Is It Okay To Use Moldy Potting Soil? – The Funky Substrate Smell Explained

All plant lovers have been there. You’ve had a thriving plant container, and it all seemed to be going great – until you noticed the pale fuzzy growth on the top of the soil.

In another common scenario, you’ve left a bag with good potting soil for a while. Once you opened it to plant something, you noticed the weird, moldy smell.

In both cases, your senses and instincts have informed you well – the soil has gone moldy!

But what are “molds” exactly anyway? Everything you’ve heard about them signals health hazards – from regular moldy food to the invisible aflatoxin. Is mold equally harmful to plants, and could the moldy soil be cleaned, sterilized, and repurposed?

Let’s cut to the chase.

Potting soil in a damp location
Potting soil in a damp location

Can You Use Moldy Potting Soil?

Yes, moldy potting soil can be used on plants. The mold strains in the soil are harmless to plants in the large majority of cases.  You can use and re-use the moldy potting soil without having to worry about your plant’s health. However, you must take care of the mold proliferation causes, which is excessive moisture.

What is Mold?

Mold is a type of fungal growth, usually flat, powdery, or fuzzy. It appears on degraded or decaying organic matter in damp conditions.

What we call “mold” are various species of filamentous fungi that don’t produce mushrooms.

It is estimated that there are over 100,000 types of mold-forming fungi; some well-known examples include the penicillin mold (Penicillium digitatum or P. chrysogenum) and black mold  (Stachybotrys chartarum).

However, these are not molds found in soil and flower pots.

What causes mold in soil

There are several reasons you can get moldy potting soil, but the first one is major, so pay attention.

Excessive moisture (Overwatering)

The appearance of molds is an indicator of high substrate moisture and/or excessive atmospheric humidity.

No matter the species, a mold-forming fungus requires moisture and darkness to thrive. And this is precisely where the confusion about the harm of pot molds arises.

 This is where the confusion happens.

Many people will think that their plants have become diseased or died because of the mold that appeared on the top, but in reality, the soggy conditions caused the plant to start rotting. Other microorganisms might have added to their demise, but almost never was it the fungus growing on top of the soil.

Poor drainage

Heavy soil or the lack of holes at the bottom of the pot can make the substrate waterlogged even with normal watering levels. Well-drained soil is one of the main conditions for healthy plants, so.

Lack of Sunlight

As I suggested priorly, molds prefer dark conditions. No mold will grow in direct sunlight (which is why it is recommended to put your clothes or linen in full sun to get rid of fungal spores). That is why pots in the shade and indoors are at greater risk of mold appearance.

Poor air circulation

Stagnant, humid air is an ideal environment for mold proliferation and usually goes hand in hand (or pot in pot) with high humidity.

Decaying Organic Matter

Some types of substrate contain organic particles prone to mold growth. In most cases, these are semi-decayed wood debris. Different wood-decaying fungi love to grow on these – but won’t touch your plants.

In fact, these molds are likely helping because by degrading the wood, they release the nutrients trapped inside of it and make it more available to your plants.

Dead decomposing leaves are also food for molds but can be less benign. Because it will attract fungi types that can grow on the particular plant that has shed leaves in the first place, there is a greater risk that the molds could spread on damaged parts of the plant.

Can Soil Mold Hurt My Plants?

White fungus growing on the top of potting soil
White fungus growing on the top of potting soil

In most cases, white or whitish pot mold is harmless. As I explained in the case of mold-powered wood debris decay, sometimes these fungi can help your plants get more nutrients from the parts of the substrate that haven’t fully degraded.

Take it from the late Larry Hodgson, a legendary Canadian gardening writer and blogger:

“This kind of fungus is saprophytic: it lives off decomposing particles of organic matter (wood, peat, leaf mold, etc.) found in the potting mix. It isn’t pathogenic or directly harmful to your plants … nor is it harmful to people or pets. In fact, it actually helps your plants, as the organic matter it digests contains minerals that are then freed up. Your plants can therefore absorb and use them for their growth.”

Larry Hodgson

So, most molds are harmless, if not beneficial, and the visible white cover-forming ones are certainly not dangerous.

However, not all fungi in the substrate are like that.

Pathogenic Molds

The soil, even in pots, is an ecosystem teeming with mostly invisible microscopic life. In accordance with that, many types of mold appear in the soil.

