Fungus Gnats: How to Get Rid of Them Organically

Ah, the perks of an indoor garden in winter: Fruit, flowers, fragrance, and…fungus gnats? If you are the parent of any indoor plant, the chances are good that you have already met fungus gnats. These small flies are non-buzzing and non-biting, but an annoyance in large numbers because they are attracted to moisture, even if is from your breath, or around your eyes. Swat, swat. Worse, if you do see a lot of the tiny, winged varmints, it means that their even tinier but much hungrier larvae are living in your pots, eating organic matter that includes the roots of your plants, which they damage.

Getting rid of fungus gnats is a three-step process. This is what has worked for me.

Photography by Marie Viljoen.

Fungus gnats are a warning sign: They thrive in moist environments, and their presence indicates you might be overwatering your plants. Long-term, this can lead to their slow death. So take the gnats’ presence as a helpful hint, and then banish them. While the adults do not feed on anything, they do lay eggs. And it is their larvae that do the quiet, subterranean damage.

Where do fungus gnats come from? Like scale insects indoors, they seem to materialize from the ether. A possible source of fungus gnats in your home is the new plant you just bought, whether it’s a seasonal poinsettia, Christmas amaryllis, or your kitty’s fresh wheatgrass from the pe(s)t store. The insects are a common pest in professional greenhouses. Because new plants might be carriers of fungus gnats, if it is practical, keep them apart from your established, unaffected plants for a period of three weeks. (The four-stage fungus gnat life cycle is about three weeks, from egg, to larva, to pupa, to adult fly.)

They could also be present in your growing media, in egg or larval form. Most potting mixes are sterilized, but it’s hard to know for sure.

Here are the three steps to getting rid of fungus gnats.

Step 1: Do not overwater.

This is a permanent care-protocol in eliminating fungus gnats: Only water your plants (deeply) when they have come close to drying out. I have been an indoor grower now for over a decade and you’d think I’d know better, but I am still prone to overwatering. A moisture meter’s long probe is able to give me a better sense of what is happening in the soil, not just in the top inch or so.

Allowing your pots to dry between waterings helps prevent a fungus gnat infestation, and it also keep your plants healthier: Root rot is caused by overwatering and can be fatal. It’s hard to turn that around, so the pesky gnats are the canary in the coal mine for indoor growers. Unlike the poor canary that keels over in bad air, they thrive in the unhealthy environment.

Step 2: Use yellow sticky traps.

Adult fungus gnats are attracted to yellow. Place sticky yellow traps in your pots. The traps effectively sequester the adult gnats, interrupting their relentless life cycle by preventing them from laying eggs in your soil. I choose traps that are the least offensive aesthetically (pretty shapes!) and change them when I can’t stand seeing the bodies pile up. Aside from catching the bugs, the traps are a good indicator of infestation, even when you have controlled the problem. I keep them to warn me of potential fungus gnat re-emergence.