Soil Fungus: What Is It and What to Do About It?

Soil fungus is not only unsightly, but it can also harm your plants. What do you do about it?

There are fungi in your potting soil, even if you can’t see them. With over 70,000 distinct species worldwide, it’s a safe chance that your containers include a variety of species. However, if you notice anything weird growing on the soil surface, you should look into it further. That fungus might be fine, or it might not.

Soil fungus is an important aspect of healthy soil because it stores carbon, helps to bind soil particles together, increases root uptake, and breaks down organic material to release nutrients for plants.

The catch is that not all soil fungi are good. When certain species are allowed to reproduce, they can cause harm to the plant and its roots.

Understanding the prevalent species and knowing what to do when discovering evidence of fungus development on the soil are essential for sustaining healthy plants.

Soil Fungus Basics

Soil fungi are categorized according to how they obtain energy. Decomposers, mutualists, and pathogens are the three broad functional groupings. Beneficial fungi such as decomposers and mutualists play important roles in the soil. Pathogenic fungi are extremely dangerous and can hurt or even kill plants if left untreated and allowed to proliferate.

  • Decomposers break down organic matter in the soil to produce biomass, carbon dioxide, and tiny molecules such as amino acids and organic acids. They also aid in decomposing the carbon ring structures seen in some common contaminants.
  • Mutualist fungi colonize plant roots and are also known as mycorrhizal fungi. They create symbiotic connections with plants and invade their roots, assisting in water and nutrient intake. They also aid in the solubilization of soil phosphorus.
  • Pathogens infiltrate roots and other organisms, causing production to decline or death. In some circumstances, these fungi aid in disease control by infecting host organisms. However, these pathogenic organisms are frequently harmful to plants.
A soil infested with a white mold.

What Causes Soil Fungus?

Soil fungus is naturally occurring and is in no way an indicator of a dirty home. The air is filled with fungal spores. When these airborne spores land on a surface and have the right conditions, they start reproducing and growing. Unfortunately, the potting soils in our container plants are a prime location for fungal growth, and indoor conditions are a catalyst.

However, some things accelerate the growth of soil fungus.

Too Much Moisture 

It’s no surprise that overwatering plants considerably contributes to fungal growth. Most people associate damp conditions with mold or an overgrowth of other microorganisms. Overwatering is especially dangerous in regards to root rot; it only takes one or two events to cause the fungus behind it to flourish. Always give the soil time to dry out before watering.

Lack of Oxygen

Lack of oxygen typically occurs when the potting soil’s pore space is filled with water. This excess water forces the oxygen out of the soil. If your soil doesn’t have enough oxygen, it becomes a breeding ground for pathogenic fungi and bacteria. This means the beneficial organisms can’t compete with the harmful kinds, and the pathogens take over. 

Contaminated Growing Media

Sometimes, there isn’t anything in your home or how you are caring for your plants that cause fungal growth. While most potting soils are sterilized during manufacturing, there is always the chance you have contaminated growing media in your container. It may have come from the manufacturer, or you may be storing it improperly. 

Stems of plant surrounded by white mold.

Is Your Plant’s Soil Fungus Beneficial, Benign, or Harmful?

Fungal growth tends to make many people uneasy. When they see it, their first thought is they need to get rid of it. But this isn’t always the case. The first step is to take a little bit of time and determine what kind of mold is on the soil and if it needs to be remedied. 

Soil fungus tends to show up in one of two ways. It may look like stringy white strands or fuzzy areas on the soil surface. The strands are called hyphae and are made up of long threads of microscopic fungi cells. This type of fungi is almost always beneficial. 

When you see fuzzy mold (a type of fungus), it’s more cause for concern.  

  • White mold is usually a saprophytic fungus that poses little to no harm to your plants. 
Molds developed around seedlings of a plant.
  • Grey mold is due to Botrytis spores and needs to be treated quickly to keep it from transferring to the foliage.
A bunch of mold cultured on a dish plate.
  • Black, sooty-looking mold is related to an infestation of sap-sucking insects that excrete honeydew. The sugars in the honeydew feed fungal growth on the soil and leaves.
Black molds covering the leaves of a plant.

Treating Soil Fungus

Chemical Fungicides

Chemical treatments are the first line of defense for some people. These products stop the overproduction of pathogens and protect the plant’s roots from attack. If you opt to go this direction, purchase a product specifically targeted to the type of soil fungus you’re dealing with, and it is safe for your plant. Always follow label instructions carefully.

Natural Remedies

Looking past synthetic and organic chemical fungicide, there are some natural ways to treat soil fungus. Sometimes these are good options to try before pulling out the big guns. These remedies are considerably cheaper than buying chemicals, and they pose little threat to you when you’re applying them. They also have a minimal environmental impact.

