Fungus Gnats: How to Get Rid of Them in Seedlings and Pots

To get rid of fungus gnats, first let the surface of the potting mix dry between waterings: females lay eggs in moist potting soil and organic debris. Yellow sticky traps may reduce the adults, and Bti gives temporary control of larvae. Adults are a nuisance, but many larvae can damage roots of seedlings and young plants.

Fungus gnats are the tiny dark flies that lift off a seed tray or a houseplant pot when you water or move it. They breed in damp mix, so the cure starts with the watering can, not a spray. This guide draws on twelve pages from ten universities, mostly extension pages, listed at the bottom; where their numbers differ, it gives the range or says how they split.

Fungus gnats at a glance

Adult	Usually around 1/16 to 1/8 inch, though a few species reach 1/2 inch; slender and dark, with long legs and antennae
Larva	Legless, clear to whitish body with a shiny black head; about 1/4 inch long when grown
Breeds in	Damp potting mix, compost and other decaying organic matter
Egg to adult	About 17 days at 75 F by one count; others say it can take 3 to 4 weeks
Harm	Adults neither bite nor damage plants; many larvae can stunt seedlings and young plants
First step	Let the top of the mix dry before the next watering (all 8 control pages we cite)
For adults	Yellow sticky traps, to catch adults and to count them
For larvae	Bti in water, poured on the mix, repeated if needed about every 5 to 7 days
Larva check	Raw potato pieces on or in the mix for a few days

What are fungus gnats?

A fungus gnat is a tiny dark fly, usually 1/16 to 1/8 inch in length, whose larvae live in damp potting mix. The common ones are Bradysia and Orfelia species, the darkwinged fungus gnats (family Sciaridae), and they look like small, frail mosquitoes.

To tell them from other small flies, look at the antennae and the wings. The antennae are longer than the head, while shore flies, which also turn up around wet mix and algae, have short, bristly ones. Under a magnifying glass the wing veins show a rounded Y-shaped fork. People often take them for fruit flies, which are a separate insect.

The adults fly weakly and seldom go far. They often stay near the pots, running over the mix and resting on the leaves, and since light draws them, the first ones you see may be at a window. The larvae are the stage that lives in the pot: legless and clear to whitish with a shiny black head, about 1/4 inch long when full grown. Where they are many and the mix is very wet, they can leave shiny slime trails on the surface, like the trails of tiny slugs.

Generations turn over fast. At 75 F the eggs take about 3 days to hatch, the larvae feed for about 10 days and the pupae take about 4 more: roughly 17 days from one egg-laying female to the next, and faster when it is warmer. Other counts say egg to adult may take 3 to 4 weeks, at 65 to 75 F or under ideal conditions. One female can lay up to 200 eggs. Indoors generations tend to overlap, with every stage in the pot at once.

Are fungus gnats harmful to plants?

The adults do no harm to plants, and one guide calls larva damage rare in homes, but a heavy infestation can set back seedlings. The adults neither damage plants nor bite people. The larvae feed mostly on fungi and decaying organic matter but chew roots too, and when there are many of them, they can damage roots and stunt growth, especially in seedlings and young plants.

How often that happens is where the guides differ:

• Seedlings and cuttings. The larvae can directly damage seedlings, and young seedlings and new cuttings are the plants most at risk.
• Houseplants. One guide reports serious root damage, even dead plants, in houseplants and indoor plantings where big populations met moist mix rich in organic matter. Others count the larvae as minor houseplant pests, or say fungus gnats typically do not harm plants.
• Outdoors. The larvae feed on roots in the garden as well, but they usually do no serious damage there; heavy losses happen more in greenhouses and nurseries.

A wilting plant is more likely thirsty, waterlogged or rotting than eaten. Overwatering, underwatering, root decay fungi and poor drainage are much more common causes of wilting than gnat larvae, so check the mix before you blame the gnats.

Why fungus gnats show up in seed trays and houseplants

They come for wet mix with plenty of organic matter in it. Fungus gnats go with overwatered soil and with soil high in organic matter, and moist mixes heavy in peat moss draw the egg-laying females in particular.

