Fungus gnats: those tiny flies around your plant soil, and how to get rid of them

Small black flies drifting up from the pot every time you water. They don't bite, they don't seem to do much, and they're everywhere. Those are fungus gnats, and while the adults are only a nuisance, the larvae in the soil are eating your plant's fine roots.

The good news is they're the easiest pest to get rid of, because they have one weakness and it's a big one.

What they are

Fungus gnats are tiny, weak-flying insects about 3mm long, dark grey or black, with long legs. They look a bit like small mosquitoes. You'll see them walking on the soil surface, sitting on the inside of the pot, or flying in a lazy zigzag when you disturb the plant.

The adults live about a week and lay eggs in the top layer of damp soil. The larvae hatch within a few days and feed on fungus, decaying matter, and tender root hairs for about two weeks before pupating. A pot can go through a full generation in under a month, which is why a few flies in March becomes a cloud by May.

The one thing they can't survive

Dry soil. Fungus gnat larvae need the top inch or two of the soil to stay damp. If it dries out, they die. That's the whole treatment.

Most gnat problems in Pakistani homes come from watering on a schedule instead of checking the soil. Every third day, every Friday, whatever the habit is. In a room with the AC on, or in winter, the soil in a 10 inch pot can stay wet for a week or more, and the gnats breed the whole time.

So the first fix is to stop watering until the top two inches are dry to the touch. Push a finger in. If it's damp, wait. For most houseplants this is also just better watering, and you'll see the plant improve for reasons that have nothing to do with the gnats.

If the plant can't cope with drying out that much, water from the bottom instead. Stand the pot in a tray of water for twenty minutes, let the soil pull up what it needs, then take it out. The bottom stays moist, the top stays dry, and the gnats have nowhere to lay.

Catching the adults

Yellow sticky traps are cheap and they work. Push one into the pot so the yellow card sits just above the soil. Within a day it'll be covered in gnats. That doesn't fix the problem on its own, because the eggs are already in the soil, but it stops the adults laying more and it tells you how bad the infestation is. When new traps stay mostly clean for a week, you're done.

If you can't find sticky traps, a shallow dish of apple cider vinegar with a drop of dish soap in it, placed next to the pot, catches a fair number.

Dealing with the larvae

Drying the soil handles most of them. For a stubborn pot, or one where you can't let the soil dry fully, two things help.

Neem cake in the soil. This is why The Master Blend has neem cake in it. Scratch a tablespoon into the top inch of soil and water it in once. The bitter compounds in neem cake stop larvae feeding and developing. It also feeds the plant, so it's not a wasted step. If you'd rather use it on its own, plain neem cake powder does the same job at the same rate.

A neem oil drench. Mix water-soluble neem oil at 5ml per litre and water the soil with it, enough to wet the top few inches. Do it once, then again a week later. This reaches larvae that are deeper than the neem cake gets.

Stopping them coming back

Gnats usually arrive in a bag of potting soil or a new plant from a nursery. Fresh compost and coco peat often have eggs in them already. When you repot, let the mix dry out before you use it, or spread it in the sun for an afternoon.

Remove dead leaves and any decaying matter from the soil surface. That's what the larvae feed on when there aren't enough root hairs.

A thin layer of dry sand or fine gravel on top of the soil, about a centimetre deep, makes it hard for adults to reach the damp soil to lay eggs. It also looks tidy.

And keep checking the soil before you water. That one habit prevents more gnat problems than every product combined.

How long it takes

Let the soil dry, put in a trap, add neem cake. Adults stop appearing within about ten days, because that's how long the larvae already in the soil take to finish their cycle with nothing to feed them. If you're still seeing plenty of new flies after three weeks, something is keeping the soil wet: blocked drainage, a saucer full of water, or a pot with no drainage hole at all. Fix that and they go.

Not sure if what you've got is gnats or something else? Send a photo of the flies on the soil to WhatsApp at 0370-9145600.