Fungus Gnats on Houseplants: Identify, Control & Prevent

Fungus gnats are about 2–4 mm long, black, and look similar to fruit flies. You’ll often spot them flying around the soil or sitting on the rim of the pot. The real danger, however, comes from their larvae living in the moist substrate and nibbling on the roots.

• Visible signs: Small black flies around the plant
• Damage symptoms: Wilting leaves, stunted growth, rotting roots
• Typical environment: Moist soil, dense substrates without drainage

As soon as you notice an infestation, you should act immediately. The goal is to interrupt reproduction and take away the moist environment that larvae need to survive.

• Reduce watering: Let the top layer of soil dry out well. Larvae need moisture to survive.
• Use yellow sticky traps: These attract the adult fungus gnats and visibly reduce the population.
• Repot severely infested plants: Remove all old soil completely, carefully rinse the roots, and use fresh substrate.

For long-term plant health, you should use eco-friendly methods. These effectively target larvae without harming your plant.

• Nematodes (Steinernema feltiae): These microscopic roundworms actively seek out larvae in the soil and destroy them.
• Bacillus thuringiensis israelensis (BTI): A natural bacterium that reliably kills larvae in the substrate. Apply in liquid form via watering.
• Fine clay granules, quartz sand, or vermiculite: A layer on the soil surface prevents gnats from laying eggs.

Some methods may provide short-term relief but can harm your plant in the long run or even worsen the infestation.

• No chemical sprays: Many chemical agents harm beneficial microorganisms or are unsuitable for indoor plants.
• No plastic bags over pots: This causes oxygen deficiency and promotes mold!
• No cinnamon or vinegar: These home remedies are too aggressive and disrupt the soil environment.

The best strategy against fungus gnats is a healthy plant environment. From the start, pay attention to the following:

• Well-draining substrate: Use soil with high air and water capacity, for example, with perlite or coconut fibers.
• Drainage in the pot: A layer of clay granules or broken clay shards prevents waterlogging.
• Adjust watering habits: Better to water thoroughly and less often rather than keeping the soil constantly moist.
• Check new plants: Inspect the soil before introducing them to avoid bringing in eggs.

How quickly do nematodes work against fungus gnats?
Nematodes start working within a few days after application. Usually, you’ll see a noticeable reduction in the gnat population after 1–2 weeks.

Why do I keep getting fungus gnats?
This is often due to overly moist soil or bringing in new plants that are already infested. Always check new plants and adjust your watering habits to make the environment less attractive for larvae.

Can I get rid of fungus gnats permanently?
Yes! With a combination of preventive measures (like a layer of sand or clay granules) and beneficial organisms such as nematodes, you can keep your plants permanently protected.

Does coffee grounds help against fungus gnats?
No! Coffee grounds can actually make the problem worse because they keep the substrate moist and nutrient-rich — perfect for larvae. They can also promote mold. So it’s better not to use them.

Does cinnamon or vinegar help against fungus gnats?
Cinnamon or vinegar are often recommended as home remedies, but they rarely deliver lasting results. Cinnamon can disturb the soil balance, and vinegar may repel adults but doesn’t kill larvae. Plus, they can stress your plant. It’s better to rely on proven natural solutions like nematodes or BTI.

How often should I apply nematodes?
Usually, one application is enough. For severe or recurring infestations, you can repeat the treatment after about 4 weeks to target any newly hatched larvae.

Do yellow sticky traps work against larvae?
No. Sticky traps only catch the flying adult fungus gnats. To control the larvae, you need nematodes or BTI (Bacillus thuringiensis israelensis).

Fungus gnat infestations are annoying but not a reason to panic. With the right mix of drier soil, sticky traps, and natural allies like nematodes, you’ll get the problem under control quickly. At the same time, healthy substrates and proper watering strengthen your plant’s natural defenses.