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Why Mold Grows on Potting Soil and How To Fix It

Recently, I had a reader let me know they had mold growing on their potted Persian lime tree’s soil. After getting mold quite a few times myself (on my microgreens, basil, and compost bin), I decided to put this issue to bed and add the tips that worked for me into a blog post. So, why does mold grow on potting soil, and how we can get rid of it?

Mold grows on potting soil because of consistent and excess moisture. In nature, the sun and wind help dry the top of the soil out between waterings, but this can be difficult for indoor plants. Most soils will have mold spores, so by simply keeping your potting soil damp or wet, mold is encouraged to grow.

So, while mold growing on potting soil can be a frustrating event, what are some more details as to why it happens, and how can we get rid of it?

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Why Mold Grows on Potting Soil

moldy potting soil on my growing tray

Mold is a vital part of nature as its job is to decompose matter in the soil. Almost all soils will have mold spores of some kind. The only exception to this is if you’re using sterilized soil. By providing a consistently wet growing medium, like potting soil, mold will likely grow and will be hard to kill.

Like mosquitos, it can be hard to think that mold has a place in the circle of life (seriously, what purpose do mosquitos really have?). Still, it can be helpful to know that mold is a perfectly normal and helpful occurrence in nature.

Mold typically occurs on wet soil and is designed to decompose organic soil matter, further breaking down the nutrients into smaller pieces. These smaller nutrients can be used by beneficial microbes and better absorbed by the roots of plants. In short, mold helps provide nutrients for your potted plants!

“By decomposing organic matter, molds play a big part in material biodegradation, enabling decay and rot necessary in all ecosystems.”

Science Daily

However, even though mold in the wild is good, it doesn’t mean that it should be allowed to grow indoors. This can pose risks to your plant (and those in the home).

Is Moldy Soil Bad for Plants?

The type of white mold found on potting soil typically doesn’t harm the plant, but it can compete with and overpower smaller seedlings. While not as common, some types of mold can introduce disease to your plants. Because of this, if you see mold growing on your potting soil, it’s best to take care of it sooner rather than later.

On the other hand, if you notice that mold is growing on the plant itself (see image below), then it will likely start consuming the plant. In this case, you can make a homemade spray with baking soda or vinegar and spray the leaves. The baking soda should dry out the mold, and the vinegar’s acid should kill it.

Additionally, you can try other ingredients as a spray for leaf mold. If the mold isn’t lessening, consider pruning the infected leaves to limit the rate that it spreads.

a moldy leaf

How To Get Rid of Moldy Potting Soil

You can get rid of mold by removing its food, water, or oxygen. Most commonly, by allowing your potting soil to completely dry between waterings, the mold will die off. You can also remove the oxygen by covering the mold with new soil, or remove its food by repotting the plant. However, this should be a last resort.

We know that we can extinguish a fire by simply taking away one of the three things it needs (heat, oxygen, and fuel), but can we do the same for mold?

Yes, we can!

Mold needs three things to grow: food, water, and oxygen. It often grows on the surface of the potting soil because there’s usually not enough oxygen below the surface (unless there are air pockets in the soil). By keeping the top of the soil moist, we’re creating the prime conditions for the mold spores to grow and take over.

By removing either food, water, or oxygen, mold will cease to grow. Let’s take a look at all the ways we can do this.

1. Allow the Soil To Dry Between Waterings

The easiest way to get rid of mold is to remove the water it needs to grow. We can do this by allowing the potting soil to completely dry between waterings.

Initially, it can seem scary to allow the plant’s soil to dry. After all, we want to kill the mold—not the plant!

However, watering only when the soil is dry is actually a best practice for most plants.

Watering only when the soil is dry is good for plants because it mimics rainfall—an occasional heavy watering.

Many gardeners feel the need to keep their plant’s soil moist (I once did this too), but providing constant, shallow watering is actually not good for plants as it encourages shallow root growth and can expose the plant to mold or root rot.

On the other hand, less frequent, deep watering encourages the roots to grow deeper to access the water further down the soil. So, not only do the deep roots better anchor the plant, but they will help it become more drought-tolerant. Meaning, you can water your plant less and you might not need someone to water it when you’re out of town!

A good rule to follow when watering most plants is to only water when the top 2-4 inches of soil dries. Of course, this is a general rule, as some plants will require different soil conditions, but it surprisingly applies to almost every plant out there.

If you’re not a fan of letting the soil dry out completely, there are a couple of other ways that you can water the plant without getting the surface soil wet and promoting mold:

  • Use a bottom-watering pot (available in 6″-12″ pot sizes, see image below)
  • Partially fill a long-neck water bottle and push it into the soil lightly

While the self-watering (or bottom-watering) pot is the easiest and most effective choice to reduce mold, you can also use a water bottle or other device to deliver the water underneath the soil (see the image below).

watering our plants with a water bottle
You can try using a long-necked water bottle to avoid watering the top of the soil

Without water on the surface of the potting soil, the mold should die off fairly quickly.

