4 Reasons Why Mold Develops on Grow Bags (Get Rid of It!)

If you’ve been gardening for a long time now, you’ve probably come across several planter alternatives as I have. One of the most commonly used is fabric grow bags. Honestly, are not my favorite as they can develop some unpleasant-looking mold. However, they popped up because I was doing something wrong. What? Well, keep reading!

Mold can develop on fabric grow bags due to 1) the presence of organic matter, 2) high humidity, 3) lack of ventilation, and 4) insufficient sunlight exposure. Harmless, beneficial, and disease-carrying spores can grow on fabric bags but they are not easy to differentiate from each other. Hence, it’s best to remove mold and prevent reemergence to avoid plant death.

Wondering whether or not you can get rid of mold without using harsh chemicals? The answer is, you definitely can. In fact, I have 2 natural options for easily getting rid of mold patches on grow bags. Stay until the end to find out how!

1. Organic Matter

The organic matter in the growing medium and the material the fabric is made of can result in mold development on grow bags. Such things serve as food for mold.

Like most living things on the planet, mold needs sustenance for it to grow and thrive. Unfortunately for us, these pesky fungi aren’t that picky when it comes to food.

More specifically, all the plants and other organic matter filled into grow bags provide a lot of food for them. Mold can even feed on materials like fabric, carpets, and upholstery.

So mold growth on fabric grow bags isn’t that surprising if we really think about it. For the most part, grow bags are primarily made of different plant fibers.

Common materials used to make fabric grow bags include fibers of bamboo, cotton, jute, soy, and vegetables. These may be mixed with recycled plastics and other organic materials like feathers.

In other words, under ideal conditions for mold growth, they can become visible on your fabric grow bags in as little as 1–2 days!

Additionally, using soil amendments that contain mycorrhizae (like the one below from Amazon) can lead to the formation of fuzzy white root-like fungi on grow bags. These, however, are not like the mold we normally picture—but more on this a bit later!

2. High Humidity

Consistently high humidity, roughly above 70%, can foster mold development in fabric grow bags.

One of the other common reasons why mold develops on fabric grow bags is the same reason why they form on clothes: damp and humid conditions.

Growing Peppers in Grow Bags - Pros and Cons - Pepper Geek

This is also more likely to happen when you overwater the plants you cultivate in textile pots and they don’t properly dry up throughout the day.

Even though grow bags are primarily designed for improved airflow—which will theoretically speed up drainage and drying—high humidity hinders this.

As a result, water will stay in the fabric walls of your grow bag instead of dissipating. Spores are likely to land on and stick to the soaking wet grow pot leading to mold development.

I’ve had acquaintance experience this during the monsoon season, where there’s heavy rainfall all day long for several days at a time. The grow bags they had outside in their garden ended up molding over just one day after all that.

3. Lack of Ventilation

Lack of ventilation can lead to mold growth on grow bags. Using fabric containers in enclosed spaces with restricted airflow will increase the likelihood of this happening.

Don’t place your filled fabric grow bags too close to each other. Otherwise, it’ll take more time for your pots to thoroughly dry out. Especially avoid them being in contact with each other as this will reduce drastically the airflow and also facilitate the spread of the spores.

You’re even more likely to experience such a problem if you use fabric plant containers for indoor gardening—especially when you placed them together in tight clusters.

Not having cool fresh air move through enclosed rooms doesn’t allow for the natural removal of excess moisture on your cloth containers. This is more so the case when it’s already quite humid at home or in your greenhouse.

Humid air can’t really pick up any more moisture compared to dry air. To make it easier to understand, this is why you’re more likely to sweat a lot when the humidity is high.

The mold will likely start forming from the thickest parts of the grow bag, right at the seams, and where it receives the least airflow, at the base.

4. Insufficient Sunlight Exposure

Grow bags will develop mold if it does not get enough exposure to sunlight because it thrives in dark and damp places.

If your filled grow bags are overcrowded, then mold growth is to be expected considering how much of them are not receiving enough sun exposure.

This is connected to the previous reasons I’ve already discussed. Less sunlight basically equates to higher humidity and even less airflow.

Many of my friends have actually experienced this during the first winter that they’ve tried using grow bags—regardless of whether or not they leave it outdoors or keep it inside.

Dark Conditions Lead to Moldy Grow Bags
Dark Conditions Lead to Moldy Grow Bags

One friend of mine who lives up north kept her grow pots in 2 tight rows right next to the wall by the back of her house facing her north-facing garden. The problem is, during the colder months of the year, it gets even shadier in that area.