A seed tray fits that picture. Commercial seed starting mixes are usually peat and vermiculite, and the tray is kept damp, often under a clear cover, until the seeds sprout. Trays also sit indoors for weeks at a stretch. One Upper Midwest schedule keeps leeks and onions inside for 14 to 15 weeks and tomatoes for 5 to 6, with cauliflower and peppers in between. Crops that transplant easily include tomatoes, peppers and eggplant from the nightshade family, and broccoli, cabbage and kale from the brassicas.

Houseplants follow the seasons. The nuisance tends to peak in late fall and winter, for a few possible reasons: plants that spent the summer outside often come in carrying gnats, and people stay indoors more and may notice them. Above all, the mix stays wetter, because shorter, cooler days slow growth and water use while the watering routine stays the same. Old mix adds to it, since broken-down mix tends to hold more water. A newly bought plant is the other way in.

How to get rid of fungus gnats: dry the surface first

Water less often, so the surface of the mix dries between waterings. Guides call this the most important step for houseplants, or the most effective one: in drier mix fewer eggs and young larvae survive, and females find it less inviting for laying.

How dry is dry? The figures run from just the surface to the top 2 inches: two guides say the top inch, two the top 1 to 2 inches and one the top 2 inches. They also put the larvae at different depths, from the top 1/2 inch or so to usually the top 2 to 3 inches.

In our reading the split is about depth, not direction: all of them want the surface dry before you water again. Start with the top inch, and go to 2 inches, the deepest figure given, for plants that tolerate it.

• Water from below. Standing the pot in water lets the roots drink while the surface stays drier. Switch to it for a while if you worry the plant will dry out, and do not leave water standing in the saucer.
• Top the mix with sand or gravel. You can cover the mix with coarse sand or fine gravel, 1/2 to 1 inch deep, which helps keep the surface drier and less inviting for eggs.
• Clear out their food. Pick dead leaves and spent flowers off the mix, and take out any pot full of rotting plant matter such as decayed bulbs or roots.
• Repot in fresh mix every so often, and especially once the old mix has broken down and stays soggy; new soil helps.
• Keep a bad pot apart. Do not move an infested plant somewhere new, where the flies can hatch and spread to other pots. In some cases a badly infested plant may be worth throwing out; a favorite plant could instead be washed to bare roots and repotted in fresh mix.

Give it time. Getting them in check may take 3 to 4 weeks of changed watering plus the sand or gravel layer. Some plants will not take a dry surface even with bottom watering; for those, move on to the other methods below. With succulents and other plants that tolerate dry soil, less water alone often clears up the problem.

Sprays come last, if at all. Three of the guides advise against them: one calls insecticides unnecessary and ineffective for fungus gnats in houseplants, another says they are rarely warranted around the home, and a third says spraying the adults generally does not work. Another allows that an insecticide may be needed if the gnats are still there several weeks after the watering changes. This guide names no products; with any pesticide, read the label and follow it closely.

Yellow sticky cards for adults

Yellow sticky cards catch adult gnats and can show how many there are, but on their own they will not clear an infestation. The adults are drawn to yellow and can get stuck in the glue.

• Size and placement. Small 2 by 2 inch traps suit a pot, or cut cards into small squares and fix them to wooden skewers pushed into the pots. Set them near or under the leaves, or on the rim of the pot. Many come with little stakes for pots.
• Change them. Replace the traps every few days.
• Read them. A card is a counter. If more and more adults turn up stuck to it, more control may be needed, and swapping cards regularly helps show the trend over time.
• Skip vinegar. Vinegar traps and other fruit fly methods do not work for monitoring fungus gnats.

Cards can do a little more than count. Sticky traps may reduce the nuisance at home, and catching many egg-laying females may mean fewer larvae in the next generation.

Bti for the larvae in the soil

Bti, short for Bacillus thuringiensis subspecies israelensis, is a bacterium that kills fungus gnat larvae in the mix but does not affect the flying adults, or the eggs and pupae. It is one of three biological controls for larvae in pots, and Bti may be the most convenient of the three for home gardeners.

Check for larvae before you treat. Set pieces of raw potato in the mix, cut side down; 1/4 inch slices work. Larvae come to the potato to feed, usually within a few days, and turning a slice over shows them on the underside. Check after a couple of days, or after 3 to 4. If a slice shows many black-headed larvae, it may be time to treat that pot, and fresh pieces every few days can also draw larvae away from the roots.

