Fungus Gnats in Houseplants: How to Get Rid of Them for Good

Those tiny black flies drifting up from your potting soil cost you nothing to ignore for a day and quite a bit if you ignore them for a season. The fix is mostly free: it comes down to changing how you water. Buying a shelf of gadgets rarely helps, so this guide ranks the options by what they actually do and what they cost you in money and effort.

Are fungus gnats actually harmful?

Mostly they are a nuisance, but not entirely harmless. According to UC IPM's fungus gnat guide, the adults do not damage plants and do not bite people. The larvae are the problem, because they can damage roots and stunt growth, especially in seedlings and young plants. The same page points out that wilting is more often caused by too much or too little water, or by root decay, than by larvae.

For a healthy, established plant, a few gnats are usually an irritation rather than an emergency. For cuttings, seedlings and plants already struggling with soggy roots, treat them sooner. UC IPM's houseplant page adds that larvae and adults can spread plant pathogens, and that the adults leave tiny black droppings that show up on light surfaces.

Why they showed up (and why free fixes come first)

Females lay eggs in moist organic matter such as potting soil. At 75°F, UC IPM reports that eggs hatch in about 3 days, larvae take about 10 days to become pupae, and adults emerge roughly 4 days later, so one generation takes about 17 days. Adults live only about 7 to 10 days, according to UC IPM's houseplant problems page, but because they keep laying eggs, the population rebuilds quickly if the soil stays wet.

That is why the cheapest lever is also the strongest one. Both UC IPM pages recommend letting the soil surface dry between waterings and cleaning up standing water. If you are not sure when a pot is actually ready for water, our guide to telling when a houseplant needs water walks through checks that cost nothing.

A budget-ranked plan

Work down this list in order. Each step costs more than the one before it, and many infestations clear up before you reach the end.

I am deliberately not quoting prices here, because they vary by store and product. Check the label for how much each product treats before you buy, since a larger package is only a better deal if you will use it.

Dry the soil, the right way

Skip the temptation to let a plant wilt on purpose. Instead, water thoroughly, empty the saucer, and wait until the top layer of soil is dry before watering again. Plants that struggle with this pattern, such as those in pots without drainage holes, are the likeliest to keep a gnat problem going. If yellowing leaves come with the gnats, soggy roots may be the shared cause, which our yellow leaves guide covers.

Trap the adults

UC IPM suggests using yellow sticky traps for adults: cut them into smaller squares, attach them to wooden skewers or sticks, and place them in the pots. Traps help you see how bad the problem is and catch some adults, but they do not touch the larvae in the soil. On their own they are a monitor, not a cure.

Find the larvae with a potato

UC IPM describes a cheap monitoring trick: place raw potato chunks cut side down on the soil, not peels. Larvae are drawn to them. After a few days, remove and discard the chunks and replace them with fresh ones. It costs about as much as a potato, and it tells you whether larvae are still present.

Treat the soil with Bti

For the larvae, UC IPM lists Bti (Bacillus thuringiensis subspecies israelensis), sold under names such as Gnatrol, as a control that can work in many cases when applied repeatedly. For potting media infestations, the fungus gnat page suggests repeating applications at about 5-day intervals, and notes that Bti does not reproduce or persist indoors. UC IPM gives no numeric dose, so follow the product label exactly.

Repot only when it makes sense

UC IPM recommends using sterile potting soil when you transplant and removing old plant material and debris from pots. Repotting costs more than the other steps, so save it for badly infested pots, or plants whose soil stays wet because it has broken down. The fungus gnat page also mentions improving drainage by raising the share of perlite or sand in the mix, though it does not give a ratio.

Stopping the next round

• Inspect new plants and plants returning from outdoors before they join the others
• Use screens on windows and patio doors, as UC IPM recommends
• Keep standing water out of saucers and cachepots
• Put a few yellow sticky traps out as an early warning system

What not to spend money on

UC IPM's houseplant page says other insecticides are rarely warranted in homes. That covers a lot of aisle space. Before buying a spray, confirm you are dealing with fungus gnats and not another pest, and fix the watering first. Spending money on larvae treatment while the soil stays soggy just feeds the next generation.

Frequently asked questions

Are fungus gnats harmful to people?

No. UC IPM states that the adults do not bite people. The droppings are a cosmetic annoyance on light surfaces, and the larvae are the real concern for plants.

Do sticky traps work?

They catch adults and are useful for monitoring, but they do not stop the larvae in the soil. UC IPM lists them as a way to reduce populations by trapping adults, so pair them with drier soil and a larvae treatment.

How long until they are gone?

Plan on several weeks. At 75°F, one generation takes about 17 days according to UC IPM, so a single treatment will not end it. Keep the soil drying out between waterings and repeat treatments on the label schedule.

Sources

• Fungus Gnats — UC Statewide IPM Program, University of California
• Houseplant Problems — UC Statewide IPM Program, University of California