How to Germinate Microgreens: Part 1 The Weighted Blackout Method

In this 4-part series on how to germinate microgreens, I will explain the most common methods and which method to use depending on the variety of microgreens you want to grow.

Most microgreens are easy to germinate. But it’s more involved than throwing seeds down and misting them with water.

Although the throw and grow method will work, the results might be less than ideal.

I will explain the weighted blackout method below in this first article of this four-part how to germinate microgreens series.

Weighted Blackout Method Defined

The weighted method involves covering microgreen seeds with a hard surfaced material and placing weight on that material to push the seeds onto the growing media.

wheat grass with weight
covering up microgreens for blackout

Above is a tray of microgreens with a tray separator, weight and eventually covered with a towel to put the seeds in the dark.

The tray separator (explained in detail below) doesn't have to be black as you can use a towel or something else to exclude light to the seeds.

how to germinate microgreens part 1 the weighted blackout method

How To Germinate Microgreens: The Weighted Blackout Method

Before we get to the weighted method, let’s define some terms and discuss the pre-blackout steps.

A Basic Assumption

The discussion below does not discuss how to fill a tray with soil or a grow mat and place the seeds on the growing media.

The assumption is that you have your tray prepared with a grow media and that the microgreen seeds are spread on the surface of the media. Furthermore, the seeds and growing media have been misted and moisten. 

If you aren't quite sure how to do that, it's ok, you can check an article I wrote called Growing Microgreens for the First Time or download the Free Guide to see all of the steps prior to germination. 

Blackout Period

All microgreens, except for lettuce, should be grown in a blackout period. You can click the link to read more about the blackout period. But in essence, the blackout period is the length of time that microgreen seeds are placed in the dark.

The blackout mimics nature as most seeds of microgreens varieties are buried in soil or under detritus before germinating. 

The length depends on the variety of microgreens and the environmental conditions in the area around the microgreen tray. 

The variables are discussed and will be updated in the previously referenced article and in the Home Microgreens upcoming courses.

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Information on the specific length of the blackout period can also be found in articles focused on how to grow particular varieties.

Home Microgreens has dozens of published articles on how to grow specific microgreens, and an index of those articles can be found on the Home Microgreens Article Directory.

All the methods we use to germinate microgreens (except lettuce) involve a blackout period.

The difference between the methods is the treatment of the seeds within the blackout.

What is the Weighted Blackout Method?

The weighted blackout method is most commonly used, and to my knowledge, is a method that can used to grow any variety of microgreens satisfactorily.

When in doubt, apply the weighted blackout method to any seed you wish to grow as a microgreen.

The weighted method involves covering microgreen seeds with a hard surfaced material and placing weight on that material to push the seeds onto the growing media.

Weighted Blackout in 35-seconds

Yeah, the video opening frame is wrong, I took a guess.

Here's a quick visual of the weighted blackout. 

In the following photos I will show you what equipment you need and how to apply the weighted blackout method to your microgreens. 

So you probably you thought of a few questions, such as those below.

  • What is a hard surfaced material?
  • How much weight is placed on that material?
  • Why would I want to weigh down those tiny little seeds and plants?
  • How does this help to germinate microgreens?
  • And, won't the weight hurt the microgreens?

I’ll get to the last two questions later in the article, but let’s first explain the materials we need for the weighted method.

The How to Germinate Microgreens Series

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

What is a Hard Surfaced Material?

I guess I couldn’t come up with a better descriptor off the top of my head. If I do, or one of you does, and adds a comment, I’ll change this text, and we won’t have to worry about that terrible descriptor.

I call these hard surface materials tray separators. As they separate microgreen trays from the weight or another tray when stacked.

A tray separator is something that can withstand weight without deforming. A material that has some flex is good, but it can’t fold or hinge around the weight.

Some flex also helps the tray separator apply pressure as evenly as possible on the soil or grow media and the seeds.

Also, seeds can not stick or become embedded into the material's surface.

As the seeds germinate, they can’t be caught in divots or depressions in the separator, or the seedlings may die.

Good examples of tray separators are the plastic lids of the Home Microgreens Tray.

How to put a home microgreens tray lid on seed

Another material that I use for larger trays and am moving towards even with the Home Microgreens Tray is coroplast.

coroplast for blackout period

Coroplast is a corrugated plastic that is commonly used as signs people place in their yards around election time).

coroplast for tray separator

Both materials will bend under the weight applying pressure to the grow media but not fold or deform around the weight. 

The surface is also hard and slick, so seeds will not be embedded, and plants can slide underneath the plastic as they grow without sticking.

Stacked microgreen trays

There are, of course, other materials that come to mind. One is the bottom of another microgreen tray. In fact, many growers use the bottom of another tray to apply weight to microgreen seeds in the lower tray. Thus, adding a hard surface and weight simultaneously to the tray.

However, I don’t like this practice because the bottom of most microgreen trays are ribbed and impede the microgreens from growing laterally. Also, seed husks from the germinated microgreens and soil can stick to the bottom of the tray, making a mess if the upper tray is set down.

microgreen trays

I prefer a separate material that can easily be removed and placed vertically in the sink or a waste container and dealt with afterward.

Using another microgreen tray for weight is fine as long as it is placed on the tray separator keeping the bottom clean.

Overall, I like coroplast as a tray or separator.

What to Use as a Weight?

Now I use weight plates as weights for my microgreen trays. Weight plates are metal weights that slip onto barbells. 

weighted blackout method

I like these because they are flat, come in a lot of different weights, can be found at garage sales (used & cheap) or many other stores (new and not as cheap), and have a low center of gravity so nothing can knock them off the tray separator.

