The second most common question we receive in our inbox is about mold on microgreens.
There are almost always pictures attached to those emails and a panicked question (at least that's how I read them).
"Is that mold on my microgreens? Do I have to throw them away?"
Luckily, almost always, the answer is "No."
In fact, 85% of the time, the white fuzzy material around the microgreens isn't mold but are root hairs.
Beautiful, white, fuzzy root hairs are growing out of the root radicle and searching for moisture and nutrients.
Sometimes, there's both. Like in the tray of borage seeds we grew shown below. Click the image to expand it for better viewing.
In the close-up photo, you can see the oddly-shaped borage seed. There's a fuzzy mass of white mold growing on both the seed and the soil.
The borage's root radicle is searching for soil and is covered in another white mass made up of little individual root hairs.
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Is That Mold on Microgreens or Microgreen Root Hairs
We want to save you some time and worry right off the bat.
Most of the time, the white fuzzy's you see are just root hairs. Especially if you're growing the more common microgreens like broccoli, kale, cabbage, or radishes.
The smooth, hard-shelled seeds like those rarely become moldy. There can be exceptions, and usually, the problem isn't the seed. We'll discuss these problems more later on in the article.
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The Microgreens Podcast Episode 014
Microgreens Seeds Susceptible To Mold Growth
We most commonly see mold form on seeds with one or more of these characteristics, softer shells, a rough surface, or soaked before planting.
These include microgreen seeds like sunflowers, peas, cilantro, and borage. We also think that the damping-off disease in our Swiss chard and beets is caused by spores attached to their seeds' porous and rough surface.
Is It Called Mold or Fungus?
We didn't know either, but after some research, we found both are technically correct. Though calling it mold is more precise.
Mold is a type of fungus that grows in the form of threads called hyphae. There are many species of molds that show a diverse outward appearance.
For some reason, I like the word fungus better, but mold it is!
What You Think Is Mold On Microgreens Could Be Root Hairs
Of all the photos sent to me asking if there's mold on their microgreens, only about 15% are actually moldy microgreens.
The majority are root hairs, and that is normal for most microgreens.
We understand the freak-out. That blackout period has us all excited to see what's growing and when we take that cover off, we want to see our plants growing beautifully.
And what do we see?
White, everything has white all over it.
Maybe because we know we've added water, put the tray in a dark place, thrown weight on top of the soil trapping in that moisture, that we expect mold.
Or, more likely, we've never seen root hairs because we've always covered our seeds with soil in the garden.
More than likely, what you see are root hairs.
Let's Look At Some Root Hair Examples
Many of these pictures were sent by our readers. We've asked permission to use the photos. You can click on the photo to expand the image for better viewing.
The white filaments you see are root hairs extending off the seed radicle. Notice that all of the fuzzy white is attached to the radicle.
The seeds and seed hulls have nothing fuzzy growing off of them -- only the white thick radicle.
Even though the fuzzy white spread on this tray looks more like mold on microgreens, they are only fine root hairs that, in some cases, are growing over nearby seeds. A bit later in the article, we will show you how to help identify root hairs from mold.
Another Way To Tell They're Root Hairs
Mist them with water.
Don't spray them under a lot of pressure. Only mist the white fuzzy things.
If they are root hairs they will disappear. Well, they don't cling to the radicle, but you won't see them.
Mold, on the other hand, will just mat down. You'll still see it after misting.
Root Hairs Will Disappear in a Couple of Days
The root hairs will disappear in a couple days as the root grows down into the soil.
Mold will not.
If you grow microgreens on mats, the root hairs will be visible for a more extended period. The radicles are never drawn down as much as with soil. We also think that there's more moisture available at the surface when using mats.
So the root hairs remain since they can perform their function in the current location.
So Your White Fuzzy Mass Doesn't Look Anything Like Root Hairs
Then in all probability, it is mold.
Sometimes mold on microgreens is obvious. As shown in the photos below.
