Fungus Gnats in Houseplants: How to Get Rid of Them

[ГОЛОВНЕ ФОТО (прев’ю): дрібні чорні мошки над горщиком з кімнатною рослиною]

The tiny black flies that rise from a pot when you water are usually fungus gnats. The adults are just a nuisance, but their larvae in the soil can damage the roots of seedlings and young plants. Here is how to tell what you have, how to get rid of gnats in houseplants step by step and how to keep them from coming back.

More on this topic: why houseplants get sick, houseplants: names and care, how to repot a houseplant.

Where gnats come from

Eggs and larvae arrive in bagged potting mix or on a new plant, and adults fly in through open windows.

They breed in constantly moist soil rich in organic matter: larvae feed on fungi and plant debris and, when that runs short, on fine roots.

So the main causes are frequent watering, water left in saucers, pots without drainage and homemade fertilisers such as tea leaves or coffee grounds.

To check for larvae, lay a slice of raw potato on the soil for 1–2 days; larvae gather underneath.

[фото: грибний комарик крупним планом на листку й личинки в ґрунті]

How to get rid of gnats step by step

Step 1. Let the top 2–3 cm of soil dry out completely before watering. Larvae die in dry soil and females won't lay eggs there.

Step 2. Stick yellow sticky traps near the pots to catch adults and show whether numbers are falling.

Step 3. Top the pot with 1–1.5 cm of dry sand, fine grit or decorative gravel so there is nowhere to lay eggs.

Step 4. Water from below via the saucer and pour away what's left after 20–30 minutes.

Step 5. For heavy infestations, drench the soil with a biological product based on Bacillus thuringiensis israelensis or a systemic houseplant insecticide, following the label. Repeat after 7–10 days.

Step 6. Last resort: repot. Shake off the old soil, trim rotten roots and use fresh mix; see how to repot a houseplant.

[фото: жовта клейка пастка в горщику з кімнатною рослиною]

Home remedies: what works

A hydrogen peroxide drench (1 part 3% peroxide to 4 parts water) kills larvae on contact. It helps but doesn't replace letting the soil dry.

A trap of cider vinegar with a drop of washing-up liquid catches adult gnats and fruit flies.

Matches pushed into the soil, garlic or potassium permanganate have weak or doubtful effect, and strong permanganate can burn roots.

[фото: шар піску чи декоративної крихти на поверхні ґрунту в горщику]

How to keep gnats away

• Water only once the top layer is dry and never leave water in the saucer.
• Use pots with drainage holes and a drainage layer at the bottom.
• Keep new plants apart for 2 weeks; quarantine reveals gnats and other pests. See why houseplants get sick.
• Don't feed plants tea leaves, coffee grounds or meat water; it's food for larvae.
• Keep opened bags of potting mix sealed; suspect soil can be heated in the oven for 30 minutes at 80–90 °C.
• Take extra care with moisture-loving plants like the peace lily.

A living plant as a gift

Looking for a living gift that lasts for months? A potted orchid is a great choice for home or office.

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See houseplants and orchids.

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FAQ

Are fungus gnats harmful to people?

No. They don't bite or carry disease; they are just annoying.

How do I get rid of gnats fast?

Let the soil dry, use yellow sticky traps and top-dress with sand; for heavy infestations drench with a biological product or insecticide.

Why do gnats appear in winter?

Plants drink less in winter but are often watered out of habit, so the soil stays wet.

Are the white jumping bugs gnats?

No, they are springtails: mostly harmless, but a sign of overwatering.

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Author: Maria Svitlovir

Graduated from Taras Shevchenko National University of Kyiv, Institute of Biology and Medicine (2001). BSc in Biology, MSc in Horticulture. Writes about plants, works at a garden centre selecting and repotting plants, and contributes to plant reference guides.