Fungus Gnats in Houseplants: Causes, Fixes, and Soil Solutions

Fungus gnats are often mistaken for fruit flies. Fruit flies gather around ripening produce, while fungus gnats are found near damp potting soil. University of Wisconsin Horticulture Extension describes adults as delicate, dark brown or black flies about 1/8 inch long. Their larvae look like pale worms with black heads, up to about 1/4 inch long when mature.

The life cycle is quick. Females lay up to 200 eggs in clusters in moist soil. Eggs usually hatch within three to six days, larvae develop over one to two weeks, and adults live roughly a week. Under good conditions, the cycle from egg to adult can take three to four weeks.

The common thread is soil that stays wet. Extension guidance names overwatering as the primary cause.

Watering on a Schedule Instead of by Soil Condition

Pots dry at different rates depending on size, material, light and season. A fixed schedule can keep the top layer of soil damp, which is where gnats lay eggs. Checking the soil first fits the plant better than a calendar.

Poor Drainage

Pots without drainage holes, or compacted soil that drains slowly, hold water around the roots for longer than the plant needs. If water pools or the soil stays saturated for days, fix the drainage as well as the watering.

Decaying Organic Matter

Larvae feed on fungi and decaying organic matter in damp soil. Dead leaves or other debris on the surface give them more to feed on, so remove them.

Treat the adults and the larvae together, and keep going long enough to cover a full life cycle.

1. Let the top inch of soil dry out between waterings. This is the step that removes the damp conditions larvae need. For plants that dislike drying, water from the bottom instead of the top.
2. Set yellow sticky traps. They catch adults and show whether your numbers are dropping. Replace them when they fill up.
3. Remove debris from the soil surface. Fallen leaves, spent blooms and other decaying matter feed larvae.
4. Add a layer of coarse sand or fine gravel. University of Wisconsin Extension suggests a 1/2 to 1 inch layer on the soil surface.
5. Consider a biological treatment for larvae. Products containing Bacillus thuringiensis subsp. israelensis (Bti) target larvae. Extension guidance describes several applications about five to seven days apart. Follow the product label.
6. Keep it up for several weeks. Because eggs keep hatching, one round rarely finishes the job.

Traps and treatments deal with the current infestation. Gnats come back if the soil stays wet.

Rebuild Drainage

Check that the pot has a drainage hole. If you like a decorative pot without one, keep the plant in a plain pot with holes and set it inside the decorative one, then empty any water that collects. If the soil is compacted or drains slowly, repot into a more free-draining mix. Perlite, pumice or coarse sand can improve drainage.

Change How You Water

Water thoroughly, then wait until the top of the soil has dried before watering again. In fall and winter, when growth slows, plants usually need water less often.

Gnats often arrive in the potting soil of new plants. Keep new plants apart from the rest for a couple of weeks and watch for adults before moving them next to your collection.

A layer of coarse sand or fine gravel on top of the soil is a simple barrier, and it dries faster than bare potting mix.

If gnats keep appearing after several weeks of drier soil, look for another source of moisture: a saucer that does not empty, a pot that sits in standing water, or soil that is saturated deeper down than the surface suggests. In that case, repotting into fresh, well-draining mix can help. Extension sources reserve insecticides such as pyrethrin products for severe infestations and recommend using them together with the non-chemical steps above.

Start by letting the soil dry, clearing debris and adding traps. These changes address the cause, and they usually matter more than any product.

• University of Wisconsin–Madison Division of Extension (Horticulture), "Fungus Gnats on Houseplants"