Mold In Potting Soil Bags – Causes & Solutions

Pile of tied-up, used potting soil bags with various printed labels, indicating garden use and recycling.

Potting soil bags come sealed and in different sizes & quantities. These useful bags contain nutrient-rich soil and potting mix that is favorable to different plant species. Some homeowners choose to grow vegetables directly out of the potting bags.

Others prefer removing the content and using it on pots and containers. Potting soils, however, can be infested with mold growing and other stubborn fungi if proper care and maintenance are not taken.

Here, we will discuss the causes and prevention mechanisms you can use to get rid of molds in a potting soil bag.

Mold In Bag Of Potting Soil – Causes

Because potting soils come in bags that are completely sealed, there tends to be too much moisture inside the bags to favor mold growth. If you choose to transfer the soil into a container or pot, you should notice the growth of molds on the container/pot after a few days.

Below is an insight into understanding the common causes of mold in potting soil bags.

1. Moisture

Moisture is just water. However, this water is in very small particles/quantities that are not visible to the human eye at room temperature. When the potting soil is being fed into the bags, a small percentage of air gets trapped inside. Because the air is trapped inside the bag plus the soil, the growth of molds begins immediately.

When the atmospheric pressure is higher than the pressure inside the bag, the air is condensed and transformed into vapor. Vapor/moisture is a key ingredient that supports mold growth. Usually, the mold is fuzzy, and it assumes a whitish appearance when you open the potting bag.

These molds are, however, not harmful to the plant if it is in small amounts.

2. Lack Of Sunlight In The Potting Soil Bags

When the potting soils leave the company, their final stop is either the supermarket shelves or the gardening retail stores. In these locations, the bags are not left out in the sun. And without the sun, molds thrive fast. Molds solemnly depend on cool and wet environments to thrive well.

With a lack of enough sunlight to reach the potting soil bags, the delivery will contain molds when opened.

3. Availability Of Nutrients

The potting soil acts as enough nutrients for molds. With all the favoring factors in place, the molds will thrive easily. Potting soils contain organic matter like peat moss and pine bark. Organic matter plus moisture inside the bag increases the growth of molds significantly.

Common Types Of Molds Found In Potting Soil Bags

There are different species of mold found in potting soils. These species are not harmful to your plants if they are in small amounts. However, leaving the mold to continue thriving can be catastrophic.

The mold will compete with your plants for nutrients and result in poor plant health.

Here are the different kinds of molds found in potting soil bags.

The saprophytic mold is the most common type of mold found in potting soils. These mold species are not harmful to the plant.

Saprophytic mold groups:

  • Penicillium sp.
  • Aspergillus sp.
  • Mucor
  • Trichoderma sp.

Note: Five Important facts that you should know about potting soil bags

After purchasing and using the potting soil, you should tightly seal the remaining soil in the bag and store it in dry and low-humid areas. Keeping your open soil bag allows air and moisture to get inside and increase mold growth. Also, make sure that before purchasing any potting soil, you need to choose only the premium quality product from the recommended gardening store. Premium quality products guarantee efficient results and enhanced plant growth.

When the potting soil is not used for an extended period, it will go bad. The peat that is present in all potting soil bags will start to decompose, and molds can be easily visible on the soil.

Ensure you check the expiry date of the potting soil on the bag. Good soil should not exceed six months if unused. Purchase only soil bags that are between 3-5 months old.

Note: Used potting soils should be replaced every one or two years

Solutions For Mold In Potting Soil Bags

Once you notice that there is a bit of mold in the potting soil bags, need not panic. There are different effective ways you can use to get rid of the molds and allow your plants to grow well.

Here are three effective ways to get rid of mold.

1. Add Natural Anti-fungal And Anti-bacterial Products

When you open the potting soil bag and notice the presence of molds, you should transfer it into a sizeable container and add natural anti-fungal products. Natural anti-fungal products can include baking soda, cinnamon, and apple cider vinegar. Adding small amounts of these natural anti-fungal products not only kills the bacteria but is safe for plant roots.

Other industrial products can leave your potting soil with small amounts of poison that often harm your plant roots.

