Fungus Gnats: How to Get Rid of Gnats in Houseplant Soil

If tiny black flies pop out every time you water or bump a pot, you’re almost certainly looking at fungus gnats (family Sciaridae, also called sciarid flies). They aren’t dangerous to you or to most established plants. They are annoying, though, and around fresh cuttings or seedlings they can do real damage.

The good news: fungus gnats are one of the few houseplant pests where changing your watering habits plus a biological control almost always does the job. You rarely need an insecticide. Below: how to confirm what you have, how to measure the problem, and a step-by-step plan to make them stop coming back.

How to identify fungus gnats

Adults are small dark flies, 1/16 to 1/8 inch long, with slender legs and segmented antennae longer than the head (UC IPM). The RHS describes them as dark grayish-brown flies, mostly 3 to 4 mm long, often seen running over the soil surface or flying slowly around plants.

Larvae are slender, whitish-to-clear, legless maggots with a shiny black head, up to about 6 mm long (UC IPM, RHS). They live in the potting mix, mostly near the surface, and feed mainly on decaying organic matter.

Fungus gnats or shore flies?

Shore flies can also hang around pots. UC IPM separates them by the antennae: fungus gnats have antennae longer than the head, while shore flies have short ones. For houseplants it doesn’t change much, since the fix starts the same way: less water sitting at the soil surface.

Why fungus gnats show up

The RHS says fungus gnats thrive in damp potting mix high in organic matter, and Clemson HGIC notes they’re a particular problem in organic-rich potting soil. A constantly wet surface is the perfect place for females to lay eggs.

Common triggers:

• watering “just in case” before the mix has dried,
• water left standing in saucers or cachepots,
• keeping a summer watering schedule into winter when growth slows,
• a pot so large the mix stays wet for a long time,
• an open bag of potting mix stored somewhere damp.

Are fungus gnats harmful?

• Adults don’t damage plants (RHS); they’re a nuisance.
• Larvae in large numbers can damage roots and stunt growth, especially in seedlings and young plants (UC IPM). The RHS notes they can harm seedlings and soft cuttings.
• Established plants are unlikely to be damaged by the larvae (RHS).

So pay the most attention when you’re propagating. If you’re rooting cuttings in soil, for example following our guide on how to propagate pothos, don’t ignore gnats.

Fungus gnats and propagation

Cuttings and seedlings are more vulnerable because they have few roots and sit in mix you keep on the damp side. A few practical rules:

• Fresh, pasteurized mix for every new batch; don’t reuse old soil.
• Small containers with drainage holes, so the mix dries faster.
• A yellow sticky card next to every propagation tray as an early warning.
• Water propagation as an alternative where the plant tolerates it. Pothos roots well in water, and gnats have no soil to lay eggs in.
• Preventive Bti or nematodes if you’ve had gnats recently. Follow the product instructions.

How big is the problem?

UC IPM recommends three simple monitoring methods:

1. Visual checks for adults on the plant and soil surface.
2. Yellow sticky traps. Stick them just above the soil. The weekly count is a good gauge.
3. Raw potato chunks. Set a slice cut-side down on the soil, lift it after a few days, and check the underside for larvae.

Use the same methods after treatment to see whether it’s working.

Step 1: Fix your watering (the most important step)

UC IPM calls cultural control the most important part of managing fungus gnats:

• Let the soil surface dry between waterings (UC IPM, RHS). Clemson HGIC notes that dry conditions kill the larvae.
• Empty saucers and cachepots, and fix anything that leaks or drips (UC IPM).
• Water when the plant needs it, not on a schedule. Check with a finger or a wooden skewer. More in our guide on how to water houseplants.
• Use pasteurized potting mix, as UC IPM recommends; Clemson HGIC recommends commercial potting soil. Keep opened bags sealed.

For plants that need consistently moist soil, lean more on biological control and sticky traps.

Step 2: Sticky traps for adults

Yellow sticky traps reduce adult numbers (RHS). On their own they won’t solve the problem, because larvae in the soil keep developing. The RHS notes traps can also catch non-target insects, which rarely matters indoors.

Step 3: Biological control for larvae

There are several biological options for larvae in the potting mix (UC IPM, RHS):

Organism	How it works	Note
Bacillus thuringiensis subsp. israelensis (Bti)	bacterium that targets larvae	watered into the soil (Clemson HGIC)
Steinernema feltiae nematodes	parasitize larvae in the soil	UC IPM notes season-long control
Hypoaspis / Stratiolaelaps predatory mites	hunt in the potting mix	listed by UC IPM and RHS
Macrocheles robustulus	predatory mite	listed by RHS
Atheta coriaria rove beetle	predatory beetle	listed by RHS; more of a greenhouse option

Bti products and nematodes are sold by biocontrol suppliers and garden centers. Use every product and live organism exactly as the label or supplier directs, including storage and application temperature.

Step 4: Repot into fresh mix

For heavy infestations, the RHS suggests replacing old potting mix with fresh. It’s especially worth doing when the mix has broken down, compacted, and stays wet for days. See how to repot houseplants, and for aroids pick a chunky, airy blend like the one in our aroid soil mix guide.

What about insecticides?

UC IPM says insecticides are rarely needed for home gardeners dealing with fungus gnats. Watering changes and biological control usually do it. If you still reach for a pesticide, use only one registered in your state and labeled for houseplants, and take the rate and repeat interval from the label.

Home remedies from the internet

You’ll find plenty of tips online: cinnamon on the soil, hydrogen peroxide drenches, garlic, apple cider vinegar traps. The extension and RHS sources this guide is based on don’t back them, so we don’t recommend them. If you want to experiment, try it on one less valuable plant first. The proven, cheap steps are different: a dry soil surface, empty saucers, sticky traps, and Bti.

Treatment timeline

When	What to do
Day 0	empty saucers, hang yellow traps, place potato slices
Week 1	water only after the surface dries; apply Bti or nematodes if needed
Weekly	count gnats on traps, check potatoes
Following weeks	continue until counts drop to zero

Because gnats develop in the soil, you’ll keep seeing some adults for a while after you start. Watch the trend: falling trap counts mean it’s working.

Common mistakes

• Relying on sticky traps alone. They catch adults while larvae keep maturing.
• Watering on a schedule. In winter or after moving up a pot size, the mix dries more slowly.
• Water hiding in a cachepot. A decorative pot without drainage is a common hidden cause.
• Throwing the plant out. With fungus gnats that’s almost never necessary.
• Impatience. Adults keep emerging for a while after treatment.

Prevention

• Water only after the top layer of mix has dried.
• Don’t leave water in saucers.
• Buy good bagged potting mix and keep it sealed.
• Check the soil surface on new plants and quarantine them.
• Keep a yellow sticky card near propagation trays.