Fungus gnats in an urban planter box are usually a sign of persistently moist soil and rich organic matter. The adults are tiny black flies that hover around the soil, while the larvae live in the top few inches and feed on fungi, algae, and soft roots. They are often more annoying than deadly, but a heavy infestation can stress seedlings and young plants.
Start by letting the top 2 to 3 cm of soil dry out between waterings. Fungus gnat larvae need moisture, so this is the most important step. Check that your planter box drains freely, empty any saucers or trays, and make sure water is not sitting at the bottom. If possible, water from the bottom or water thoroughly and then wait until the surface is dry again. Increase airflow around the box with a small fan or by moving it to a breezier spot, and remove fallen leaves, algae, and decaying plant debris from the surface.
Use yellow sticky traps near the soil to catch adults and reduce egg-laying. Traps alone will not solve the problem, but they help you monitor progress. For the larvae, apply a Bti product such as mosquito bits or dunks according to the label. Soak the product in water as directed and use that water on the soil, repeating as recommended. Beneficial nematodes, especially Steinernema feltiae, are another option in containers. Avoid harsh home remedies that may harm roots or beneficial soil life.
Adjust your soil and watering habits to prevent a comeback. Do not use garden soil indoors or in balcony boxes, and use fresh, well-draining potting mix. If you add compost, make sure it is fully finished. After the surface has dried, a thin top-dressing of coarse sand or grit can help discourage adults from laying eggs, but it is not a substitute for proper watering. In a severe case, repot the plants with fresh mix, wash the container, improve drainage holes, and discard the old infested soil away from other plants. With consistent drying, trapping, and Bti treatment, fungus gnats are usually manageable within a few weeks.