• Mix it and pour it on. Mix Bti with water and apply it as a soil drench, or spray it onto the mix, following the label. One way: mix the product with water, let it soak 30 minutes, then pour it on the soil. Granules go on the surface and get watered in; use enough water to help the Bti reach the larvae.
• Repeat it. Bti does not multiply or last indoors, so repeat it if needed, about every 5 to 7 days, until the infestation is under control.
• Expect weeks, not days. Bti can be effective over several weeks; one guide expects the numbers to fall after 4 to 5 weeks, with adults still flying in the meantime.
• Get the right Bt. Bti is a different subspecies from the Bt sprayed on leaves against caterpillars, and the caterpillar kind does not work on fly larvae. Look for israelensis on the label.

Where to find it is where the guides split. One says several Bti products are readily available at retail nurseries and garden centers. Another points to the pond supplies at farm and garden stores, because the same products are also labeled to control mosquitoes in standing water. A third says Bti sold for fungus gnats is generally not carried in retail stores, and the one product it names goes to commercial greenhouses and large indoor plantings. Wherever you find it, choose a product labeled for use on houseplants.

Other larva treatments come up too. Steinernema feltiae, a nematode applied as a drench, is the species singled out against fungus gnats: infected larvae die within 3 to 4 days, and it works at 60 to 90 F in moist mix but must be mail-ordered live and applied right away. The third biological option, Hypoaspis mites, probably would not do well in a single houseplant. One guide also pours 1 part 3% hydrogen peroxide mixed with 4 parts water over the soil, then lets the soil dry; none of the other pages we read mentions hydrogen peroxide.

How to keep fungus gnats out of seed starting mix

Start with fresh pasteurized mix and keep the surface from staying wet once the seeds are up. Buy and use only pasteurized mix, and keep it in closed containers off the ground until you use it. Most commercial mixes are already heat treated; store yours away from any infested plants.

1. Use a seed starting mix. Sow seed in sterile soilless mix, not bagged topsoil, garden mixes or potting mixes. Fresh commercial seed starting mixes are generally free of insects, weeds and disease; one guide calls them sterile. Keep half-finished compost out: it often carries gnats unless it has been pasteurized.
2. Use containers that drain. Every seed starting container needs drainage holes. A tray without holes catches drips like a saucer, but take care not to overwater an undrained flat.
3. Uncover the tray once seeds are up. Guides time this differently: one takes the cover off as soon as sprouts appear; another takes the bag off a single sown container when seedlings appear, but keeps the cover on a flat until all the seeds have germinated; a third lifts a dome when the seedlings grow tall enough to touch it.
4. Water from below. Set the tray in water 1 to 2 inches deep for 3 to 10 minutes, then put it back under the lights. Larger cells can look dry on top while still moist lower down.
5. Let the top nearly dry between waterings. Wait until the top of the mix is nearly dry and keep it moist but not wet; avoid heavy watering while the plants are small or when algae shows on the surface.

Those seedling rules were written for healthy roots, not for gnats. In our reading they ask for the same thing the gnat guides do: a surface that dries between waterings.

Keep gnats from walking in. Keep new plants apart from the others for a few weeks with a yellow sticky trap beside them, checking daily. Screening and caulking leaky windows and doors also helps keep them out. If you blend your own mix with garden soil for potting up, you can solarize it: moisten the soil, bag it in clear or black plastic no more than about 8 inches deep, and leave it on a raised surface in the sun for about 4 to 6 weeks.

Fungus gnat traps and supplies

What you need: fresh seed starting mix, yellow sticky cards and, only if potato checks turn up larvae, a Bti product used as its label directs.

• Yellow sticky cards small enough for a pot, such as 2 by 2 inch traps.
• A Bti product, liquid or granules, labeled for houseplants.
• Pasteurized seed starting mix, stored closed and off the floor.
• A tray without holes for watering from below.
• Coarse sand or fine gravel to top houseplant pots.
• A raw potato for larva checks.

What you'll need for fungus gnats

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• 

  MAXGUARD Yellow Stake Sticky Traps, 30 Pack

  Yellow sticky cards small enough for a pot, such as 2 by 2 inch traps.

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• 

  Summit Mosquito Bits, 30 oz

  Only if potato checks turn up larvae: a Bti product labeled for fungus gnat larvae, used as its label directs.

