not at the computer.

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