Your Bag of Potting Soil Is Moldy: Is It Still Usable?

Mold is a common problem with potted plants. It can be widespread in potting soil, which can be difficult to clean. But I’m here to tell you that it’s not necessarily the end of the world when your potting soil gets moldy! 

Your potting soil is still usable if moldy, but only if it goes through a strenuous sterilization process. For some, depending on the size of the bag, it may be easier and more time effective just to throw your bag away. However, if you’re on a budget, you can sterilize thoroughly before using. 

In this article, I’ll explain why your moldy potting soil needs to be dealt with before being added to a pot. Additionally, I’ll discuss how potting soil gets moldy and a few ways to avoid mold. Lastly, we’ll discuss easy ways to sterilize your potting soil and support its overall nutrient richness. 

Moldy Potting Soil Should Not Be Used As Is

So, you’ve just got some new flowers ready for planting and you want to ensure they grow to be the best possible versions of themselves. You can imagine yourself doing all the right things: you’re watering them, giving them light, and listening to your mom when she tells you not to overwater or overfeed. 

You’ve come prepared, with a bag of potting soil leftover in the shed from last season. However, once opened, you’re hit with a nasty smell and something that looks a lot like mold. Can you still use it? 

The answer to the question is a tad complicated. It’s a good idea to avoid using moldy potting soil for your plants because it can damage them and stunt their growth. You should not use your moldy potting soil. However, if you’d like to go through a sterilization process, whether because you don’t have the budget for new soil or the time to get a new bag, that changes the answer.

How Potting Soil Gets Moldy

If you’ve got a bag of potting soil that’s been left out in the rain or in a damp environment, mold will likely grow on the surface of the soil. Mold follows moisture, so leaving your potting soil outside over a season in the snow may have caused mold growth.

Additionally, mold, in particular, is spread by spores. If you’ve got your bag of potting soil from a gardening center or even online, there’s a chance it was contaminated with spores before reaching your home. For this reason, you should always be vigilant about mold, whether you’ve had the bag for a year or an hour. 

Identifying Mold in Potting Soil

You can usually do a visual investigation to see if your potting soil is moldy. Mold will appear as spots or streaks on the soil’s surface, especially if it’s dry outside or there’s little ventilation inside your house (such as during the winter). It might also appear as irregularly-shaped patches of discoloration within the soil itself. Inspect the potting soil carefully for signs of mold growth.

The most common types of mold that grow on potting soil are:

  • Penicillium. This type of mold is greenish-gray in color and tends to grow in clumps rather than individual spots. Penicillium can grow on the surface or inside the container where the potting soil was stored.
  • White mold. Mold looks like fuzzy white or yellow patches that may spread across the soil. It might also have a blue-green color if it has been exposed to sunlight for a long period. 

If you see any of these signs on your potting soil, there is likely mold growing in it. However, if unsure, you can take a sample of the suspect area and send it off to an independent lab for analysis using molecular techniques like PCR (polymerase chain reaction). This will tell you whether mold is present or not—and what kind!

How Mold Affects Soil and Plants

Some mold is not dangerous or lethal, but all mold affects your plant growth. Mold can harm your plants and your garden because it spreads quickly. Hence, you must learn how to prevent it from happening in the first place.

When a plant is affected by mold, it grows slowly and has trouble absorbing nutrients. The leaves are covered in black spots and may fall off if they’re too badly infected. If you see any of these symptoms, you should inspect the soil for mold growth and remove it as soon as possible.

Additionally, mold growth may be telling you something. It can mean that the area you have left your bag in is damper than you originally thought, or if there’s mold on your plants, you’re watering too much. 

How To Avoid Molding Potting Soil

If you’re worried about moldy potting soil and want to protect your plants from it, the first thing to do is check the packaging for any indications of mold or moisture when you purchase it. It’s common to find mold present, even if there are no visible signs of mold or moisture. 

To avoid mold when storing, be conscious of what you’re buying. Make sure you’re using the right kind of potting soil. Make sure not to buy potting soil in large bags if you’re not going to use it up (even if it seems like a better deal)—they’re unlikely to be fresh enough by the time you use them again. 

Finally, never store your potting soil in a plastic container—you’ll just trap moisture inside and promote mold growth! Some gardeners will let their soil sit in the sun while in their plastic bins to sterilize. I suggest this only if you know there’s no moisture in the bin or the soil. 

Sterilizing Your Potting Soil for Use

If you do not sterilize the soil before you use it, you risk introducing mold spores into the soil. These spores can take root and produce new mold colonies. This will make it nearly impossible for you to grow healthy plants in your garden because they will be competing with the mold for nutrients, water, and sunlight.

First and foremost, you should consider throwing out the potting soil and starting over with fresh new potting soil if you find mold. This is honestly the easiest way to deal with a mold problem. If you leave it, the fungus will continue to grow and invade your indoor plants. The soil is essentially unusable. 

However, it doesn’t have to be a total loss if you’ve got a budget or don’t want to run back to the store. You can use one of the following methods to sterilize your soil:

Don’t worry about losing nutrients by sterilizing your potting soil. If you want to learn more about the effects of sterilization on the nutrient availability in your potting soil, you can read more about it in my other article: Does Sterilizing Soil Remove the Nutrients?

You Can Use the Microwave To Sterilize Potting Soil

Though microwaving isn’t my favorite method, it’s one of the most accessible (even college kids have access to microwaves–just don’t use the community microwave to sterilize soil!). To do so, place two parts water to one part potting soil in a microwave-safe container. Add something on top to ensure nothing gets onto the microwave. 

Microwave on low for one minute, stirring after every twenty seconds or so.

After you’ve done this, leave your potting soil out to dry in a sunny spot. This method is a combination of the boiling method and oven methods (we talk about both below). The hot water will kill any fungus or mold in the soil, and drying it out ensures no room for new growth.


Molding potting soil is a problem that many gardeners have to deal with at some point in their lives. Luckily, it’s not unsolvable, but you may have to throw your potting soil away and start over if you don’t want to deal with mold.

Alexander Picot

Alexander Picot is the principal creator of, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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