Baking Soda

Sodium bicarbonate, or ordinary household baking soda, is touted as a safe, effective treatment for many different fungal species. Research shows it appears to diminish the effects of common fungal problems when applied to soils or plants. Baking soda is an alkaline product raising the soil pH, preventing or slowing fungal growth but not eradicating it completely.

The most effective treatment calls for four teaspoons per gallon of water, mixed thoroughly. Once mixed, saturate the soil with the solution. You can also sprinkle the baking soda directly on the soil where you see fungal growth but do so sparingly.

Keep in mind, though, that it will kill the beneficial fungi too. 

Hydrogen Peroxide

Hydrogen peroxide effectively treats soil fungus, mold, and insect pests. It works because it increases the oxygen concentration of the soil. As mentioned above, when soils lack oxygen, the harmful pathogenic fungi outcompete the beneficial species and proliferate. To treat your soil, use one teaspoon per cup of water and then drench the growing substrate.

A growing mold contaminating the soil.

“Treatments” to Avoid


Vinegar can sometimes kill garden fungus, but it can also damage your plants. Vinegar is often used as an organic weed killer, especially when mixed with soap and salt. Once inside, it acts as a desiccant by breaking down cell membranes, which pulls water out of the foliage to kill plants.

Boiling Water

Yes, boiling water will kill soil fungus if the water is hotter than 140-160°F. If you want to sterilize potting soil that doesn’t have plants growing in it, this method is acceptable. However, water this hot also severely damages the roots if you pour it on the soil of a potted plant. 

Preventing Harmful Soil Fungus

Remove Diseased Leaves From Plants

While we aren’t talking about fungal diseases that plague the foliage, this is still an essential aspect of preventing problems in the soil. If your plant has foliage severely affected by fungal issues, it’s best to remove the leaves from the plant before the spores transfer to the soil. Once you remove diseased leaves, dispose of them away from your plants. 

Keep Soil Surface Clean

Debris that accumulates on the soil surface helps create a perfect environment for fungal growth, especially if fallen leaves were infected with fungal diseases. Regularly pick up any debris that falls to the soil surface and dispose of it away from your plants. Cleaning up the debris also prevents insect problems that can spread fungi to healthy plants.

Repotting healthy soils in a  large container.

Carefully Control Soil Moisture

Overwatering is one of the leading culprits of fungal growth. Unless your plant needs the soil consistently moist, allow the top of the soil to dry out before watering. Make sure every container has drainage holes in the bottom, and never let your plant sit directly in standing water if there is a saucer under the pot. 

Improve Growing Conditions 

  • Make sure all of your plants are getting enough light. Strong sun rays can hinder the growth of some species. Natural sunlight is always best, but supplemented light helps too.
  • Keep the humidity lower around your plants, especially if you live in a humid climate. A dehumidifier can help bring humidity levels lower.  
  • Arrange plants, so there is adequate air circulation between them and through the foliage. You can always use a small fan to create a gentle breeze.

Don’t Overfertilize

The nutrients in fertilizers not only feed your plants but also provide a readily available food source for any fungi living in the soil. If you are struggling with soil fungus, it is best to stop applying fertilizer until the problem is under control. Then you can resume a regular fertilizer regime, making sure not to overfertilize plants.

Clean Equipment

It doesn’t matter if you’re working with healthy plants or those showing signs of problems. It’s imperative to clean and sterilize any equipment you use on your plants to keep from unintentionally transferring fungal spores from one plant to another. This is one of the cases where “an ounce of prevention is worth a pound of cure.”

  • After pruning plants or taking stem cuttings, clean your cutting tools with rubbing alcohol to disinfect the cutting blade.
  • When reusing containers, scrub them with soapy water using a soft-bristled brush to remove residual potting soil. Rinse well and then soak them for fifteen minutes in a solution of one-part bleach and nine parts water. Take the containers out of the bleach solution, do not rinse them, and allow them to air-dry. 

Sterilize Potting Soil

If you choose to reuse potting soil over and over again, it’s recommended you periodically sterilize the growing media to get rid of harmful pathogens and fungal spores. Sterilizing it is relatively straightforward and can be done using the oven in your kitchen. The soil is heated to a minimum threshold where the harmful organisms cannot survive.

Natural Antifungals

Look no further than your trusty spice rack or kitchen cabinets if you’re looking for a natural product to minimize fungal growth. To naturally inhibit growth, sprinkle a bit of baking soda, cinnamon, or apple cider vinegar on the surface of the potting soil to inhibit growth. When used sparingly, they don’t pose risks to your plants.

Inspect and Quarantine New Plants

Each time you bring a new plant into your home, whether you bought it yourself or it was gifted from a friend, inspect it thoroughly for signs of pests or fungal diseases. If any problems are spotted, treat them immediately. Many people also quarantine new plants for a couple of weeks before adding them to the collection just to be safe.

Carley Miller
Carley Miller is a horticultural expert at Bustling Nest. She previously owned a landscaping business for 25 years and worked at a local garden center for 10 years.
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