How do I get rid of fungus gnats in my houseplants?

Fungus gnats are a nuisance, not a danger. They do not bite people or pets, and they are not living on your child, so there is no need to treat hair or skin. They breed in damp compost, and they hang on for months because most fixes hit only the adults or only the larvae. You need both at once, kept up for a few weeks.

If you have been at it for a year, you have not failed. The usual tips are just aimed at the wrong half of the problem.

Why they keep coming back

Adult gnats lay eggs in the damp top layer of compost. The larvae feed there, out of sight, then hatch as new adults. Swatting, vacuuming or trapping adults thins the cloud, but the next lot is already growing in the pots. Repotting into new compost helps for a while, until it gets wet and new eggs go in.

Moving the plants outside does not clear the gnats already in the room. Those adults live out their short lives on ceilings and windows. With no damp compost indoors to breed in, they fade.

The plan that works

1. Let the top of the compost dry out between waterings. Push a finger in: if the top feels damp, wait. Water from below by filling the saucer, then tip away what is left after a while. Do not leave pots standing in water.
2. Catch the adults on yellow sticky cards. Push one into each problem pot or hang it near the plants. They catch the fliers and show you, week by week, whether numbers are falling. Swap them when they are full.
3. Treat the larvae with a biological control. The established options are beneficial nematodes, microscopic worms sold for fungus gnats and watered into the compost, and products based on Bti, a bacterium used against gnat larvae. Follow the label on how much, how often and how to store it. One dose is rarely enough: repeat as the label directs.
4. Do every pot at the same time. One untreated plant will keep supplying adults to the rest.
5. Check the extras. Drip trays with standing water, a decorative outer pot holding water at the bottom, an open bag of compost, seed trays. Keep spare compost sealed and dry.

Keep going until the cards stay mostly empty for a couple of weeks. Then stay strict about watering, because damp compost will draw them back.

Kids and pets: yellow sticky cards have no poison, but the glue grabs fingers, hair and fur. Put them high in the pot or on plants out of reach, and keep curious cats away from them. Read the label of any biological control for children and pets, and store it out of reach. Lice or skin treatments do nothing for gnats, so leave them out.

What people try, and why it falls short

• Cinnamon or matchsticks in the soil. Popular tips, but they are not established treatments. A year of trying them is a good sign to switch to the plan above.
• A layer of sand or grit on top. A dry surface can make pots less inviting for egg laying, but it does nothing for larvae already in the compost. Use it with the plan, not instead.
• Drying out and then soaking. Drying works only if it becomes the new habit. One dry spell, followed by a big watering, sets the cycle going again.
• Insect spray. It hits the adults in the air, not the larvae in the pot. If you use any product on plants, pick one labelled for houseplants and follow the label.

If they still will not go

After a few weeks of dry tops, sticky cards and a full course of biological control, numbers should be well down. If the cards keep filling, check whether these really are gnats. Small flies in a bathroom or by the sink may be drain flies, which need a different fix. See how to tell the small flies apart, and if it is the drain, how to clear drain flies.

A good garden centre can also advise on your specific plants and which biological control suits them. Bring a caught gnat on a card.