In the photo above, mold is blanketing cilantro seeds. To have mold infestation this large, the source of the spores is most likely the soil.
The next photo of mold on microgreens is a common occurrence, especially in the age of CoVid.
It's not literally related to CoVid, but the cause could be associated with the incomplete composting of the soil material before it was bagged and shipped.
All good potting mixes contain some composted materials. Furthermore, as the potting mix ingredients are brought together, they will start to again compost.
Whenever nitrogen sources are mixed with carbon sources, the microbes will consume the nitrogen and produce heat.
If the new potting mix is bagged before the composting has, for lack of a better term, equalized, the mix will produce heat.
Warm & Moist = Greater Chance of Mold on Microgreens
The added warmth of the working potting mix might not be enough to trigger the growth of mold. But when we put microgreens in a blackout period with a solid lid, we have essentially created an oven.
The extra warmth from the composting and the constant moisture due to the cover makes a perfect environment for mold to form.
You will also see a pattern like this if the room temperature is too warm, especially during the summer months in a non-air conditioned home. Or even when using heating pads during the winter.
The microgreens growing on the outside edges, where it was cooler, will grow nicely, but in the dead center of the tray, there will be germination issues and damping-off disease issues.
We are currently working on perfecting the use of heating mats with completely covered microgreens trays.
So during the warm months, or are using heating mats, we suggest not using a solid lid on the top. Instead, use a plant tray with ridges so heat and moisture can escape from the middle of the tray.
Mold on Microgreen Seeds and Stems
Root hairs will always be on the radicle -- the first part of the seed that roots. Yes, sometimes the root hairs of adjacent seeds will grow over other seeds.
But when the fuzzy white stuff is growing on the seed husk itself or on the plant's stem, that is mold.
Root hairs will not form on the stems of microgreens like on the mustard microgreens above. We usually see mold-like this form on microgreens grown under a dome and rarely from those grown using the weighted method.
Mold on microgreen seeds can also spread to the soil, like with the borage seeds below.
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This is a tray of borage we grew (you'll see how we fixed this problem in an upcoming article) for commercial sale. You can see that the potting mix is wet. Often, I'm too liberal with the watering and soaked the tray too much when I planted it.
The moisture escapes from the soil and condenses on the cover, over-wetting the soil surface. With rough husked seeds like borage, mold is often the result.
How to Stop Mold on Microgreens Before It Forms
Mold spores are everywhere; we are always in a spore storm.
Therefore, the best way to prevent mold from forming is to stop creating ideal conditions for them to thrive.
Below Are 5 Ways to Reduce Mold Growth
Let's start stopping mold before we even plant microgreen seeds.
1. Start with the Soil
Soil generally comes in plastic bags, and this is good and bad.
The plastic bags are necessary to keep the soil dry from outside moisture. They're also more durable than paper or plastic-lined paper bags.
Commercial soil bags do have holes in them to help regulate the moisture (but bad for fungus gnats and mold spores) and to stop the bag from bursting if it's dropped or something is dropped on it.
However, plastic does form a barrier. Any moisture in the soil will condense and concentrate when the bag goes from cooler to warmer temperatures.
Always open up a bag of soil as soon as you get it home or it's delivered to you.
Then purchase a storage tote and dump the soil into it some there is more air circulation. We like the Home Depot HDX black totes with the yellow tops. They are sturdy, won't bend or fold, come in many sizes, and will stack.
A 2 cubic foot bag of soil will fit into a 17-gallon HDX storage tote. If you're buying smaller 8-quart bags or even Home Microgreens Potting Mix, at the least open the bag to let out moisture. Both ideally get a small container that allows for some headspace for storage.
Even if it dries out, that is ok. It will re-wet, especially the Home Microgreens potting mix since it's coconut coir based.
2. Sanitize Trays
Microgreen trays should be washed and sanitized after using them. Mold could have formed on the planting media during the last batch and dropped spores on the trays. Or even the soil can contain spores that didn't grow in the previous microgreen batch.