Want to learn more about this? Check out our article on sunlight in north-facing gardens!

More importantly, since the pots were against the wall, her plants received little to no sunlight at all. Unfortunately, she only noticed that something was wrong only when the bottom half of all her cloth containers were moldy.

Plants grown indoors using cloth pots aren’t safe from mold either—especially because they already receive very little natural sunlight as it is.

What Molds Can You Find on Used Fabric Grow Bags?

Fuzzy patches of the mold with various colors, like white, green, and black, can develop on fabric grow pots. These may include common indoor molds that are relatively harmless for plants like Aspergillus. Disease-carrying spores like Botrytis, however, can greatly damage and even kill plants.

Molds on grow bags come in a wide array of vibrant and dark colors:

Hence, they may or may not be noticeable depending on the color of the grow bag. Other signs of molding also include sliminess and a musty smell. So if you notice any of that, you likely have a moldy grow bag in your hands.

Common molds that develop on grow bags and plants include

  1. Cladosporium
  2. Penicillium
  3. Aspergillus
  4. Chaetomium
  5. Alternaria
  6. Stachybotrys
  7. Sclerospora
  8. Botrytis
  9. Capnodium
  10. Fumago
  11. Scorias
Mycorrhizal Fungi vs Molds on Grow Bags
Mycorrhizal Fungi vs Molds on Grow Bags

In reality, though, mold isn’t always harmful to plants. Numerous different molds and fungi, such as Cladosporium and mycorrhizae, are even beneficial for plant development.

Mycorrhizal fungi improve growth and increase yield for many plants such as corn and tomatoes. Their presence also protects plants from pests and disease. Plus, they also boost soil quality!

In comparison, although most Cladosporium molds are harmful due to viruses and bacteria, some make plants grow better and more resistant to stresses like weeds and temperature.

But the big problem with mold identification is that most gardeners—even experienced ones—can’t always correctly identify which mold develops on their grow bags.

So rather than gamble on whether your plant can survive despite the presence of mold, I would recommend getting rid of it instead. Even the Centers for Disease Control and Prevention advises this!

4 Ways to Effectively Remove Mold in Grow Bags

Colonies of mold can be effectively removed from fabric grow bags using 1) lemon juice and salt, 2) vinegar and baking soda, 3) diluted hydrogen peroxide, and 4) diluted bleach solution.

If you catch mold development early on, you don’t really have to worry about your grow bag all that much. With proper cleaning and disinfection, it’ll be good as new!

Pro Tip: Wear glasses, masks, and rubber gloves when removing mold from grow bags. They will protect you from getting irritated by the spores.

When possible, it’s also best to do all of this outdoors, in a well-ventilated space to prevent the further spread of spores in the house.

1. Lemon Juice and Salt (7 Easy Steps!)

Let’s start the list with the safest and most available cleaning materials you can use to kill off the mold on your grow bag—lemon juice and salt.

Remove mold from grow bags using lemon and salt paste:

  1. Let the moldy pot dry out under the sun.
  2. Scrub off the mold patches with a dry stiff brush.
  3. Mix together 2 parts lemon juice with 1 part salt.
  4. Cover mold stains with lemon juice and salt paste.
  5. Leave the pot under the sun again for at least 1 day to bleach off the moldy patches.
  6. Water the plant and bag thoroughly with clean water to rinse off the paste.
  7. Place the bag in a bright and windy spot to let it dry.

This is perfect for you if you want an all-natural method of mold removal that has little to no chance of negatively affecting your precious plants and the environment at large!

2. Vinegar and Baking Soda (6 Easy Steps!)

Another organic and eco-friendly option you can try using to clean and fix moldy grow bags is using a paste made of vinegar and baking soda.

Remove mold from grow bags using vinegar and baking soda paste:

  1. Let the pot dry out directly under the sun so the mold spores dry out.
  2. Rub off the mold patches with a dry stiff brush or abrasive sponge.
  3. Mix together 2 parts white vinegar with 1 part baking soda.
  4. Scrub mold stains with vinegar and baking soda paste.
  5. Rinse it off with clean water to remove all the grime.
  6. Dry the pot outdoors again, with direct sunlight for about 1 day.
DEEP CLEANING FOR FABRIC POTS — remove mould, algae and nutrient deposits

If there is no longer any plant in the grow bag, you can also soak it in a bucket filled with water, vinegar, and baking soda for at least one day.

