Gnats in houseplants are annoying. Known as fungus gnats, they’re actually small flies drawn to moist potting soil and decaying plant material at the base of indoor plants. While they look similar to mosquitoes, they don’t bite.
Because gnats in houseplants typically appear when the potting mix contains too much moisture, the best way to prevent gnats from taking over your home is to avoid over-watering in the first place.
But what if the damage is already done, and you’re dealing with a swarm of pesky flies surrounding your plants? Here, we walk you through the best ways to get rid of gnats in your houseplants.
KEY TAKEAWAYS
- Fungus gnats are caused by excess moisture in the soil
- They don’t bite, but their larvae can damage plant roots
- Adult gnats are attracted to light and moist soil
- The most effective way to prevent gnats is to avoid overwatering
- There are three simple natural solutions to get rid of gnats: DIY vinegar traps to catch adult gnats, sticky traps and indoor fly traps.
- Eliminating adult gnats is only a short-term fix.
- Drying out the soil helps kill eggs and larvae.
What are fungus gnats?
Fungus gnats (family Sciaridae, most commonly Bradysia species) are small, mosquito-like flies about 2–3mm long. They’re dark grey or black with long legs, long antennae, and a distinctive Y-shaped vein pattern on their wings.
Their life cycle has four stages: egg, larva, pupa, and adult, and it’s the larval stage that causes the most damage. Fungus gnat larvae live in the top few centimetres of moist potting mix, feeding on fungi, organic matter, and sometimes plant roots.
Adult gnats don’t feed on plants at all, but they do lay eggs, starting the cycle all over again.
What causes fungus gnats in your house?
Overwatering and moist soil
The number one cause of fungus gnats in houseplants is overwatering. Fungus gnats are drawn to damp potting mix because it creates the perfect conditions for the fungi and organic matter their larvae feed on. If your soil stays wet for days after watering, you’ve created an ideal breeding ground.
New plants bringing larvae indoors
Fungus gnats often hitchhike into your home on new plants purchased from nurseries or garden centres. The eggs and larvae are invisible to the naked eye and can be present in the potting mix even when the plant looks perfectly healthy. This is also common when plants that have spent the summer outdoors are brought back inside in autumn.
Organic matter in potting mix
Standard potting mixes are rich in organic material, bark, peat, and compost, which fungus gnat larvae love. Even if you’re watering correctly, a particularly rich or moisture-retentive potting mix can contribute to the problem.
How to identify gnats in indoor plants
One way to tell if you’re about to run into a gnat problem is to look for eggs. Fungus gnats lay eggs in the soil, and these eggs become larvae, which feed on fungi in the soil of plants. Fungus gnat larvae are around 1/4-inch long, with a shiny black head and an elongated, whitish-to-transparent body.
In addition to fungi, they like organic matter and will sometimes eat plant roots or seedlings; the plant will appear wilted. Look for a slime trail similar to the kind slugs and snails leave behind. If you can see a trail, there are likely gnats in your indoor plants.
They also like light, so you may notice them around your windows, particularly if houseplants are nearby.
Gnats vs fruit flies
Gnats are often confused with other garden pests, like the common fruit fly, but the two insects are completely different. Fruit flies will linger mostly around fruit, and unlike fungus gnats, are tan in colour and look like oval, miniature versions of the house fly.
Are fungus gnats harmful to plants or people?
Fungus gnats don’t bite humans and aren’t harmful to people. However, a heavy infestation can damage your plants. Fungus gnat larvae feed on the fine root hairs that plants use to absorb water and nutrients, which can cause wilting, yellowing leaves, and stunted growth, especially in seedlings and younger plants.
Mature, healthy plants can usually tolerate a mild infestation, but it’s still worth treating the problem before populations build up.
How to get rid of gnats
If you notice signs of gnats, it’s important to take steps to get rid of them right away.
While you could play the long game and buy a few carnivorous house plants to combat the problem, there are three easier ways to quickly eradicate adult gnats flying around your indoor plants.
1. Create a DIY gnat trap using vinegar
- Place a tablespoon of sugar in a bowl.
- Add 2-3 drops of sweet-smelling dish soap and a cup of white vinegar to the same bowl. Cover the bowl tightly with cling wrap.
- Poke several small holes in the cling wrap. Place the bowl near the house plant and let it catch flies overnight.
2. Use sticky fly traps
If the smell of vinegar puts you off creating your own DIY gnat trap, another option is to buy sticky gnat traps. These traps can be hung from a plant branch or placed directly in the soil to catch unwanted critters. By reducing the adult population, you’ll also reduce the number of eggs laid in the soil of your plants.
Check the traps every few days and replace them when they become covered with gnats. Avoid touching the plant leaves with the sticky paper.
Keep an eye out for non-toxic, double-sided traps for maximum effectiveness.
3. Let the soil dry out
This is the single most effective thing you can do. Fungus gnat eggs and young larvae cannot survive in dry soil.
Allow the top 3–5cm of your potting mix to dry out completely between waterings. For most indoor plants, this won’t cause any harm, and it will kill off a significant portion of the larval population within a week or two. Be consistent: even a single premature watering can restart the cycle.
4. Use an indoor fly catcher
Not your first time dealing with gnats in your houseplants? Then you may want to invest in an indoor fly-catching device.
These devices are usually USB-powered and use a combination of LED lights and fans to attract and suck flies into the trap.
How to prevent gnats in indoor plants
While getting rid of adult gnats is a great first step, that’s often only a short-term fix. More adults will appear from the larvae in the soil. A better approach is to target the larval stage of their life cycle by allowing the soil to dry out. Because gnats lay their eggs in the moist soil around plants, reducing excess moisture is a key to getting rid of these nuisances for good.
- Avoid over-watering – Allow the soil to dry between regular watering, not to the point that your plant begins wilting, but enough that the soil isn’t continually moist. The eggs and larvae usually die in dry soil.
- Choose a pot with good drainage – A pot with good drainage will prevent gnats from laying eggs in the soil and will also protect your indoor plant against common problems such as root rot. Remember to drain any excess water that may have accumulated in saucers.
Other ways to prevent fungus gnats
Fungus gnats are often more noticeable in autumn. It may be that they hitch a ride on plants when they’re brought indoors at the end of summer.
Before bringing plants inside, check them to make sure they’re free of insects. Before you purchase new plants, examine them to make sure there are no insect infestations. Use a sterile potting mix when planting or re-potting.
Fungus gnats FAQs
The fastest results come from combining two approaches: letting the soil dry out to kill larvae and placing yellow sticky traps to catch adults.
The most common reason gnats appear is overwatered houseplants. Moist potting soil is the perfect environment for fungus gnats to lay their eggs. Other causes include bringing a new plant indoors that already had eggs in its soil, or moving outdoor plants inside at the end of summer.
Gnats are repelled by strong scents, including peppermint, lavender, eucalyptus, cinnamon, neem oil, and citronella. Cinnamon is the most useful option for indoor plants, specifically.
