Tiny black flies hovering around your plants, crawling on the soil or bumping into your screen? Those are almost certainly fungus gnats. They're annoying but harmless to people, and with a few weeks of consistent effort you can get rid of them.

What are fungus gnats?

Fungus gnats are small, dark, mosquito-like flies about 2–3 mm long. They're weak fliers and tend to hang around the soil surface. (Fruit flies, by contrast, are tan with red eyes and hang around your fruit bowl.)

The adults are just a nuisance. The problem is their larvae, tiny clear worms that live in the top few centimetres of damp soil and feed on fungi and organic matter. In large numbers they can nibble on the fine roots of seedlings and young plants.

Why you have them

Fungus gnats need consistently damp soil to breed. That's why they're so common in winter, when plants use less water and soil stays wet longer. They can also arrive in new plants or bags of potting mix that have been stored open.

How to get rid of them: the 3-week plan

A gnat's life cycle is about 3–4 weeks, so you need to keep at it for at least that long to catch every generation. Combine these steps:

1. Let the soil dry out

This is the most important step. Let the top few centimetres of soil dry completely between waterings. Larvae can't survive in dry soil. A moisture meter helps you water only when the roots actually need it.

2. Bottom water

Instead of watering from the top, set the pot in a tray or sink of water for 15–20 minutes and let it soak up from below. The roots get a drink while the surface stays dry and unwelcoming for egg-laying adults.

3. Trap the adults

Stick yellow sticky traps into the soil of affected plants. They catch the flying adults before they can lay more eggs, and show you how the problem is going.

4. Treat the soil surface

Sprinkle diatomaceous earth over the dry soil surface. It's a natural powder that dehydrates larvae and adults. Reapply after watering from the top.

5. Add a top layer

A 1–2 cm layer of sand, fine gravel or bark on top of the soil makes it hard for adults to lay eggs. (Ask about sand at our Soil Bar.)

Bad infestation?

If a plant is swarming, repot it into fresh, dry mix, gently shaking off as much old soil as you can. A chunkier, faster-drying mix like Chunky Monkey (for aroids) or Arid Mix (for succulents) is much less gnat-friendly. Many gardeners also use a biological larvicide (BTI, sold as "mosquito bits") mixed into watering water.

How to keep them from coming back

• Water less often, and only when the soil needs it, especially in winter
• Use pots with drainage, and empty saucers and cache pots
• Store potting mix in a sealed bag or bin (our Soil Bar bags are resealable)
• Check new plants and keep them apart from your collection for a week or two
• Keep a sticky trap or two in your plants over winter as an early warning

Still seeing gnats after a month? Bring a sticky trap or a photo to the shop at 515 Chatham St #103 and we'll help you figure out what's going on. More winter tips in Winter Houseplant Care in Victoria.