Others have also added this paste into the wash when they want to clean their grow bags properly.

3. Diluted Hydrogen Peroxide (5 Easy Steps!)

Now, if you want to use something stronger without the risk of ruining or killing your plants, go with a diluted hydrogen peroxide spray instead made with something like this one from Amazon.

Hydrogen peroxide can kill various microorganisms including mold spores while also promoting dense root growth for plants. Its effects can be seen immediately after application—as quickly as just 5 minutes!

Remove mold from grow bags using hydrogen peroxide spray:

  1. Dry out the mold spores on the bag under the sun.
  2. Brush off the moldy spots using a dry stiff brush or abrasive sponge.
  3. Dilute 1 part 3% hydrogen peroxide with at least 9 parts water.
  4. Spray the diluted hydrogen peroxide solution directly on the leftover mold stains.
  5. Repeat this until the mold disappears completely.
How to Kill Mold in a Plant Pot : Indoor Planting

You can also do this regularly as a preventive measure against mold growth. What one friend of mine likes most with this spray, however, is that there’s no need for rinsing!

4. Diluted Bleach Solution (6 Easy Steps!)

Out of all the solutions here, this is the only one you can’t do with the plant still inside the moldy grow bag. You’ll have to transplant it to another clean pot before you use bleach on the grow bag—even if it’s already diluted!

Also, this is where protective gear becomes especially important since bleach is a super strong cleaning agent.

Remove mold from grow bags using bleach solution:

  1. Keep the moldy bag outdoors to fully dry the spores under direct sunlight.
  2. Remove the dried spores using a dry stiff brush or abrasive sponge.
  3. Dilute about 0.25 to 1.50 parts regular bleach with about 16 parts warm water.
  4. Scrub leftover mold stains with a sponge that’s been dipped in the bleach solution.
  5. Wash the bag with clean water and gentle laundry detergent afterward.
  6. Hang it out to properly dry the grow bag under the sun before using it again.

If the mold colony on your grow bag is significant, you can opt to soak it in the solution instead before letting it run for at least one cycle in the wash.

Just make sure that it’s okay to put it in the laundry machine. Otherwise, it’s best to have it sent to the dry cleaners.

9 Tricks to Prevent Molding Grow Bags

The formation of mold colonies on fabric grow bags for plants can be prevented by:

  1. Reducing the watering frequency and amount
  2. Cleaning off damp debris and dirt
  3. Washing after each growing season
  4. Elevating using pot stands
  5. Monitoring humidity using a hygrometer
  6. Spacing out each grow bag
  7. Placing a dehumidifier or fan nearby
  8. Airing out the room when used indoors
  9. Exposing all sides to sunlight or grow lights

Grow bags can get pretty nasty after several years of continuous or repeated use. But if you keep these tips in mind, you can make the most out of your eco-friendly fabric grow bags.

Also, since mold spores exist all around us, it’s better to prevent their spread and development than wait until it becomes a problem before doing anything.

This is because although mold won’t necessarily kill your plants, it is—first and foremost—a sign of unfavorable growing conditions for your plants like overwatering.


What does salt buildup look like on grow bags?

Over time, salt and mineral buildup is likely to become obvious on dark-colored grow bags after regular watering as the nutrients in the soil get flushed down and seep into the fabric. Though it’s sometimes mistaken for white mold, salt buildup forms a crusty white layer that can be easily scraped off.

How long do fabric grow bags last?

A fabric grow bag should last for at least 2–3 of continued use. But with great care and proper storage, a cloth container can last for more than 10 years. Ultimately, however, this will depend on what materials were used to create the bag. Grow bags made entirely from biodegradable materials are likely to only last a few years.

Summary of Why Mold Develops on Grow Bags

It’s normal for grow bags to develop mold colonies because of the presence of organic matter they can feed on, humidity levels consistently over 70%, lack of ventilation between and around the pots, and lack of sunlight—or full-spectrum grow light—exposure.

Most ordinary home gardeners can’t readily tell apart beneficial and harmful molds that form on grow bags. One factor that makes this complicated is their variety in color. Another factor is the wide variety of species under each genus, each having specific effects on plants.

Organic methods of mold removal on grow bags include the use of lemon juice and salt, as well as vinegar and baking soda. These can generally be used without removing the plants from the bag. Cleaning agents like hydrogen peroxide and bleach can also be diluted to get rid of mold patches on these cloth containers.


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