How to Get Rid of Fungus Gnats on Houseplants

Fungus gnats (sciarid flies) are the most common pest we see in UK houseplant collections, including in our own nursery. Those tiny black flies hovering around your pots aren't fruit flies. They're fungus gnats, and while the adults are mostly harmless, their larvae feed on organic matter and fine roots in damp compost. Left unchecked, a small nuisance becomes a full infestation in under a month.

The good news: fungus gnats are very treatable. This guide covers everything from identification through to the most effective treatments, including the methods we use at GrowTropicals when they crop up in the glasshouse.

What Are Fungus Gnats?

Fungus gnats are small flies in the family Sciaridae, which is why they're also known as sciarid flies in the UK. Adults are dark-bodied, about 2–3 mm long, with long legs and a single pair of wings. They're weak fliers and tend to hover just above the soil surface or run across the top of the compost. You'll often spot them when you water or disturb the pot.

The adults don't bite and don't damage plants directly. The problem is underground: females lay up to 200 eggs in moist soil, and the larvae (small, translucent-white, with a distinctive shiny black head capsule) feed on organic matter, fungi, and fine root hairs in the top few centimetres of compost.

How to Tell If You Have Fungus Gnats

Look for these signs: small dark flies hovering near the soil rather than around fruit or food (that's fruit flies), tiny white larvae visible when you disturb the top layer of compost, and unexplained yellowing or wilting in seedlings or small plants. In mature houseplants, a moderate infestation usually causes little visible damage, but heavy infestations can slow growth and stress the plant.

Life Cycle: Why They Spread So Fast

The entire fungus gnat life cycle, from egg to larva to pupa to adult, completes in just 3–4 weeks at typical indoor temperatures. That's why a handful of flies can become hundreds in short order. Eggs hatch within 4–6 days, larvae feed for about 14 days, pupation takes 3–4 days, and adults live for roughly a week. In that week, each female can lay another 200 eggs.

This speed is why a multi-pronged approach works best: you need to target adults and larvae simultaneously, and keep at it for at least 4–6 weeks to break the cycle completely.

Prevention

Preventing fungus gnats is easier than treating them. Most infestations start because the conditions are right, primarily consistently moist soil.

Watering Habits

The single most effective prevention method is letting the top 3–5 cm of compost dry out between waterings. Fungus gnat larvae need moisture in the upper soil layer to survive, and females won't lay eggs in dry soil. For moisture-loving plants like Calathea and Fern, consider bottom watering: place the pot in a tray of water for 20–30 minutes, then remove. This keeps the top layer dry while the roots still get what they need.

Soil Choice

Peat-heavy composts retain more moisture and contain more organic matter for larvae to feed on. Switching to a well-draining mix with plenty of perlite, bark, or pumice reduces the hospitable environment. For plants that tolerate it, growing in LECA eliminates the organic matter that larvae feed on entirely.

Quarantine New Plants

New plants, especially those from garden centres or supermarkets, are the most common source of fungus gnats entering a collection. Keep new arrivals isolated for 2–3 weeks and inspect the soil surface before placing them near existing plants.

Good Hygiene

Remove fallen leaves from the soil surface promptly. Don't leave saucers full of standing water. Keep compost bags sealed. If you have a worm bin or kitchen compost nearby, move it away from your plants. It's a breeding ground.

Treatment: Yellow Sticky Traps

Yellow sticky traps are the first line of defence against adult fungus gnats. They're attracted to the yellow colour and stick on contact.

Place the traps at soil level or just above, not at the top of the plant. Fungus gnats hover close to the compost, so a trap 30 cm above the pot won't catch many. Use one trap per 2–3 pots in a cluster, or one per larger pot. Replace them every 2–3 weeks or when they're covered.

Sticky traps alone won't eliminate an infestation. They only catch adults. But they dramatically reduce the breeding population while other treatments tackle the larvae below the surface.

Treatment: Neem Oil Soil Drench

Neem oil contains azadirachtin, which disrupts the growth cycle of fungus gnat larvae. It's an effective organic treatment when used as a soil drench rather than a foliar spray. The larvae are in the soil, not on the leaves.

How to apply: Mix 5 ml of cold-pressed neem oil with 1 litre of lukewarm water and a drop of washing-up liquid (this helps the oil emulsify). Water the mixture directly into the soil as you would a normal watering. Repeat every 7 days for 3–4 weeks to catch successive generations.

We stock Pepin Neem Oil, which is cold-pressed and works well for this. Avoid neem-based leaf shine products; they're not the same thing.

Treatment: Hydrogen Peroxide Soil Drench

A hydrogen peroxide drench kills fungus gnat larvae on contact by oxidising them, while also adding oxygen to the soil. It breaks down into water and oxygen, so it's safe for plant roots.