2. Take It Outside To Receive Enough Sunlight

When the potting soil bag contains fungus, another natural remedy that does not have a negative impact is to take it outside. When the sun is out, and the temperatures are high, taking your moldy potting soil outside helps kill the mold instantly. Molds thrive in saturated moisture environments and die in low-humid environments. In winter, when sunlight is not enough, you can use artificial lighting sources to kill all the present mold.

3. Apply The Recommended Fungicides In Small Amounts

Quality fungicides do not harm plants. Inferior fungicides, on the other hand, can add toxins to the soil that will later harm your plants. If you only use the recommended fungicides to treat your moldy potting soil, it will be good for use any day. When applying the fungicides, note you should do it outside where there is ample air circulation. Adequate fungicides prevent the growth of new molds.

4. Aerate The Potting Soil And Remove Any Physically Present Mold

This is a much simpler and straightforward method. The first thing you ought to do when you notice mold in the potting soil bag is to remove the visible mold spores physically. After you remove the visible mold, aerate the remaining soil in a dry and non-humid environment (preferably outside).

Do not let the potting soil get into any contact with water or moisture. Additionally, be careful not to breathe any mold spores in because some of them can be mildly hazardous.

A person wearing a plaid shirt and gloves is potting soil into a container, with a green garden in the background.

How To Prevent Mold From Growing Back After Using The Soil

Once you have dealt with the potting soil mold, you can then proceed to use the soil and grow your vegetables/plants. However, you can still notice mold growth after a while.

So, what can you do to make sure that there is no more mold?

Do Not Overwater The Soil

Overwatering an already moldy potting soil will cause new mold formation. The presence of water and nutrients can cause mold to form again, even if you had applied an anti-fungal or anti-bacterial. The top two inches of soil should be completely dry before watering.

Wait until the top quarter inch has dried on smaller containers (one gallon or smaller) before starting a new watering schedule. You can check the moisture content with your finger. Additionally, never allow pots to sit in water-filled saucers for longer than five minutes.

Remove any extra moisture by draining.

Increase Ventilation And Reduce Humidity

You should place moldy potting soil in a well-ventilated space where there are low humidity levels. If the area where you stored it was dry, it may have previously been exposed to sunlight, which would have caused any moisture to rise to the top of the bag. Without airflow or a chance to dry out, mold would have started to grow.

However, keep in mind that it will be mold that was already in the bag, but it got a head start on all the other microorganisms. A moldy plant stands at a high risk of new mold formation as opposed to dry potting soil. In case the humidity levels remain high, you can use an artificial fan to push humid air away from your plant.

The microorganisms will work things out and restore balance if the affected soil is left in an area that is open to the air and has some moisture.

Sterilizing The Potting Soil

Although they say that there is no need to sterilize the potting soil when it comes out of the bag, you should at least sterilize yours and avoid mold growth. There are different methods of safely sterilizing your potting soil.

Below are a few effective ways:


Steaming is an old-fashioned sterilizing method that is easy and effective. By doing this, you always get rid of mold that might have started to grow. Using a pressure cooker, you should steam the potting soil to a maximum of 82°C (179°F). Without a pressure cooker, you can still use a sterilizing cup and achieve successful outcomes.

Note: Before starting the sterilization process, make sure to use a guide or a DIY step-by-step process.

Baking In An Oven

Baking is another simple solution to sterilizing your potting soil. There are different ways to bake your gardening soil. However, the simplest of them all is by using an oven.

Procedure: Transfer your potting soil into a container. Cover the container using a thin-layered foil. Bake the soil at 82°C (179°F) for about 30 minutes. Allow the soil to cool until room temperature before use.

Using A Microwave

Even without an oven, you can still sterilize your potting soil with a microwave. Microwaves work effectively to kill minute microorganisms and mold spores. Procedure: Add a few pounds of potting soil on polythene/container each time you want to sterilize it.

Other sterilization methods include:

  • Chemical treatments and
  • Solarization
  • Boiling water technique

Final Thoughts

Proper care and maintenance are important factors to consider if you want your plants to grow healthy. The above insight has looked at easy and efficient ways to deal with stubborn mold that come in potting soils. Follow the guidelines stated for healthier growth.

Happy Planting!

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