How to Get Rid of Fungus Gnats in Houseplants

Small, dark flies drifting around your houseplants are almost always fungus gnats. They are annoying but mostly harmless to people, and in many cases they are harmless to healthy mature plants too. They are, however, a sign that your soil stays wet, and their larvae can damage seedlings and stressed plants. The good news is that you can get rid of them, as long as you tackle the whole life cycle.

Know the enemy

Fungus gnats are small, weak fliers, about an eighth of an inch long. You will see them near soil, windows, and lamps, or crawling on the surface of the pot. Adults do not harm plants. The larvae are the problem. They are tiny, translucent, with a dark head, and they live in the top few inches of moist soil, where they feed on fungi, organic matter, and sometimes fine roots.

The life cycle is short. Eggs hatch in a few days, larvae feed for roughly two weeks, then pupate and emerge as adults, and the whole cycle takes about three to four weeks at room temperature. Adults lay many eggs in moist soil. That is why one or two flies can turn into a crowd quickly, and why a single treatment rarely fixes it.

Why you have them

Fungus gnats are drawn to constantly damp soil rich in organic matter. Common triggers include overwatering, potting mix that stays wet, a pot with poor drainage, and bringing home a new plant that already carries eggs. See overwatering versus underwatering.

Step 1: Let the soil dry out

This is the most effective single step. Larvae need moisture to survive. Let the top inch or two of soil dry out between waterings, and for severe cases let it dry deeper, as far as the plant can tolerate. Use a metal chopstick or your finger to check, and see how often to water houseplants. Do not do this to plants that cannot handle drying out, like ferns, and instead use the other steps.

Step 2: Use yellow sticky traps

Yellow sticky traps catch adult flies, which breaks the cycle by stopping them from laying eggs. Place them flat on the soil surface or upright in the pot. Traps also help you judge progress. If you catch fewer flies each week, you are winning. Traps alone will not solve the problem, because they do not touch the larvae.

Step 3: Water from the bottom

Bottom watering keeps the top layer of soil dry, which discourages egg laying and larvae. Set the pot in a few inches of water for about twenty minutes, let the soil soak up what it needs, then drain. This works well with drying the surface between waterings.

Step 4: Treat the larvae

If drying out is not enough, you have several options.

• Bti (Bacillus thuringiensis israelensis). This is a naturally occurring bacterium that kills fungus gnat larvae and is sold as mosquito dunks or in gnat-specific products. Soak a piece of a dunk in water, then use that water to irrigate the plants. It works on larvae and is considered low risk for people and pets when used as directed. Follow the label.
• Beneficial nematodes. Microscopic worms that attack larvae in soil. They are sold live, and you apply them as directed.
• Hydrogen peroxide drench. A diluted solution is sometimes used to kill larvae in the soil, though results vary and it can stress roots if too strong. If you try it, dilute it well and test on one plant.

Repeat treatments over several weeks to catch each generation.

Step 5: Cover the soil

A half inch to an inch layer of coarse sand, fine gravel, or horticultural grit on top of the soil makes it hard for adults to lay eggs and keeps the surface drier. It is a good preventive measure for plants that tend to attract gnats.

Step 6: Replace badly infested soil

If the problem will not clear, or the soil smells sour and stays soggy, repot into fresh, well-draining mix. See how to repot a houseplant. Remove as much old soil as possible, and check the roots for rot.

Preventing fungus gnats

• Water only when the plant needs it.
• Use pots with drainage holes, and empty saucers.
• Choose a well-draining mix, with perlite or bark.
• Quarantine new plants for a couple of weeks and check the soil.
• Do not leave decaying leaves on the soil surface.
• Keep a few sticky traps in the room as an early warning system.

What if it is not fungus gnats?

Not every small fly is a fungus gnat. Fruit flies hang around ripe produce and are usually found in the kitchen. Shore flies are stockier. If the flies are not near soil, or the traps are not catching them, look elsewhere. If your plants also show webbing or speckling, it may be spider mites.

Be patient. Because of the life cycle, it usually takes three to four weeks of consistent effort to clear an infestation. If the problem keeps returning, our plant care service can help you track down the cause.