However, drying out the top of the soil can be difficult if there’s not enough drainage in the first place.

2. Improve Drainage

If you find that you’re watering less often, and the soil still isn’t drying out, the issue here is likely drainage.

You can improve your plant’s drainage by:

  • Drilling more holes in the bottom of the pot
  • Placing a thin layer of rocks or gravel on the bottom to prevent soil from blocking the drainage holes
  • Repotting the plant with new soil (more on this later)

Increasing the drainage of your pot can be tricky if you’ve already planted your plant. To reduce transplant shock, it’s best not to disturb the soil unless you have to. Because of this, use the above suggestions on drainage only if you’re potting your plant for the first time or you have to repot it.

3. Increase Air Circulation

Another way to dry out the surface of your plant’s potting soil, and kill the mold, is by increasing the amount of airflow around it. Often, there’s little to no airflow indoors, which means the soil stays wet for longer than it should. This is an opportune time for mold to grow.

There are a couple of ways you can increase the airflow and combat mold:

  • Place the plant outdoors for the wind (and sun) to dry the soil
  • Point a fan on a low-speed setting towards the plant’s soil

Note that while the fan is a good option (I was able to get rid of mold in my compost bin this way), it can also blow mold spores around your home. For this reason, consider placing your potted plant and fan in the garage, or ideally—outside.

Another option regarding airflow is to suffocate the mold by placing 1-2 inches of fresh potting soil on top of the moldy soil (I use Espoma’s). Without oxygen, the mold will die off. While mold can grow through the new soil, this method has worked for me previously.

4. Increase Sunlight

Even though mold can grow in sunlight, it prefers darker environments. I don’t know about you, but I rarely see mold growing in broad daylight, if ever.

A big reason why mold doesn’t grow in sunlight is that the sun effectively heats and dries out the top of the soil. Because of this, if you place your potted plant in the sun for a few days, and let the top of the soil dry a bit, it should kill off the mold.

5. Repot the Plant With New Soil

Repotting the plant with new soil will isolate the moldy soil from the plant, so you shouldn’t have to worry about it regrowing in the new soil. However, as mentioned, there are mold spores in every environment (including the air), so if the new soil remains too wet, there’s a chance mold will grow again.

While repotting the plant is a great way to resolve mold and stagnant water (it recently saved my Kaffir lime tree from root rot), this should be used as a last resort since the plant can go into transplant shock.

I’d suggest exhausting the other options on this list first, unless your potting soil is smelling like a swamp or is completely collapsed. If it does smell like a swamp, then root rot (a type of fungus) is likely taking hold, and repotting is the best choice.

You can also try sterilizing your potting soil before using it, but this isn’t a guaranteed way to eliminate mold spores. While it may work in the short term, mold spores can be reintroduced rather easily.

If you are interested in sterilizing your potting soil before planting, you can bake the soil on an oven-safe sheet or container for at least 30 minutes on 250ºF. Again, I’m not 100% confident this will kill mold spores. However, it did kill the fungal gnats in the potting soil!

More Tips To Prevent Potting Soil From Growing New Mold

When I had mold on my potting soil, I tried a lot of things and at one point, I just wanted to throw in the towel. However, I found that increasing the airflow and drying out the surface of the soil was the solution that worked best for me.

Ideally, performing all of these options at once is the best choice in fighting the mold: watering only when dry, improving drainage, and increasing airflow and sunlight. If this doesn’t work, repotting can be a last-ditch effort.

If you’ve tried the above options and nothing is working for you, or if you’d like some creative solutions, I have a few more ideas for you. Here are some other ways you can kill and prevent potting soil from growing new mold.

  • Vinegar
  • Baking soda
  • Cinnamon

I’ve used a homemade vinegar spray to kill mold before, but it typically doesn’t stay dead for long. For my moldy compost bin, I ended up using the vinegar spray to initially kill the mold and then placed a fan pointed at the top of the soil. This worked in just one day and the mold hasn’t been back since. However, if you’re using vinegar on your potted plant’s soil, know that too much vinegar can kill the plants, so be careful when trying this as a solution.

Baking soda is great at drying materials out (I sometimes use it as a dry shampoo), and sprinkling a little bit on the mold can dry it out and kill it. Again, if the soil is constantly wet, baking soda won’t do much good. You should address the wetness first as it’s most likely the primary cause of the mold.

While I haven’t tried cinnamon personally, I’ve heard anecdotal evidence that it works well as an anti-fungal. You can simply sprinkle some over the spots of mold and wait a couple of days to see if it dies down.

If you’d like more information about drying out waterlogged or moldy soil, check out my recent post “The 7 Quickest Ways to Dry Out Soil“.