That doesn't mean they won't the next time.
How to Sanitize Trays
There are many ways to do this.
Small trays like the Home Microgreens Planting and Watering Trays can be rinsed and thrown in the dishwasher. Or sanitized using the methods below.
Bleach, Vinegar, Hydrogen Peroxide Soak or Spray
Rinse any remaining soil off the trays and soak or spray the trays with any of these liquids. They will kill or reduce the number of mold spores that may be on the trays.
For bleach, use about a cap full in 16-ounces of water.
When using vinegar, use two tablespoons in 16-ounces of water.
We think it's safe to use the drug store brown bottle of 3% hydrogen peroxide when sanitizing trays. The stabilizer will be destroyed when the spray dries and won't be absorbed by roots later on.
However, only use food-grade hydrogen peroxide on living microgreens or soil. We use this brand. This 12% peroxide is much cheaper, but it will need to be diluted before use. Don't use 12% hydrogen peroxide on anything directly.
3. Sanitizing Seeds
Microgreen seeds are often the source of mold. As mentioned, sunflower, peas, cilantro, borage, and Swiss chard seeds with rough and irregular husk are very susceptible to carrying mold spores.
How to Sanitize Seeds
All varieties of seeds can be sanitized before planting. But it's a pain to spread out small wet seeds, and they usually don't provide much of a mold risk as it's hard for the mold spore to attach to the smooth round seed.
But for the others mentioned above, soaking them for 10 or 15 minutes in a 3% solution of food-grade hydrogen peroxide will help reduce the mold spore count.
Rinse the seeds afterward with fresh water. You can repeat this step a couple of times if you think it's necessary. For instance, if you had mold issues with this seed lot before.
Before we sanitize seeds, we always try to grow a batch first. If the planting doesn't produce mold, we won't sanitize that lot until we have problems.
4. Use Soil with Good Drainage
Soil that contains perlite, aged forest products, volcanic rock, and such is a good thing for drainage. Irregular shaped particles and materials with different porosity will help channel water down and away from the plants.
Using soil media with particles of consistent size, shape, and porosity will retain water.
We don't have to remind you to use trays with holes as planting trays, do we?
I hope not.
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5. Don't Pre-saturate the Soil
At Home Microgreens, we preach to not saturate the soil while sowing the seeds.
We only wet the upper 1/3 of the soil profile when planting microgreen seeds. Seeds don't need the soil wet all the way to the bottom of the tray.
Wetting the upper third of the soil is enough during the germination phase. Soaking the whole soil profile causes saturation and heating of the soil interior.
Creating a perfect place for mold to grow.
6. Don't Over-seed or Clump Seeds Together
Many people have questioned Home Microgreens seeding density. The belief is that we under-seed our trays.
This isn't the case. We test all of our seeding density recommendations and base the final rate on many factors related to growing microgreens in the home.
One of the main reasons I started Home Microgreens was the amount of seed other people recommended. The plants were too crowded, the microgreens were all stems and little leaf, and continuously damp or wet in the tray's interior.
Over-seeding causes disease issues not only as the microgreens grow up with restricted airflow but also during germination.
Mold appears to form in areas where a lot of seeds have come together. Whether that is from bulk over-seeding of the tray or places where seed congregate. Like in depressions in the soil or around the edges of the trays.
Seed to seed contact retains moisture and since most mold is reported to come from the seed, to begin with, clumping them together makes it worst.
Spread seed as evenly as possible to reduce mold issues.
7. Small trays Over Large Trays
Small trays will help improve airflow across the microgreens. Mold is more prevalent in the middle of trays. The outside edges have much better airflow and dry conditions than those growing in the middle.
Instead of growing a large 1020 tray of microgreens, grow a couple 1010's or smaller trays. Increasing the edge area of the microgreen mass reduces the chance of developing mold on microgreens.
