Fungus Gnats in Potted Herbs: How to Eliminate and Prevent Them

Fungus gnats are one of the most frustrating pests for anyone growing herbs indoors. They’re small, fast-moving, and seemingly come out of nowhere. While adult gnats don’t damage plants directly, their larvae feed on organic matter in the soil, including delicate herb roots. This can lead to stunted growth, yellowing leaves, and even plant death in severe cases.

These gnats thrive in damp environments and often become a problem when herb pots are overwatered or lack proper drainage. Fortunately, there are effective, chemical-free ways to eliminate them and prevent their return.

How to Identify a Fungus Gnat Problem

Fungus gnats are tiny, mosquito-like flies that hover near the soil surface. If you disturb the pot and see a few small flies take off, or if you notice them crawling on the surface, it’s likely you’ve got them.

Unlike fruit flies, which are attracted to sugary surfaces and food scraps, fungus gnats focus on moist soil and organic-rich potting mix. Their larvae are tiny and live beneath the soil surface, where they feed on roots, fungi, and decaying organic matter.

One clear sign of a serious problem is if your herb starts growing slowly, wilts unexpectedly, or begins to yellow without any clear reason. At that point, the larvae may already be damaging the roots.

How to Get Rid of Fungus Gnats Naturally

Start by reducing how often you water your herbs. Fungus gnats thrive in moist conditions, so allowing the top inch of soil to dry between waterings is one of the most effective ways to disrupt their life cycle. Switching to bottom watering can help the surface stay dry while still hydrating the roots.

Sticky traps are another useful method. Yellow sticky cards placed near the soil can catch adult gnats before they lay eggs, which interrupts the reproduction cycle. You can place them vertically or lay them flat on the soil’s surface.

If the infestation is persistent, consider adding a layer of coarse sand or horticultural grit over the soil. This creates a dry barrier that makes it difficult for adult gnats to lay eggs and for emerging larvae to reach the surface.

For a more active treatment, dilute hydrogen peroxide (3%) with water in a 1:4 ratio and pour it over the soil. It will kill larvae on contact but is safe for the plant when properly diluted.

Neem oil can also help, but it must be applied as a soil soak, not just as a foliar spray. Mix it with water and a few drops of mild soap and apply every few days until signs of gnats disappear.

Preventing Future Infestations

Good drainage is key. Make sure every pot has drainage holes and avoid using overly dense, moisture-retaining soil mixes. Adding perlite or sand can improve aeration.

Only water herbs when the top layer of soil is dry to the touch. Never let your herbs sit in standing water.

If you're growing from seed, use sterile seed-starting mix to reduce the risk of bringing in larvae from the start. When bringing new plants indoors, isolate them for a few days to monitor for pests before placing them near your other herbs.

Lastly, be cautious with compost or organic matter in indoor pots. While great for feeding plants, they can attract gnats if used in excess or not properly decomposed.