How to Germinate Microgreens Part 3: The Domed Blackout Method

This article on the Domed Blackout Method is the third in a four-part series on germinating microgreens.

There are links to the other articles in the series below.

The Domed Blackout Method is for microgreens with thin stems that don’t handle the weighted blackout method very well.

Some microgreens are also vertically-challenged, and the dome elongates the stems, making them easier to harvest.   

The Domed Blackout Method Explained

Fewer people are confused by the domed blackout method than the weighted blackout method.

It makes perfect sense as most people are familiar with growing garden sets and place a clear dome over their seeds and seedlings to help with germination.

The concept is the same, except the dome is black instead of transparent.

The dome helps retain moisture and humidity and keeps a constant temperature as the seeds germinate and the fragile seedlings grow.

As mentioned in the first article of this microgreen germination series, all microgreens can be germinated using the weighted blackout method.

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So, Why Use the Domed Blackout Method?

Thin-stemmed Microgreens

A few microgreen varieties have skinny and weak stems that cannot withstand the constant pressure the weighted method places on them for any length of time.

Under the weight for too long, the plants will not recover and grow irregularly or, in some cases, die under the constant pressure.

The Amaranth in the photo below did not recover from the weighted blackout method. They didn’t grow straight even after being placed under the lights.

Microgreens that start as this will grow but are hard or impossible to harvest.

Besides Amaranth, many mustards have thin stems and grow better using the domed blackout method instead of the weighted method.

Vertically Challenged Microgreens

Microgreens that naturally have short stems, like, arugula, sorrel, and cress, can be grown using the domed blackout method to increase the microgreen height. 

The dome’s height allows the stems to elongate as the plant grows taller, hoping to push through the dome into the light. 

However, the Hybrid Blackout Method can also be used and is often more effective for shorter microgreens. Therefore, I will discuss the Hybrid Blackout Method in the fourth part of this microgreen germination series.

How to Germinate Microgreens Using the Domed Blackout Method

The video below shows a snapshot of the domed method focusing on preparing the tray and showing the results after the microgreens germinate.

Videos in the Home Microgreens Fundamental Microgreen Growing Course show the process at a slower speed with a descriptive narrative. 

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Preparing the Trays for the Domed Blackout Method

Before starting, find a tray, sheet pan, or something larger than the microgreen tray you will use. 

The green tray below is a Dough Tray.

I love this tray as it is heavy-duty, sturdy, large enough to fit a 1020 inside, and still lets me fling the soil around with my hands and not spill.

Soil is not messy if a couple of precautions are taken. Notice I made the video in my living room on my coffee table. I have no worries about a mess. 

Step 1 Fill the Tray with Potting Mix

Fill the tray with soil and brush off the excess before compacting and smoothing the soil surface.

Add more soil and compact and smooth the surface again if the level is far from the lip of the tray.

A good potting mix will only compact a little. The smooth surface makes it easier to evenly spread the seeds, so most are not touching each other. Also, it makes it easier to move the seeds around with your finger if they are too close.

Step 2 Wet the Soil Surface

Notice I said wet the soil surface. Don’t drench the soil. 

Microgreen seeds don’t need the soil wet all the way to the bottom of the tray. Wet only the upper 1/3 of the soil profile. 

That is enough water for the seeds to germinate.

If you watched the video, you saw that I placed the microgreen tray inside another tray (the white one) to wet the soil.

The reason is that I still needed to finish filling trays with soil, and I didn’t want the soil tray wet so the potting mix would stick to it when I dumped the excess back into the soil bin.

Step 3 Spread the Seeds Evenly

Once the potting mix has been misted, spread the seeds as evenly as possible on the soil surface.

The seeds shown are Red Garnet Amaranth and are very small. That is why I use a shaker jar to spread the seeds, as I can distribute the seeds easier with the jar.

Step 4 Mist the Seeds

Once the seeds are spread and evenly spaced, I mist the seeds to wet them.

Not only does this wet the seed to start the germination process. In addition, it settles the seeds into the soil for better seed-to-soil contact.

Be careful on the first go around, as misting can blow the seeds out of the tray. Once they are wet, you can increase the misting to make it go faster.

This is an essential step in the Domed Blackout Method, as the seeds need water and be settled into the soil.

Notice that the seeds are not “perfectly” spaced. That is ok as long as they aren’t bunched up or a quadrant with the tray has very few seeds.

The lower right side could use more, but as you will see later, the tray grew densely.

Step 5 Placing the Tray into the Domed Blackout

Now remove the tray and place it somewhere out of the way, but not so much that you forget about it.

Once the tray is in place. Place a dark dome over the top.

I use the watering tray as a dome for the Home Microgreens Tray shown.

The lid goes under the planting tray to catch any soil that wants to escape from the drainage holes (more precisely, the watering holes).

If your tray does not have this convenient feature, use a solid dome, lid, pot, or box over the planted tray.

It is best if the dome is similar in size to the planting tray to keep moisture and humidity high.