Have you heard of fungal plant diseases? Some molds can most definitely attack plant tissue and need to be managed. However, it is important to establish that there is a difference. Not all potting soil molds are bad – but some may be.

Still, pathogenic molds rarely (if ever) form any visible structures on plant soil.

Types of mold found in soil

Let’s see what types of mold may appear in an average, moist plant pot.

White Mold (No Harm / Beneficial)

The most common type of mold you’ll find on the surface of your potting soil is the white fungus or white mold.

This mold-forming fungus is saprophytic, meaning it feeds on decaying organic matter. The wood debris and other poorly degraded plant parts found in the soil are its “food.” Thus, it won’t attack your plant and actually may increase the number of available nutrients in the pot.

However, as with all other molds, the white mold signals that your plant may be overwatered or suffer from poor drainage.

Yellow Mold (No Harm / Beneficial)

The yellow fungus has similar growing habits to the previously described white mold. However, it tends to form smaller structures – the soil may look only speckled with yellow dots instead of being overgrown with mold.

Yellow molds are also saprophytic, meaning they don’t go after the healthy plant tissue – you can relax.

Some subtypes of yellow molds, such as Fuligo septica or dog vomit slime (yes, that’s its official name), may look worrying due to the color intensity and the shapes it can form. Still, it rarely appears in potted plants and is equally harmless.

Rhizoctonia solani (Harmful)

Rhizoctonia solani is one of the most common soil fungal pathogens, able to cause some form of plant disease in nearly all cultivated plants (what a reach). It is transferred through contact and air.

Some of the diseases it causes include Rhizoctonia root rot, stem rot, damping-off in seedlings, and the aerial blight of leaves. Also, it often causes rot in cuttings in high-humidity environments.

Although the pathogenic Rhziochontia species grow on top of the soil, they don’t form any visible mold layer. Unfortunately, the only visible thing will be plant damage, such as stem rotting at the base or wilting.

Should You Try to Remove Mold from Soil?

Removing mold from the top of soil
Removing mold from the top of soil

Since visible mold is not a disease-causing agent, you don’t have to take extreme measures to remove it. In fact, you don’t have to touch it at all.

If you are bothered by it for aesthetic or any other reason, you can just scoop the upper layer that it’s clinging to and discard it by putting it in the compost or directly in the garden (because wasting nutritious soil is a pitty).

Rather than pushing to remove the mold itself, try to take care of the conditions that help it appear (listed above). That way, you’ll prevent further appearance and proliferation of any mold in your pots.

How To Use Soil That Has Grown Mold

The moldy soil that previously had healthy plants can be reused for planting without much preparation. If for any reason, you feel uncomfortable using it again, you are not obliged to.

Still, don’t throw the soil in the garbage if you want to discard it. You can add it to your compost pile or garden. However, never do this if you suspect a plant has died of fungal or bacterial disease, as it will most likely spread from the contaminated soil onto the previously healthy plants.

What to do with moldy potting soil in a bag

If you are worried about the funky, moldy smell from the bagged potting soil, one of the things you can do to decrease it (and automatically reduce mold) is to spread it out over a plastic sheet and leave it exposed to direct sunlight for a few hours.

 This way, you’re reducing the number of fungi in the soil. However, don’t let the soil dry out completely, as too much heat and drying can change the substrate’s physical properties.

Moldy Soil Safety Precautions

When handling moldy soil, you should wear gloves and preferably a mask. This way, you lessen the chance of spreading molds and inhaling the spores. While there is no evidence that inhaling fungal spores from soil mold can negatively impact your health, I’d err on the side of caution and avoid inhaling any fungal spores whatsoever.

Final thoughts

I hope that after reading this article, you realize that mold in potting soil is not something to be worried about, at least when it comes to plant health. Plants love all sorts of organic matter in their soil but if the plant dies, you should not directly re-use the potting immediately

Most molds appearing in potting substrates degrade the decaying organic matter, and have no interest in going for your plants.

Certainly, there are plant diseases caused by other types of mold. However, these pathogenic fungi do not form a moldy or slimy film on the soil’s surface.

The only thing to watch out for is what mold appearance indicates. Too much moisture, humidity, and poor air circulation make the molds thrive but may kill your plant. That way, pot molds can become your filamentous little helpers in discovering poor growing conditions.

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