However, in the past, I have used rocks (I’m a geologist, and we always have some lying around), thick books, boxes of shotgun shells (they work great!), and sealed containers of water will also work.

You can click an image to expand it for better viewing.

adding weights to microgreens trays during the blackout period
using books for weight
roots in grow mat allow plants to lift weight

How Much Weight to Use?

The weight doesn’t have to be exact. But most people not familiar with the method would think obsessive.

  • I generally use 2-½- to 3-pounds on small trays like the Home Microgreens Trays.
  • For 1010 trays, I use 5- to 7-pounds.
  • For larger 1020 trays, I stick with 10 to 13 pounds.

For instance, when I grow multiple 1020 trays of microgreens, I stack up two trays and then put the weight on top of the double stack.

So the top tray receives 10 pounds while the bottom tray has the extra weight of the upper tray added to the stack.

I was stacking up three trays at once, but now I stick with only two for various reasons that are not in the scope of this article.

Be Careful of Thin-Stemmed Microgreens

As mentioned, the weighted method can be used to germinate all microgreens, but some varieties can’t handy as much weight.

Microgreens such as amaranth and many smaller mustards do not do well under a lot of pressure.

So care must be exercised, or as explained in a later article in this how to germinate microgreens series, another method might work better for those varieties.

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Why Use Weight on Microgreen Seeds?

I’ve touched on why we put microgreen seeds in the dark. Remember the blackout period. 

Now let me touch on why we use the weighted blackout period.

The weight does several things.

Seed to Soil Contact

The added mass evenly spread on the soil (or any grow media) forces better seed-to-soil contact.

Because we often only spread the seeds on the surface of the grow media (see the second article in this series), the weight helps push the seeds into media where the moisture levels are consistent.

Elevated or “floating” seeds on the soil surface will have more difficulty germinating.

Many seeds are particular to their germinating conditions.

If those conditions are not met, they will either not germinate or start to germinate and then die.

Nature Places Weight & Darkness on Seeds

Seeds naturally germinate in, not on, soil, and replicating that environment is best for the seeds.

Adding weight helps replicate these conditions when the seed is only spread across the surface of the growing media.

The tray separator also helps hold moisture and keep humidity levels consistent.

Forces Deeper Rooting

Weight also forces the plant to set deeper roots. Most likely, to create leverage to lift the weight and find their way to light.

If you don’t think so, here are a couple of experiments.

Place some microgreens seeds (any will do) on soil and place a dome over the top of the tray. By the way, the domed method will be the third article in this how to germinate microgreens series.

After 3 days, lift the dome.

red garnet mustard microgreens from domed blackout period

You will seed germinated microgreens plus a whole lot of white stuff.

The white stuff is not mold; instead, those are root hairs.

The humidity under the dome is such that the microgreens can capture all the water they need from the air and are free to grow upwards under the dome. So they don’t set deep roots.

In the second experiment, partially fill a tray with soil, maybe a quarter of an inch below the edge of the tray. 

Spread the seed as before, and place a flexible tray separator and weight on the tray so that the tray separator touches the soil across the middle of the tray but not along the edges.

Again, wait for 3-days.

better rooting in the middle of the tray

When you remove the weight and tray separator, the microgreens in the middle of the tray will have very few root hairs showing. While those along the edges will have more visible root hairs.

For this reason, I have made more of a conscious effort to fill the tray closer to top.

The tray separator also keeps the humidity high enough for microgreens to capture water without setting deep roots.

Those along the edge have moisture and a bit of room to grow upwards.

However, the microgreens in the middle, which were exposed to the same humidity, have many fewer root hairs exposed. 

Why is this?

Because those microgreens did not have room to grow. The roots grew deeper to gain leverage, hopefully, enough to lift the separator and weight off themselves so they could continue to grow.

What is the point?

The point is that microgreens grown beneath weight will be stronger and more well-rooted than those without weight. 

The difference in the health of the microgreens will continue as they grow. A deep rooted microgreen can gain access to water and nutrients more easily than a shallow-rooted microgreen.

Deep-rooted microgreens will also be able to grow longer, straighter, and taller than shallow-rooted microgreens.

Won't the Weight Hurt the Microgreens?

Nope.

In fact, most microgreens will throw the tray separator and weight right off the tray. 

They have no problem lifting weight. Many are stronger than one. 

Sometimes they lean if left too long.

leaning weighted blackout method

They can also flip if left too long. Microgreens are strong in numbers. 

radish microgreens lifting off weight

As mentioned, some microgreens are thin-stemmed and will not be able to produce the power to lift much less throw the weight off the tray. 

If you use the weighted method for amaranth, mustards, and a few other microgreens you will have to check the trays more carefully if you are using the weighted blackout method.

For that reason, we have other methods that are more appropriate for the more delicate microgreens. We will touch on that method in part 3 of this how to germinate microgreen series.

The End Result Using the Weighted Blackout Method

Germinating microgreens with the weighted blackout methods will produce a healthier and easier-to-grow plant than microgreens grown by any other method.

There are benefits to the other methods I will explore in this how to germinate microgreens series the Buried Method, the Domed Method, and the Hybrid Method for certain varieties and situations, but the Weighted Method is by far the one you should use the most to germinate microgreens. 

Do You Have a Pinterest Microgreen Board?

If not, why not start one! Use this pin as the first or add to your existing boards.

how to germinate microgreens

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 MyViewFromTheWoods.com.

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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  • Thanks for another enlightening article that definitely adds valuable information to our knowledge database. I can see applying this concept for other plant seeds as well.

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