If the dome is transparent or translucent, that is okay, as you can cover it with a tea towel or two to make it dark.

The dome should have an inside height of at least 1-1/2 inches.

Step 6 Secure the Dome & Seal

I like to place a weight on the dome to help keep it on the planting tray.

The dome doesn’t have to click or lock onto the planting tray. Resting on the rim or lip is perfectly fine.

Again, it is ideal for the dome to fit on top of the planting. If not, do your best to reduce airflow to the planting tray. 

For instance, if you place the tray on a rack, like the one above, place the planting tray on a towel. Then wrap the towel over the tray and dome to prevent drafts from removing moisture from the soil.

I also like to place a towel over the tray, even with the weight on top of the dome. The towel helps reduce drafts and any light that might sneak in. 

The towel also helps keep a more even temperature in the tray as the temperature in my home often fluctuates between daytime and nighttime.

Step 7 Leave Them & Then Check Them

Let the seeds germinate and grow. There is no need to check them every day.

In 48 to 72 hours, you can check on the seeds’ progress.

One advantage of the domed blackout method is that you do not disturb the seeds and plants when lifting the dome.

However, lifting the dome will change the humidity.

When to Remove the Dome & End the Domed Blackout Method

Using the Home Microgreens Tray System, the time to remove the dome and place the microgreens under lights is when they grow and touch the top of the dome. 

Notice how the Red Garnet Amaranth below looks boxy, like the dome.

The “boxy” look is from the plants growing and filling in the space within the dome. The dome has just been removed. So the plants are whiteish, and the root hairs have yet to grow into the soil.

More on this in a minute

If your dome is taller than 1-1/2 or 2 inches. I suggest removing the dome when the plants are 2 inches tall.

They are ready for the lights at that point, and the domed blackout method is over.

Possible Issues With the Domed Blackout Method

There can also be some problems to look out for with the domed blackout method. 

But before we get into the problems, let me talk about the white on the soil surface in the photo above.

Domed Method and Root Hairs

When I post pictures of microgreens grown by the domed method, I always get the following comment.

“Is that white mold?”

The answer is no.

The fuzzies you see in the last photograph are root hairs. I have written a very popular article on the difference between root hairs and mold here. 

Root hairs are the conduits that readily uptake water into the plant’s vascular system. 

Because the humidity is so high under a proper dome, the root hairs can capture water from the air and the upper surface of the potting.

Looking closely at trays grown using the weighted blackout method, you can also see root hairs when you take off the tray separator. However, the root hairs are less pronounced than they are in the domed method.

After a day, the root hairs disappear, and all is well. This article goes more in-depth on identifying root hairs and mold.

Main Concern When Using the Domed Blackout Method

If planted correctly, the only problem that frequently occurs with the domed method is too much moisture.

The dome method is more susceptible to condensation for two reasons.

The area under a dome is much larger than under a tray separator, so there is more humid air.

The dome also has a larger surface area and is more exposed to ambient temperature changes.

So differences in temperature between the air inside the dome and the dome surface can cause condensation to form and drop onto the plants.

The temperature difference can become more significant if the tray is placed on a storage rack above the lights. Even LED lights heat up slightly, warming the soil, which drives water to condense against the cooler dome.

Too Much Condensation Isn’t Great

Plants grown by the domed method are already considered more fragile and tender. This is the main reason we grow with the domed method.

So extra moisture can quickly ruin a tray of microgreens.

I’ve done it a few times with Amaranth, by waited too long to remove the dome. My home is much cooler at night than in the daytime. So every night condensation rained down on the plants.

I also place my blackout trays on the upper shelves. I almost always have warm lights heating the trays during the daytime, and at night when they shut off, the tray cools, and condensation forms.

Solution to Condensation Problems

Condensation is why I only water the upper 1/3 of the planting tray. Extra moisture can only cause problems.

However, that isn’t enough.

Store your blackout trays on shelves without lights, or at least, on lower shelves (I don’t because I’m tall, and it makes it harder to check on the trays – lazy, I know) without lights below them. Remove a heat source.

Here is the most helpful tip after the fact: tip the dome when you remove it to check on the seedlings.

Lift one side higher than the other, so the condensation on the dome runs to the curved edge.

Then you can dump the water that pools up.

This also lowers the humidity under the dome.

Summary of the Domed Blackout Method

The domed blackout method can be used to grow most microgreens.

However, the weighted blackout method and its sister, the buried blackout method, are better choices as microgreens seem to thrive when grown with those methods.

However, the domed method shines when growing amaranth, many mustards, and other thin-stemmed and fragile microgreens.

We will use the principles of the domed blackout method in the next and last germination method of this how-to-germinate microgreens series.

Use the form below to get updated when that article and others are published. 

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In addition, we sell seeds in packets sized perfectly for trays and larger bulk bags.

Our potting mix is also among the best on the market and is highly rated by our customers.

Compare our prices with the other larger seed suppliers. You will be happy with the price, and I know our service is the best.

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