How to Get Rid of Fungus Gnats in Houseplants
Those tiny black flies around your pots are almost always fungus gnats. Why they appear and the step-by-step way to get rid of them for good.
If tiny black flies, 1 to 3 mm long, hover over your pots, they are almost certainly fungus gnats. Unlike fruit flies, which gather around food, fungus gnats lay their eggs in moist potting soil, where the larvae feed on fungi, organic matter and young roots.
Fungus gnats or fruit flies?
- Fungus gnats: black, slender and long-legged. They rise from the soil surface, fly weakly and often walk across the soil.
- Fruit flies: tan and stocky with red eyes. They gather around fruit and kitchen waste.
Adult gnats don't harm plants, but a large larva population chews the roots of seedlings and cuttings. One female lays more than a hundred eggs, so numbers climb fast.
The cause is almost always wet soil
Fungus gnats love soil that never dries out. Frequent watering, poorly draining soil, water left in the saucer and compost-rich mixes are the usual culprits. In autumn plants use less water, so keeping a summer watering schedule leaves the soil wet and the gnats multiply.
Get rid of them in this order
- Step 1, dry the soil: don't water until the top 2 to 3 cm is completely dry. Eggs and larvae struggle to survive in dry soil.
- Step 2, sticky traps: yellow sticky cards placed just above the soil catch the adults before they lay more eggs.
- Step 3, kill the larvae: a BTI (Bacillus thuringiensis israelensis) product mixed into your watering can kills larvae in the soil and is considered safe for people and pets.
- Step 4, top-dress: a 1 to 2 cm layer of coarse sand or grit makes it hard for adults to lay eggs.
- Step 5, water from below: fill the saucer and let the pot soak it up so the surface stays drier. Discard what's left after 30 minutes.
A drench of 3% hydrogen peroxide mixed 1 part to 4 parts water also reduces larvae. Fizzing in the soil is normal; test it on a small area first with delicate plants.
Keep them from coming back
- Water when the top of the soil is dry, not on a fixed schedule.
- Empty saucers right after watering.
- Keep bags of potting mix sealed — gnats lay eggs in open bags too.
- Pick fallen leaves off the soil.
Soil that stays wet also leads to root rot. If leaves turn yellow or stems go soft, check the signs of overwatering too.