How to Get Rid of Gnats in Indoor Plants | Soil, Traps & Bti

Get rid of gnats in indoor plants by letting the top 1–2 inches of soil dry, trapping adults with sticky traps, and treating the soil with Bti.

The little flies circling your houseplants are fungus gnats, and they thrive because the soil stays damp. The fix isn’t a single miracle spray—it’s changing how you water while you knock out both the flying adults and the larvae living in the potting mix. Here’s the step order that works.

Why Fungus Gnats Keep Coming Back

Fungus gnats lay eggs in moist potting soil, and the larvae feed on organic matter and root hairs. As long as the top inch or two of soil stays wet, the cycle continues. University of California’s Integrated Pest Management program notes that these pests thrive in moist conditions and recommends good drainage plus letting the soil surface dry between waterings.

That’s why the most common mistake—watering on a fixed schedule—keeps the problem alive. The soil surface tells you everything: if it’s damp to the touch at one inch down, wait.

The 4-Step Process That Eliminates Gnats

These four actions work together to break the gnat life cycle. Skip one and the infestation lingers.

1. Dry the soil. Let the top 1–2 inches dry completely before watering again. Stick your finger in the soil; if it feels damp, wait another day or two.
2. Water from the bottom. Set the pot in a tray of water for 15 to 30 minutes, then remove the excess. This keeps the surface dry while the roots get what they need. Both University of Minnesota Extension and Wisconsin Horticulture recommend this method.
3. Place yellow sticky traps. Position them near the base of the plant to capture flying adults. Replace them as they fill up.
4. Apply Bti to the soil. Bti (Bacillus thuringiensis subsp. Add a piece of a Mosquito Dunk to your watering can, or use a Bti product like Mosquito Bits or Gnatrol according to the label. Wisconsin Horticulture notes you may need to repeat applications every 5–7 days until the infestation is under control.

If you need a proven product, our tested roundup of the best gnat spray for indoor plants covers what actually works on contact and as a soil drench.

Extra Steps When Gnats Persist

If adults still appear after a couple of weeks, escalate with these measures.

Add a Dry Top Dressing

Wisconsin Horticulture recommends covering the soil with ½ to 1 inch of coarse sand or fine gravel. This dries quickly and makes the surface far less attractive for egg laying.

Repot With Fresh Mix

If the infestation is severe, repot the plant in fresh, sterile potting mix. Discard the old soil—and the old planter if it’s porous and likely holding eggs. Repotting alone won’t fix the problem unless you also change your watering habits, since gnats will reinfest damp soil quickly.

Consider Chemical Control Only for Severe Cases

Wisconsin Horticulture advises using insecticides only for severe infestations and only if the product is labeled for indoor use on houseplants. It also warns against spraying the air, which is ineffective—the larvae live in the soil, not on leaves.

The Bottom Line on DIY Traps

Vinegar-and-soap traps catch some adults, and they’re fine for a quick reduction in numbers. But they don’t touch the larvae, so the infestation continues. The reliable route is drying the soil, trapping adults, and applying Bti. Skip the vinegar trap as your only defense and go straight for the combination that breaks the cycle.

Common mistakes that sabotage control: overwatering, keeping the soil surface constantly damp, relying only on adult traps without treating larvae, and skipping the repeat Bti applications. Fix those four and the gnats will be gone in a couple of weeks.