Fungus gnats are tiny dark flies whose larvae live in damp potting mix, so getting rid of them is mostly about moisture. Let the surface of the soil dry between waterings, catch the adults on yellow sticky cards, and, if they keep coming back, treat the soil with a Bti product aimed at the larvae. Extension sources describe fungus gnats as mostly a nuisance that rarely harms an established indoor plant, but they are a clear sign that the top of the mix is staying wet.
This guide explains how to confirm what you have, why gnats show up, and which steps are backed by university extension guidance. It also covers the popular home remedies that are not.
What Fungus Gnats Are
Adult fungus gnats are about 1/16 to 1/8 inch long, with dark bodies and clear wings folded back when they rest. They are weak fliers and are drawn to light, so you will often see them around windows, lamps, and the soil surface when you water.
The part you do not see is the larva. It is whitish with a shiny black head, can grow up to about a quarter inch, and lives in the top layer of damp potting mix, where it feeds on decaying roots, fungi, and other organic matter. University of Minnesota Extension says fungus gnats rarely, if ever, damage indoor plants. Clemson Extension adds that some species attack roots and can be particularly hard on young plants, and Oregon State Extension notes the larvae can damage seedlings. So the risk is higher for cuttings, seedlings, and small new plants than for a mature plant in a large pot.
How to confirm it is fungus gnats
- Set a yellow sticky card next to the pot. Extension guidance recommends yellow sticky cards for catching adults and for detecting flying pests.
- Try the potato test. University of Minnesota Extension suggests placing a slice of raw potato on the soil surface and checking it periodically for larvae.
- Look at the pattern. If the flies hover near the soil and the mix stays damp for days, fungus gnats are the likely cause.
Why You Have Them
Fungus gnats need moisture and something to feed on. The usual causes are:
- Soil that stays damp. Larvae live in the upper half inch or so of moist mix, according to Oregon State Extension. Overwatering is the most common reason.
- Peat-heavy potting mix. University of Minnesota and Clemson Extension both note that gnats prefer damp mixes with a high percentage of peat or other organic matter.
- Standing water. Clemson advises against letting water sit in the saucer under a pot.
- Decaying material on the surface. Oregon State notes that dead leaves and spent flowers can support a population.
- A new plant. Gnats can arrive with a new plant, which is one reason extension guidance recommends keeping new plants separate for a while.
How to Get Rid of Fungus Gnats
Work through these in order. The first two do most of the work.
1. Let the surface dry between waterings
This is the first step in every extension source we reviewed. University of Minnesota Extension says to let the surface soil dry as much as possible between waterings without letting the plant wilt, and Oregon State suggests checking that the top inch is dry before you water. Clemson notes that most houseplants tolerate this, especially in winter.
Oregon State also suggests watering from below to keep the soil surface dry. If your plant prefers evenly moist soil, such as a fern or prayer plant, aim for lightly moist rather than wet, and never leave it sitting in water. Our guide to how often to water houseplants explains the finger test.
2. Empty saucers and make sure the pot drains
Water that pools at the bottom keeps the mix wet from below. Empty saucers and cachepots after watering, and check that the pot has a drainage hole. Our beginner guide explains why drainage holes matter.
3. Trap the adults
Place yellow sticky cards near the soil. Oregon State suggests small cards, replaced every few days, and cautions you to keep them away from pets and children. University of Minnesota Extension says traps are good for detecting and monitoring but should not be relied on as the only management method, because they catch adults and not the larvae in the soil.
4. Clear away decaying material
Remove dead leaves and spent flowers from the soil surface so larvae have less to feed on.
5. If they persist, treat the soil with Bti
Bti is short for Bacillus thuringiensis subspecies israelensis, a bacterium that targets fly larvae. University of Minnesota Extension lists it, applied to the soil, as an option and notes that it is only effective against larvae. Oregon State says it kills larvae but not adults, and describes soaking a product and pouring the liquid onto the soil. Clemson notes it is applied to the soil and watered in.
Because it does not touch adults, keep the sticky cards in place while you treat, and keep the soil on the drier side for as long as flies keep appearing. Choose a product whose label lists houseplants or potting soil, and follow the directions. Clemson advises checking that the label specifies indoor use before you use any pesticide indoors.
6. As a last resort, refresh the mix
University of Minnesota Extension suggests repotting into fresh soil in a clean container after washing the old soil off the roots. Oregon State recommends disposing of heavily infested plants and notes that most commercial potting mixes are already pasteurized. If you are choosing a new mix, our potting mix guide can help.
What About Hydrogen Peroxide, Neem Oil, and Other Home Remedies?
These come up often in search results. The extension guidance we reviewed for fungus gnats focuses on moisture control, sticky cards, Bti, and fresh mix, and does not list hydrogen peroxide drenches or neem oil as gnat treatments. We cannot vouch for them, so we have not built them into the steps above. If you decide to try a home remedy anyway, test it on a single plant first and watch for leaf or root damage before using it on the rest.
Common Mistakes
- Watering on a schedule. A fixed calendar keeps the surface damp whether or not the plant needs water.
- Relying on sticky cards alone. They catch adults, while the larvae keep living in the soil.
- Leaving water in the saucer. This keeps the bottom of the pot wet.
- Treating one plant and ignoring the rest. Check every pot in the room, and keep the infested plant apart from others while you work.
- Stopping as soon as the flies disappear. If the habit that made the soil wet comes back, so will the gnats.
How to Keep Them From Coming Back
- Check the soil before every watering instead of following a schedule.
- Use fresh, pasteurized potting mix and a container with drainage.
- Keep new plants apart from your collection for a few weeks, with a yellow sticky card nearby. Our guide to what to do when your plant arrives in the mail walks through the first-week routine.
- Bringing plants inside after a summer outdoors? See our guide to when to bring houseplants inside.
From the Greenhouse Bench
Growers routinely monitor for flying pests with yellow sticky cards, and the same habit works in a living room. A card tells you early whether gnats are present, and it shows you whether your changes are working. The watering advice on our own Help page applies directly: there is no universal schedule, and you should water when the top of the soil is dry for that plant in your room. That one habit prevents most gnat problems.
Frequently Asked Questions
Are fungus gnats harmful to my plants?
Usually they are more of a nuisance than a threat. University of Minnesota Extension says they rarely, if ever, damage indoor plants. Clemson notes that some species can attack roots, especially of young plants, so seedlings and cuttings deserve closer attention.
Why do fungus gnats keep coming back?
Most often because the soil is staying damp. Sticky cards catch the adults, but new adults keep emerging from the soil until the surface dries out and the larvae are dealt with.
Will fungus gnats spread to my other plants?
They can, since the adults fly. Keep the infested plant apart from the others, and use sticky cards across the room to see whether gnats have spread.
Are sticky traps safe around pets and children?
Oregon State Extension advises caution around pets and children, so place cards where they cannot reach them.
Do I need to throw out the soil?
Not usually. Start with drying the surface, trapping, and soil treatment. Fresh mix is a last step, and Oregon State recommends discarding only heavily infested plants.
The Short Version
Dry the surface, trap the adults, empty the saucer, and treat the larvae only if you need to. Then keep watering to the soil, not the calendar. For more on what happens when soil stays wet too long, read our guide to the signs of overwatering.
Sources: University of Minnesota Extension: managing insects on indoor plants, Oregon State University Extension: fungus gnats, and Clemson Cooperative Extension: common houseplant insects and related pests. Always follow the label on any pesticide product.