Background information
Plastic and wood in potting soil: how manufacturers defend themselves
by Darina Schweizer
How fungus gnats get into soil
28.11.2023
Translation: Elicia Payne
First plastic parts and now fungus gnats: I haven’t had much luck with my potting soil recently. But at least I’ve found out where and how the critters get into soil. And how difficult it is to fix.
I actually only wanted to plant indoor herbs this spring. Instead, along with the potting soil, I brought pieces of plastic and wood into the house:
Apparently, that wasn’t enough either because I recently discovered that gnats had also smuggled themselves into the soil. I immediately started asking myself questions: how did I not notice the cheeky critters getting in? And how could I have prevented it from happening?
The common reason: organic soil and too much water
Ricoter and Migros have an answer to this. «The infestation often occurs at the point of sale, especially when soils are stored next to plants or other soils,» explains Beat Sutter, Managing Director of Ricoter Erdaufbereitung AG. Organic soils with an organic base fertiliser are particularly popular. The reason: their smell is more intense and is more attractive to the insects than conventionally fertilised soils. This is also confirmed by Alexandra Heckmann, Lead CFM Houseplants and Wildlife at Do it + Garden Migros:
The scent produced when organic fertiliser is used attracts fungus gnats. The gnats then lay their eggs through the perforations in the bag.
According to Regine Hofmann, Marketing Manager at COMPO Jardin AG, just one fungus gnat is enough for a swarm. This is because the insect lays up to 200 eggs during the one to two days it lives. The real outbreak usually occurs when it’s in the hands of the end customer. Especially if they water the plant too much.
«Too much moisture is the main reason for fungus gnats,» says Alexandra Heckmann from Migros. The risk of overwatering is particularly high with peat-free soils because their surface can soon look dry. Hobby gardeners reach for the watering can, even though there’s plenty of water in the deeper layers. If the weather’s mild, as it has been this year, the fungus gnat party goes into overdrive.
Solutions needed
According to the manufacturers, it’s difficult to prevent this. «The eggs in the soil are very resistant. We’re still looking for good solutions,» says Andrea Neuenschwander, Head of Marketing at Hauert HBG Dünger AG.
Migros launched an experiment over ten years ago and sterilised houseplant soil. Immediately after production, the results looked good. The soil was free of fungus gnats. But it wasn’t all that simple. As the natural enemies of the pests were killed off in the sterilised soil, the fungus gnats were able to multiply enormously. There was a real invasion. Consequently, Migros decided not to sterilise houseplant soil in future.
Reaction instead of prevention
Basically, there’s no real solution in sight to keep fungus gnats away from plant soil. Summing up the issue, Alexandra Heckmann from Migros says, «There’s no soil that fungus gnats don’t like.» Still, the research hasn’t finished yet –it’s just´ beginning. Beat Sutter from Ricoter says:
We’re currently working with the Zurich University of Applied Sciences on a research project to effectively research fungus gnat control.
In the meantime, your only options are to use fertilised soil low in humus, water in moderation and/or rapidly deal with the gnats at home. My colleague Maike Jensen will soon be telling you how to get rid of the pests in our Houseplants for Dummies series. Until then, keep your earthy fingers crossed that the gnats will keep away – or at least won’t multiply in masses and invade your home.
What experiences have you had with fungus gnats? Which houseplant soil can you recommend? Let me know in the comments.
Header image: Shutterstock/MarcOliver_Artworks
Darina Schweizer
Senior Editor
Darina.Schweizer@digitecgalaxus.chI like anything that has four legs or roots. The books I enjoy let me peer into the abyss of the human psyche. Unlike those wretched mountains that are forever blocking the view – especially of the sea. Lighthouses are a great place for getting some fresh air too, you know?
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