Get Rid of Fungus Gnats
Fungus gnats are the most common houseplant pest. Here's exactly why you have them and a step-by-step plan to get rid of them permanently.
Fungus gnats are among the commonest houseplant pests - those tiny black flies that drift up in a little cloud whenever you move a pot. The good news: they’re harmless to people, they don’t bite, and adult gnats do little damage to plants. The annoying news: they’re persistent, and getting rid of them for good means understanding what they actually are.
What Fungus Gnats Actually Are
A fungus gnat infestation has two stages, and you must treat both:
- Adults - the tiny black flies you see. Colorado State University Extension gives them a life span of seven to 10 days, in which a female may lay up to 200 eggs. They do not bite or feed; they are mostly just annoying.
- Larvae - tiny translucent worms living in the top few centimetres of soil. These are the real population. They feed on fungus, organic matter, and decaying roots in damp soil, and in large numbers they can nibble healthy roots.
Here’s the key insight: fungus gnats are a symptom, not the disease. They breed only in constantly damp soil. If you kill the adults but leave the soil wet, a new generation hatches within days. To win, you must fix the moisture and break the breeding cycle.
Why You Have Fungus Gnats
- Overwatering - by far the most common cause. Soil that’s always damp on top is a perfect nursery.
- A new plant brought them home - they came in its soil.
- A bag of contaminated potting mix.
- Soil rich in undecomposed organic matter that stays moist.
If you have gnats, you almost certainly have a watering habit that’s a little too generous - fixing that is half the battle.
The Step-by-Step Plan to Get Rid of Them
Do all of these together. One method alone rarely works.
Step 1: Let the soil dry out
This is the most important step - Colorado State’s guide calls letting the growing medium dry between waterings, especially the top 1 to 2 inches, the most important strategy. Let the top 3-5 cm of soil dry out completely before you water again - and keep it that way. Fungus gnat larvae and eggs can’t survive in dry soil. For plants that tolerate it, letting the soil go quite dry between waterings starves the larvae directly.
Step 2: Switch to bottom watering
Water the plant from below - stand the pot in a tray of water for 20 minutes so it soaks up moisture from the bottom. This keeps the top layer of soil dry, which is exactly where gnats breed. It’s one of the most effective long-term fixes.
Step 3: Top the soil with a dry barrier
Some growers cover the soil surface with a 1-2 cm layer of something dry: coarse sand, fine gravel, or perlite. The idea is to keep the surface dry and make egg-laying harder. We found no extension trial that measures how well it works, so treat it as a supplement to drying the soil, not a replacement.
Step 4: Trap the adults with yellow sticky traps
Place yellow sticky traps flat on or just above the soil. Gnats are drawn to yellow and stick fast. This steadily cuts the adult population and lets you monitor whether you’re winning (fewer caught over time = success).
Step 5: Kill the larvae in the soil
To attack the larvae directly, use one of these:
- BTI (Bacillus thuringiensis israelensis) - a naturally occurring soil bacterium, sold as mosquito bits or dunks and in gnat-specific products. The US Environmental Protection Agency says its toxins specifically target the larvae of mosquitoes, black flies and fungus gnats, and that Bti has no toxicity to people. Apply it as a soil drench the way the product label directs. The University of California’s pest notes add that Bti does not persist indoors, so infestations may need repeated applications at about five-day intervals.
- Insect-parasitic nematodes - microscopic roundworms sold for watering into the soil. The same University of California notes say they can give relatively long-term control of the larvae.
Two things we leave out. A hydrogen peroxide drench is recommended all over the internet; none of the extension guides we read lists it. And Colorado State notes that short-lived contact sprays - pyrethrins, soaps, oils and neem - do not give sufficient long-term control of fungus gnats.
Step 6: Repeat and be patient
At ordinary room temperatures the cycle from egg to adult may be completed in three to four weeks. Keep all the above going for at least 3-4 weeks, even once you see fewer gnats - you’re outlasting every egg already in the soil. Stopping early lets the population rebound.
When Nothing Works: The Repot Option
If an infestation is severe and stubborn, the nuclear option is to repot the plant. Remove as much of the old infested soil from the roots as you reasonably can, and repot into fresh, dry potting mix in a clean pot (our repotting size calculator helps you pick the right pot). Combined with the steps above, this resets the problem.
How to Prevent Fungus Gnats
- Don’t overwater. Let soil dry properly between waterings - the #1 prevention.
- Bottom water plants prone to gnats, keeping the surface dry.
- Quarantine new plants for two weeks and check their soil before they join your collection.
- Top vulnerable pots with sand or grit as a permanent barrier.
- Use fresh, good-quality potting mix and store opened bags sealed and dry.
- Don’t leave water standing in saucers.
Frequently Asked Questions
Are fungus gnats harmful?
To people, no: the University of California’s pest notes say adult fungus gnats don’t damage plants or bite people. Large numbers of larvae can damage roots and stunt growth, particularly in seedlings and young plants.
Why do I keep getting fungus gnats?
Because the soil stays too damp. Fungus gnats breed only in constantly moist soil. If you kill the adults but keep overwatering, a new generation hatches within days. Fix the watering.
What kills fungus gnat larvae in soil?
A Bti drench, applied and repeated as the label directs, or insect-parasitic nematodes. Letting the soil surface dry between waterings takes away the conditions the larvae need.
Do yellow sticky traps get rid of fungus gnats?
Sticky traps catch and reduce the adults, but they don’t touch the larvae breeding in the soil. They’re one part of the plan - use them alongside drying out the soil and treating the larvae.
How long does it take to get rid of fungus gnats?
Plan for at least 3-4 weeks of consistent treatment to outlast the full life cycle. Stopping as soon as you see fewer gnats lets the remaining eggs rebuild the population.
Recommended Gear on Amazon
- Yellow sticky traps - browse current options and reviews on Amazon.
- Mosquito bits (BTI) - browse current options and reviews on Amazon.
- Horticultural sand top dressing - browse current options and reviews on Amazon.
As an Amazon Associate, we earn from qualifying purchases - at no extra cost to you. This helps us keep producing free, in-depth guides.
