Controlling fungus gnats with sand, wine, or vinegar

Have you noticed tiny, slow-moving flies hovering around your plant pots? They circle the leaves, multiply near the roots and seem to appear in greater numbers whenever you water. These are fungus gnats, also known as sciarid flies.

Fungus gnats can be a real nuisance, and getting rid of them completely takes some effort. They’re persistent little creatures, but with the right approach you can tackle them yourself.

Fungus gnats look similar to fruit flies, but there are some key differences. Both are small, slow fliers with a dark colour. The big difference is that fruit flies are drawn to fruit, vegetables and other organic waste, while fungus gnats stick to plants.

They’re attracted to damp soil and rotting roots. If your plant is suffering from root rot, fungus gnats will usually be the first to point it out.

An adult fungus gnat only lives for one or two days. That sounds short, but each one lays around 200 eggs, which makes the problem much longer lasting.

• Black flies slowly hovering around soil or terrarium substrate

• They appear mostly after watering

• Tiny larvae live in the soil and feed on rotting matter or roots

With houseplants the issue usually starts with soil that’s kept too wet. The damper the soil, the more eggs get laid.

Separate any infested plants so the problem doesn’t spread.

Hold back on watering for a while so the larvae have less chance to survive.

Sprinkle a thin layer of sand or perlite on top of the soil. This makes it harder for the gnats to reach the soil.

Yellow sticky traps attract the flies and reduce their numbers.

Nematodes are microscopic worms that feed on fungus gnat larvae. Within a few weeks you’ll see a clear difference.

You can buy nematodes online or in garden centres. We’ve had good experience with the ones from Ecostyle.

Some people use clove water, or a small dish of wine or vinegar. These attract adult gnats, but they don’t solve the root cause.

• Water only when the top layer of soil is dry

• Always use fresh compost when repotting

Things work slightly differently in a terrarium. The humidity is high and the soil stays lightly moist, which makes it the perfect environment for gnats.

Too much moisture can cause rotting leaves. Leave the terrarium open for a while to let the humidity drop.

Take out any rotting leaves or moss. These are a food source for larvae.

A thin layer of sand or gravel on top of the soil makes it harder for gnats to lay eggs. Keep in mind that removing this layer later isn’t as easy in a terrarium.

You can safely use nematodes in terrariums too. They’re natural and effective.

Because a terrarium is a closed ecosystem, it takes longer to get an infestation under control. Patience is key.

• Keep plants healthy and always remove dead leaves
• Open the terrarium from time to time to balance the climate

A simple but effective way to cut down fungus gnat numbers is to use yellow sticky traps. Push them into the soil of a houseplant or place them near a terrarium. The bright yellow colour attracts the flies like a magnet. Once they land, they stick firmly to the trap. This mainly targets adult gnats and prevents them from laying new eggs. On their own sticky traps won’t wipe out a full infestation, but they’re a handy way to reduce the nuisance quickly when used alongside other measures.

Yes. The best way is to avoid overwatering. Fungus gnats love damp soil and rotting roots. By only watering when needed, you reduce the risk of gnats as well as root rot.

You can also bring gnats in when repotting with old compost. Used compost, especially if stored in a damp place, may already contain pests. You won’t notice straight away, but within weeks more and more gnats will appear. Always repot with fresh compost, or use compost that’s properly sealed and stored in a cool, dry place.

Absolutely. Sticky traps are made for this. Push them into the soil of your plant. Their bright yellow colour attracts the gnats, and because the surface is so sticky they get caught straight away. This stops them from flying around your home and laying more eggs.