How to Get Rid of Fungus Gnats in Houseplants

Fungus gnats are the tiny dark flies that drift up from houseplant soil when you water or brush past a pot. Adults are mostly a nuisance, but their larvae live in the soil and, in large numbers, can damage fine roots, especially on seedlings and young cuttings. The good news: they’re almost always a sign of soil that stays wet too long, and they can be controlled without harsh chemicals if you attack both the adults and the larvae.

Quick answer: Let the top 1–2 inches of soil dry between waterings, empty saucers, and put yellow sticky traps at soil level to catch adults. For larvae, water in a product containing Bacillus thuringiensis subsp. israelensis (BTI), repeating weekly for a few weeks. Keep it up for about a month, since you need to outlast several generations. Prevent a comeback with well-draining mix, pots with drainage holes and a quarantine period for new plants.

Confirm It’s Fungus Gnats

Feature	Fungus gnats	Fruit flies
Where you see them	Around pots and soil	Around fruit, trash, drains
Flight	Weak, erratic, near the soil	Stronger, more direct
Body	Slender, long legs, mosquito-like	Rounder, often tan with red eyes
Larvae	Tiny clear worms with black heads in the top soil	In fermenting food
Main fix	Dry the soil and treat larvae	Remove food sources

A quick test: slice a raw potato, press it cut-side down on the soil for a few days, then lift it. Larvae often gather underneath.

Understand the Life Cycle

Adults lay eggs in the top layer of moist soil. Larvae feed on fungi, organic matter and sometimes roots, then pupate and emerge as adults. In warm, damp conditions the cycle can complete in about three to four weeks. Because eggs, larvae, pupae and adults are all present at once, a single treatment rarely works. Plan to keep at it for about a month.

Step 1: Let the Soil Dry Out

This is the most important step. Larvae need moist soil, so let the top 1–2 inches dry completely before watering again. Water thoroughly when you do, let it drain, and empty the saucer or cachepot.

For plants that don’t tolerate drying, such as ferns and calatheas, bottom water: set the pot in a tray of water for 15–30 minutes so it soaks up from below, then drain. The surface stays drier while the roots get moisture.

Step 2: Trap the Adults

Yellow sticky traps placed at soil level catch adults before they lay more eggs, and show how bad the problem is. Replace traps when they fill up. Traps alone won’t solve the infestation, but a steady drop in catches over a few weeks tells you the other steps are working. Keep traps away from pets and out of reach of curious hands.

Step 3: Treat the Larvae

Treatment	How it works	Notes
BTI drench	Bacterium toxic to gnat larvae, not to plants, pets or people when used as labeled	Most reliable; repeat weekly for about 3–4 weeks
Beneficial nematodes (Steinernema feltiae)	Microscopic worms that attack larvae	Effective; must be used fresh per instructions
Predatory mites (Stratiolaelaps scimitus)	Soil mites that eat larvae	Good for large collections and greenhouses
Hydrogen peroxide drench (diluted)	Kills larvae on contact	Mixed evidence; can harm delicate roots if too strong
Top-dressing with sand or grit	Dry surface discourages egg laying	Helps prevention; hides soil moisture
Cinnamon or other home remedies	Various	Little evidence of real effect

BTI is sold as liquid concentrates and as granules or “dunks” meant for standing water; for houseplants, many people soak a piece of a dunk in a watering can overnight and water with it. Follow the product label.

Step 4: Top-Dress to Discourage Egg Laying

A ¼–½ inch layer of coarse sand, fine gravel or horticultural grit on top of the soil keeps the surface dry, which makes pots less attractive to egg-laying adults. The downside is that you can’t see the soil anymore, so check moisture with a finger or a wooden skewer before watering.

Step 5: Isolate or Repot Stubborn Plants

If gnats trace back to one plant, move it away from the others while you treat it. For a severe, recurring infestation, repot into fresh, well-draining mix, shaking off as much old soil as the roots allow. Bag the old soil and discard it, or leave it to dry fully outdoors, rather than tossing it back into indoor pots.

Why Gnats Keep Coming Back

Cause	Fix
Watering on a schedule regardless of soil	Check the soil first every time
Pots without drainage or water left in saucers	Use drainage holes; empty saucers
Pots too large for the plant	Size up only 1–2 in when repotting
Dense, peat-heavy mix that stays wet	Add perlite, bark or pumice
Bagged mix stored open and damp	Store potting mix sealed and dry
New plants brought in with gnats	Quarantine new plants for a couple of weeks
Fall and winter slowdown	Water less as growth slows; see our fall and winter care guide

Plants coming indoors after a summer outside are a common source. Our guide on when to bring houseplants indoors covers how to check them first.

Prevention Checklist

• Water only when the top inch or two of soil is dry.
• Use pots with drainage holes and empty saucers after watering.
• Choose an airy mix with perlite or bark for gnat-prone plants.
• Remove fallen leaves from the soil surface.
• Keep yellow traps in plant areas as an early warning.
• Quarantine new plants and plants coming in from outdoors.

FAQ

Are fungus gnats harmful to plants?

Adults are harmless nuisances. Large numbers of larvae can damage fine roots, which mostly matters for seedlings, cuttings and stressed plants.

How long does it take to get rid of fungus gnats?

Usually about three to four weeks of consistent drying, trapping and larval treatment, since you need to outlast several overlapping generations.

Are fungus gnats dangerous to people or pets?

No. They don’t bite or spread disease to people or pets. They’re an annoyance and a sign of overly wet soil.

Does hydrogen peroxide get rid of fungus gnats?

A diluted drench can kill larvae it contacts, but results are mixed and too strong a solution can harm roots. BTI is more targeted and more consistent.