Getting rid of fungus gnats requires breaking their life cycle by letting the top inch of potting soil dry out, catching adults with yellow sticky traps, and treating the soil with Bti to kill larvae.
For the full breakdown, see our best Fungus Gnat Trap guide.
Fungus gnats appear as tiny, dark flies hovering around houseplant soil, but the real problem lives below the surface: the larvae. The adults are a nuisance, but the larvae feed on roots and can damage plants in large numbers. The good news is that you don’t need harsh chemicals. A few targeted steps that focus on drying the soil and treating the larvae will clear most infestations within a couple of weeks.
What Causes Fungus Gnats?
Fungus gnats thrive in consistently moist soil that contains decaying organic matter. Overwatering is the single most common cause. Potted plants with poor drainage or heavy, peat-rich mixes that stay wet for days create the perfect nursery. Adults lay eggs in the damp top layer, and larvae feed on fungi and root hairs below. The UC IPM program notes that avoiding overwatering and providing good drainage are the first lines of prevention.
A complete approach covers adults in the air and larvae in the soil at the same time. These steps are listed in the order that works best.
Let the soil dry out. Stop top-watering until the surface layer turns dry to the touch. For most houseplants, letting the top inch or two of soil dry between waterings is enough. This directly removes the moist habitat larvae need to survive.
Set out yellow sticky traps. Place bright yellow sticky cards at soil level or near the base of plant stems. Cornell University notes these traps are an easy and effective way to catch adult fungus gnats. Fewer adults mean fewer eggs laid in the soil. Replace traps as they fill up.
Treat the soil with Bti. Bacillus thuringiensis israelensis is a naturally occurring bacterium that targets fungus gnat larvae without harming the plant. Look for a labeled Bti product and follow the instructions on the package. Because adults lay eggs continuously, a single treatment usually is not enough. Plan on two or three applications spaced according to the product’s guidance.
Add a soil barrier. Once the top layer is dry, cover the soil surface with a thin layer of sand, grit, or decorative stones. This layer makes it physically harder for adult gnats to reach the soil to lay eggs and also helps keep the surface drier.
Repot if the infestation is severe. When the steps above do not clear the problem, remove the plant from its pot, discard the old potting mix, and rinse the roots carefully. Repot into fresh, well-draining potting mix. Dispose of the old soil in the trash rather than your compost pile.
Common Fungus Gnat Treatments Compared
| Treatment | What It Targets | How Fast It Works |
|---|---|---|
| Let soil dry out | Larvae habitat | Days to a week |
| Yellow sticky traps | Adult gnats | Immediate catch |
| Bti soil drench | Larvae | Several days per application |
| Sand or gravel barrier | Egg laying | Preventive, ongoing |
| Repot with fresh mix | Whole infestation | Immediate for that pot |
What Not to Waste Time On
Vinegar traps placed near the pot will catch some adult gnats, but they do nothing against the larvae in the soil, so the infestation continues. Bti products are more predictable and better documented for larval control. And the biggest mistake of all: watering the plant again before the soil has had a chance to dry. Unless the soil dries properly, the gnats will keep coming back.
References & Sources
- UC IPM. “Fungus Gnats.” Comprehensive guide on causes, prevention, and control methods including soil drying, barriers, and Bti.
- Cornell Cooperative Extension. “A Spooky Houseplant Pest: Fungus Gnats.” Notes on yellow sticky trap effectiveness for adult catch.
- Scotts Miracle-Gro.