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• 

  Sun Gro Seed Starter Mix, 8 qt

  Pasteurized or sterile seed starting mix, stored closed and off the floor.

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• 

  Bootstrap Farmer 1020 Trays, No Drain Holes, 5-Pack

  A tray without holes for watering from below.

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Sources we used

Each figure and control step above traces to one of these twelve pages, saved when we read them. When they disagree, the page gives the range or says how they split. We do not run growing trials or show star ratings. This guide names no pesticide products.

• "UC Statewide IPM Program: Pest Notes: Fungus Gnats" (UC ANR Publication 7448, updated 08/2013)
• "Colorado State University Extension: Fungus Gnats as Houseplant and Indoor Pests" (Fact Sheet 5.584)
• "Oregon State University Extension: I have fungus gnats everywhere. Can you help?" (revised April 2025)
• "University of Wisconsin-Madison Extension: Fungus Gnats on Houseplants" (XHT1277, revised 2021)
• "UW-Madison Insect Diagnostic Lab: Fungus Gnats: Tiny Flies Around Your Houseplants" (2020)
• "Iowa State University Extension and Outreach: Fungus Gnats in Houseplants" (2023)
• "University of Minnesota Extension: Fungus gnats" (insect diagnosis page)
• "NDSU Extension: Houseplant Fungus Gnat Woes" (2021)
• "Cornell Cooperative Extension, Allegany County: A Spooky Houseplant Pest: Fungus Gnats" (updated 2024)
• "Clemson Cooperative Extension, HGIC 1259: Starting Seeds Indoors" (revised 2020)
• "University of Minnesota Extension: Starting seeds indoors" (reviewed 2026)
• "University of Maryland Extension: Starting Vegetable Seeds Indoors" (updated 2026)

Related guides

• How to grow tomatoes, whose seed is started indoors weeks ahead.
• How to grow peppers, another crop started indoors.
• How to grow onions, among the first seeds started indoors.
• How to grow cilantro, including in pots indoors.
• Sprouting seeds, grown in a jar.
• Low light houseplants, on which plants cope with a dim room.
• Pothos care, for the trailing houseplant also called devil's ivy.
• Money tree care, for the braided Pachira houseplant.
• Wandering jew plant, the striped inch plant.

Frequently asked questions

Do fungus gnats bite?

No, fungus gnats do not bite. The adults neither bite people nor damage plants, and they do not feed, though they may drink water sitting on leaves or on the mix. They are a nuisance indoors; the larvae in the mix are the stage that can hurt roots.

What causes fungus gnats?

Mix that stays wet, especially mix rich in organic matter, is what draws and feeds them: fungus gnats go with overwatered soil and soil high in organic matter. Plants use less water in fall and winter, so an unchanged watering routine keeps the mix wet, and plants summered outdoors often bring gnats in.

Does neem oil kill fungus gnats?

Not for lasting control. Neem is one of the short-lived contact sprays, with pyrethrins, soaps and oils, that do not control adult fungus gnats for long and need repeating every couple of days, and sprays aimed at adults do not reach the larvae. For the larvae, use drier mix and Bti.

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

Plan on about a month. Getting them in check may take 3 to 4 weeks of changed watering, with a sand or gravel top layer, and one guide expects Bti to show results after 4 to 5 weeks. Adults keep flying for a while, since Bti does not affect them.

Are fungus gnats the same as fruit flies?

No, they are two different insects, though people often mix them up. Fungus gnats are delicate dark flies with long legs and antennae that breed in damp potting mix, and their wings show a rounded Y-shaped vein. Vinegar traps, a fruit fly method, fail as a way to monitor fungus gnats.

Does hydrogen peroxide kill fungus gnats?

One extension guide uses it against the larvae; the others we read do not mention it. That guide pours 1 part 3% hydrogen peroxide mixed with 4 parts water over the soil, then lets the soil dry. The others rely on drier mix, sticky traps and Bti instead.

Can fungus gnats damage seedlings?

Yes, if the larvae are many. A large number of larvae can harm roots and stunt growth, especially in seedlings and young plants, and they can directly damage seedlings. When a seedling wilts, check watering first: too much or too little water and poor drainage are much more common causes.