How to apply: Mix 1 part 3% hydrogen peroxide (the standard concentration sold in chemists) with 4 parts water. Water the mixture through the soil thoroughly until it drains from the bottom. You may see fizzing; that's normal and means it's working. Repeat once a week for 3 weeks.

This is one of the most effective treatments for heavy infestations. The key is using 3% H₂O₂. Don't use higher concentrations, as they can damage roots.

Treatment: Mosquito Bits (BTI)

Please note: BTI is not legally licensed for use in the UK. This advice only applies to those in countries where BTI is legally available.

Bacillus thuringiensis israelensis (BTI) is a naturally occurring bacterium that specifically targets the larvae of gnats and mosquitoes. It's sold as "mosquito bits" or "mosquito dunks" and is one of the most effective long-term treatments available.

How to apply: Soak a tablespoon of mosquito bits in a litre of water for at least 30 minutes (overnight is better). Strain out the bits and use the treated water to water your plants. The BTI is ingested by the larvae and kills them within 24 hours. Repeat with each watering for 4–6 weeks.

BTI is harmless to plants, pets, and beneficial insects. It only affects larvae of flies in the Diptera order. It's widely used in professional horticulture and is our go-to recommendation for persistent infestations.

Treatment: Sand, Grit or Pumice Top-Dressing

A physical barrier on the soil surface prevents adult females from reaching the compost to lay eggs. Apply a 1–2 cm layer of horticultural sand, fine grit, perlite, or pumice on top of the soil. This layer dries out quickly, creating an inhospitable surface for egg-laying.

This method works best as a preventive measure or alongside other treatments. On its own, it won't kill existing larvae already in the soil, but it stops new eggs being laid, which breaks the cycle over time.

Treatment: Home Remedies

Apple Cider Vinegar Trap

Fill a small jar or glass with apple cider vinegar and add a drop of washing-up liquid. The vinegar attracts the gnats; the soap breaks the surface tension so they drown. Place the trap next to affected pots. This catches adults and is useful alongside soil-level treatments, though it's less effective than yellow sticky traps on its own.

Cinnamon

Ground cinnamon has natural antifungal properties and can be sprinkled on the soil surface to deter egg-laying and reduce the fungi that larvae feed on. It won't kill existing larvae, but it makes the soil surface less attractive. Apply a thin, even layer and reapply after watering.

Chamomile Tea Drench

Brew a strong chamomile tea, let it cool completely, and use it to water your plants. Chamomile has mild antifungal properties that reduce the food source for larvae. It's gentle enough for regular use and works well as a complementary treatment.

Biological Controls

For larger collections or persistent problems, biological controls are the professional-grade solution.

Beneficial Nematodes (Steinernema feltiae)

These microscopic roundworms actively seek out and parasitise fungus gnat larvae in the soil. You can buy them from biological control suppliers in the UK. They arrive as a powder or gel that you mix with water and apply as a soil drench. They work best at soil temperatures above 10°C, so they're most effective from spring through autumn.

Apply in the evening (nematodes are UV-sensitive) and keep the soil moist for a few days after application so the nematodes can move through the compost. One application usually handles the problem, but a second treatment after 2 weeks ensures thorough coverage.

Predatory Mites (Hypoaspis miles / Stratiolaelaps scimitus)

These soil-dwelling mites feed on fungus gnat larvae and eggs. They establish a resident population in the soil and provide ongoing control. Put simply, they eat the gnats before they become a problem. They're particularly useful in greenhouses and large collections where prevention is the priority.

Chemical Controls

We'd always recommend trying cultural, biological, and organic methods first. But for severe infestations that aren't responding, insecticidal soap applied as a soil drench can help. Systemic insecticides are also effective but should be a genuine last resort. They can affect beneficial soil organisms and aren't necessary in most household situations.

Our Recommended Approach

After years of managing fungus gnats in our own nursery, this is the approach we recommend for most home growers:

Week 1: Place yellow sticky traps at soil level. Let the top few centimetres of soil dry out. Apply a neem oil soil drench. Top-dress with sand or grit.

Weeks 2–3: Continue neem oil drenches weekly. Replace sticky traps as needed. For heavy infestations, switch to or add hydrogen peroxide drenches.

Weeks 4–6: Maintain drier watering habits. Consider BTI-treated water for ongoing prevention. If the problem persists, introduce beneficial nematodes.

Ongoing: Bottom-water susceptible plants. Quarantine new additions. Use well-draining soil mixes. Keep sticky traps out as an early warning system.

Products That Help

These are products we stock at GrowTropicals that are directly useful for preventing and treating fungus gnats:

• Pepin Neem Oil 60ml: cold-pressed, effective as a soil drench for larvae
• LECA (10–20mm): inorganic growing medium that eliminates the larvae's food source
• Potting Mixes: our aroid and chunky mixes drain faster than standard compost

Related Reading

• Alocasia Care Guide: a genus particularly prone to fungus gnats due to its moisture needs
• Shop Houseplants
• Shop Potting Mixes