There's mould on my soil, what is it and is it harmful to my plants? By Lisa Price

What is the mould growing on my soil?

Essentially soil or potting mix is alive, there are millions of microorganisms that make up healthy soil. It’s likely the white mould or fuzz you’re seeing on the surface of the potting mix is a saprophytic fungus; these are among groups of microorganisms that break down or mineralize organic materials. 

Why does the potting mix get mouldy? 

"A person breathes in between 10,000 and 20,000 litres of air every day, and every breath contains between 1 and 10 spores," - Viviane Després of Johannes Gutenberg University Mainz. Spores are a natural part of our day and they are a normal and healthy part of potting mix. Under the right conditions - damp, cool, dark - they can grow into fungi and this is what the white fuzz or cobweb appearance on the surface of the soil is. 

If your potting mix has a dense structure, it is likely to hold on to more water and have poor aeration, this paired with more frequent watering makes the perfect breeding ground for mould spores to thrive. Opting to use a chunky, well-draining potting mix is beneficial for root health and less likely to get mouldy.

Is this mould harmful to my plant?

The mould itself is unlikely to be harmful to the plant but it is indicative of a potting mix that is remaining wet for too long and this could be detrimental to the roots of your plant and therefore the life expectancy of the plant.

How do I get rid of this mould? 

Sterilise a tool (a spoon or scoop) to scrape and remove the mould, dispose of it in the bin then sterilise your tool again. Aerate the potting mix with a skewer or small stick. If you determine that the potting mix isn’t well-draining enough you can either add perlite and pine bark to it or replace with a fresh mix. Position your plant in the appropriate light and where the air flow is good.

What are the mushrooms in my soil and how do I get rid of them?

The mushrooms are the fruiting bodies of the spores; they’re usually harmless to the plant but could be toxic to a human or pet. As a precaution you can carefully remove them (ideally before they sporulate,this is before the caps expose their gills) but it’s likely you’ll disturb the spores. I’d recommend wearing gloves to scrape off the mushroom and the top inch or so of potting mix. Improve the airflow around the plant and make sure the potting mix is well aerated and fast draining. 

References

1. Fröhlich-Nowoisky, Janine & Pickersgill, Daniel & Despres, Viviane & Pöschl, Ulrich. (2009). High diversity of fungi in air particulate matter. Proceedings of the National Academy of Sciences of the United States of America. 106. 12814-9. 10.1073/pnas.0811003106.