Why is the Soil Moldy?
Several factors can cause mold to grow in potting soil. The most common reasons are overwatering poor drainage, and high humidity. When soil stays constantly damp, it creates the perfect conditions for mold spores to germinate and mold colonies to develop. Mold needs moisture, oxygen, and a food source to thrive – and potting mix provides all of that.
High organic matter content in the soil can also promote mold issues. Soil with a high proportion of organic materials like peat moss and compost provides an abundant food source for mold and bacteria. As organic matter decomposes in the soil, it releases nutrients that mold feeds on. Potting mixes with high organic content therefore tend to mold more easily when overwatered or exposed to excess humidity.
High humidity levels, both indoors and outdoors, can lead to moldy plant soil even if the soil itself is only intermittently damp. When ambient humidity is constantly elevated, it takes less moisture within the soil for mold to develop. Any moisture that remains on the surface of the soil or that is trapped within soil pores for extended periods can be utilized by mold under humid conditions.
Limited air circulation around the plant reduces the rate of evaporation from the soil surface, trapping moisture within the potting mix for longer. This provides the sustained dampness that mold requires to thrive. Ensuring adequate airflow and ventilation around indoor plants can help reduce mold issues even when other conditions favor mold growth.
Allowing excess water to remain in the drainage saucer or tray below the plant pot provides an external moisture source that mold in the soil can feed on, even if the soil itself is not saturated. Removing water from the saucer after each watering or aerating the soil more frequently can help minimize this issue.
How to Remove Mold on Your Soil
The first step is to gently remove as much of the moldy soil from the roots as possible. Do this over a trash bag to contain the mold spores. Discard any obviously moldy potting mix.
Next, flush the remaining soil and roots with a dilute hydrogen peroxide solution or a store-bought plant fungicide. This will kill any remaining mold. Trim away any dead or moldy roots.
Finally, rinse the plant and repot using fresh potting mix in a clean pot.
How to Prevent Mold From Returning
After reporting your plant, take steps to ensure a mold-free environment going forward:
Improve drainage by adding perlite or grit to the new soil mix
Only water when the top 1-2 inches of soil is dry. Avoid leaving water in the saucer or trays
Place the pot on raised feet or pebbles to increase air circulation
Use a humidifier to reduce indoor humidity, especially at night
Replace potting mix every 1-2 years. Mold spores can build up over time in old soil
Is Mold Harmful to Plants?
Mold on soil can be harmful to plants. Mold is a type of fungus that can grow on soil, and some types of mold can produce toxins that can damage plant roots and stunt plant growth.
When mold grows on soil, it can create a number of problems for plants, including:
Root rot: Mold can infect plant roots, causing them to rot and die. This can lead to stunted growth, yellowing of leaves, and even death of the plant.
Nutrient competition: Mold can compete with plants for nutrients in the soil, reducing the amount of nutrients available to the plant. This can lead to nutrient deficiencies and stunted growth.
Toxicity: Some types of mold can produce toxins that can damage plant cells and disrupt normal plant growth.
Reduced soil structure: Mold can break down organic matter in the soil, leading to a decrease in soil structure and fertility. This can make it more difficult for plants to grow and thrive.
However, not all mold on soil is harmful to plants. Some types of mold, such as mycorrhizal fungi, can actually benefit plants by helping them absorb nutrients from the soil.
