How to get rid of houseplant flies

Houseplant flies are a menace to gardeners of any skill level. 

You may find these small, black flies (also known as ‘sciarid flies’) hovering around your houseplants. They love damp compost and seed trays, and while they won’t cause any actual damage to your plants, too many of them will become a nuisance.

Fungus gnats lay tiny larvae in the top layer of your compost (the first 5–8cm), and these worm-like creatures may feed off of your plant’s roots. 

Some plants can cope with this minor damage, but if you’re growing seedlings (or your plant is particularly weak), their larvae can be very harmful. So, if you’re ready for a deep dive on how to get rid of houseplant flies, stay with Whitaker's Garden Centre! 

We’ll answer some of your most asked questions, like ‘why do fungus gnats appear near my plants?’, and ‘will these flies go away on their own?’ (spoiler alert: no, they won’t). 

Why do fungus gnats appear near my plants? 

Before learning how to get rid of houseplant flies, you’ve got to learn the ‘why’. Why are they here, and what encourages them? 

1. Overly damp potting mix

Are you killing plants with kindness? If you’re prone to over-watering, be aware that this is the ideal environment for fungus gnats to lay eggs and for their larvae to develop. 

Moist potting mix (damp topsoil, specifically) will be very attractive to houseplant flies, particularly when the soil doesn’t have any time to dry out. 

2. Presence of decaying organic debris

A fungus gnat’s favourite pastime is nibbling away on organic debris. Any decaying leaves or rotting root matter in the soil of your houseplant will provide plentiful nourishment. 

3. Inadequate drainage

A common theme here is moistness: any damp, wet environment will be attractive to houseplant flies. 

So, overwatering or inadequate drainage traps water in the growing medium and under the pot tray: with the bottom layer of soil persistently wet, houseplant flies will come in droves. 

4. Introduction via newly acquired or outdoor plants

Have you accidentally helped houseplant flies move in? Indoor houseplants brought indoors from outdoor settings may already harbour eggs or early‑stage larvae in the potting soil. 

The fungus gnat populations may then appear soon after being moved inside.

5. Attraction to light around shaded or bright plant areas

We all want the best for our houseplants, and often, that means placing them near a bright source of natural light. Well, like plants, adult gnats are also drawn to light and may hover near plant leaves or windows where there is an abundance. 

How to prevent houseplant flies 

So, you want to know how to get rid of houseplant flies: where to start? Before working your way through this list of preventative measures, we recommend gathering the following tools and materials: 

Tools and materials 

• Yellow sticky traps or glue cards
• Mosquito bits or mosquito dunks containing Bacillus thuringiensis israelensis (Bti)
• Beneficial nematodes (e.g. Steinernema feltiae)
• Fine gravel, horticultural sand or small pebbles
• Insecticidal or dish‑soap spray (washing‑up liquid solution)
• Food‑grade diatomaceous earth

1. Water less often

First, it’s time to develop a more regular watering routine. No more daily watering: restricting the frequency to once a week (or once every two weeks, depending on the plant) ensures that the soil surface becomes dry between sessions. 

Doing this denies fungus gnats the moist conditions they need to lay eggs. If you’re in the habit of top-watering, instead, water your plants from the bottom via the pot’s saucer where possible so that the surface stays drier (and is less attractive to flies).

2. Gravel mulch

The best way to discourage houseplant flies from laying larvae on your topsoil is with a layer of fine gravel, horticultural sand or small chippings. Plus, when watering is done from below, the mulch will stay dry and unattractive to adult flies. 

3. Sticky traps 

Plant owners can position sticky traps at soil level to capture adult fungus gnats as they emerge or hover above the plant pot. 

These bright yellow sticky cards (which can be purchased from most gardening retailers) don’t reach larvae, but by removing adults, you reduce egg laying and slow population growth.

4. Biological control

When you choose biological control to tackle houseplant flies, you’re addressing the larval stage directly in the soil. 

Products containing Steinernema feltiae nematodes introduce microscopic roundworms into your soil via a watering solution that then drips into the compost. 

Once there, they locate fungus gnat larvae and naturally release symbiotic bacteria inside them, which digest the larvae from within and interrupt the gnat life cycle. Or, there are solutions containing Bacillus thuringiensis israelensis (Bti). 

This naturally occurring bacterium releases proteins that target and stop larvae in their digestive system when ingested. 

Will fungus flies go away on their own? 

The short answer is: no, they won’t. Unless you apply our methods, they can continue to breed and multiply for months on end. 

Shop for gardening tools and materials with Whitaker’s Garden Centre! 

Now you know exactly how to get rid of houseplant flies, it’s time to stock up on supplies! Luckily, Whitaker’s Garden Centre is your one-stop shop. With plenty of pest disease control products and gardening tools, you can keep on top of your houseplant care. 

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