We Use These Microgreen Trays
We use Bootstrap Farmer Shallow Heavy-duty trays when we plant 1010 or 1020 trays. We also use their deeper trays to plant peas or other uses.
These trays will never be damaged with normal use.
When we grow microgreens for ourselves we use the Home Microgreens Trays & Soil. We like the amount of microgreens these trays grow and we find we can either stagger plantings or double up trays to fit our microgreen needs.
8. Only Bottom Water
Keeping microgreens dry is very important to reduce the possibility of mold on microgreens.
Watering microgreens over the top can only create issues.
Yes, large microgreen growers do water over the top. But these facilities have way more airflow than your house does and much stronger lights. We will discuss on airflow and light can be used to remedy mold issues.
Bottom watering only when the soil is dry is the best way to reduce mold from spreading or growing after the seeds have germinated.
Soggy soil is a breeding ground for mold.
9. Good Ventilation
Having some airflow across your growing microgreens is a good practice to reduce the possibility of mold growth.
We have these USB fans attached to our growing racks. We don't always use them, but then it's hot, and the humidity is high, we have them running. They don't appear to pull much air through the fan, but they do when you consider they're running 24/7.
10. Reduce Humidity
Lowering the relative humidity in the growing area is a good idea.
Installing a dehumidifier (which also increases airflow) near your microgreens is a good idea if you're growing quite a few microgreens.
You can also cool the room, which will reduce the relative humidity.
11. Remedies for Mold on Microgreens
Once you get mold on microgreens, you might not have to throw in the towel.
We want you to use your own judgment on whether or not a tray can be salvaged. Always error on the side of caution.
Also, wash microgreens before you eat them, not before storage, but before you use them.
Increase the ventilation in the area. Air movement is your friend. Not so much that it looks like the microgreens are in a hurricane, but some leaf or stem movement is ideal.
If you can put them in sunlight or under higher-powered lights or even move them up closer to the lights, then do it.
Light and air are mold's kryptonite.
Treatments for Mold on Microgreens
There are at least three treatments you can try.
We've used two of them, and they will work when the mold issue isn't hopeless.
A tablespoon of 3% solution of food-grade hydrogen peroxide mixed with 16-ounces of water and misted on the mold can kill it. There is no smell.
You may have to mist the mold more than once. Don't worry; the hydrogen peroxide turns into oxygen and water.
You will see the mold start to fizz just like peroxide does on a wound.
We don't recommend using the drug store H2O2 as it has a stabilizer to increase shelf life.
Vinegar - Grocery Store not Cleaning
Dilute 5% vinegar - this kind sold in grocery stores to use in cooking - at a rate of 1 tablespoon in a 16-oz spray bottle.
Spray the dilution onto the mold. Again, it might take more than one misting.
It will smell, well, like vinegar. It may also attract fruit flies.
The white distilled or cider vinegar will work. There are additives in either.
Grapefruit Seed Extract
We tried this grapefruit seed extract on the borage shown in the article, and it worked great. We used a teaspoon of the extract shown to the right or below mixed in 8-ounces of water.
One treatment took care of the mold.
We will post an article on this soon.
So far, we like using the grapefruit seed extract the best. It worked quickly.
Use Your Own Judgement with Mold on Microgreens
Will not take responsibility for any call on whether or not you should trash a tray of microgreens with mold growing on it.
Most of the time, especially right out of the blackout, what you see are root hairs, and they are not a problem.
However, if you're growing one of the rough husked seed microgreens, take a close look. If it is mold, make a determination of how bad the spread is and decide if you want to treat it or trash the tray.
Spray it once with one of the treatments above and move it to sunlight or closer to the lights and see what the tray looks like on the next day.
Suppose the mold isn't beaten back considerably. In that case, I'm dumping it and making sure the trays are sanitized before using them again.
We hope this article helps and clears up a few questions you may have. If not, use the comment section below to share your experiences or use the contact page to reach out to us.