Mushrooms appearing in an urban planter box can look alarming, but in most cases they are simply breaking down dead organic matter in the soil. They are usually a sign that the potting mix is staying moist and contains something fungal-friendly, such as wood chips, old roots, or compost. The first step is not to panic and not to eat them. Remove the mushrooms safely by wearing gloves, pulling or twisting them out at the base, and placing them in a sealed bag in the trash. If you are sensitive to mold or spores, wear a dust mask and keep children and pets away until they are gone.
Next, address the conditions that encouraged them. Check your planter box drainage. Make sure drainage holes are open, remove any saucer or tray where water stands, and elevate the box slightly if possible. Water less often and allow the top inch or two of soil to dry between waterings. Urban planters often stay wet because they are sheltered from rain and wind, so improve airflow by thinning crowded plants and moving the box to a brighter, airier spot if the plants can tolerate it. Remove fallen leaves, rotting plant debris, and old mulch from the surface, because these materials feed fungi.
If mushrooms keep returning, inspect the roots and soil. Soggy, sour-smelling soil usually means the mix is waterlogged and may need replacing. Repot the plants in fresh, well-draining potting mix, clean the container, and add perlite or coarse material if drainage is poor. Do not use fungicides as a first response; they are rarely necessary for harmless decomposer mushrooms and can disrupt the soil. If the mushrooms are large, unusual, or growing near edible plants and you are unsure what they are, take a photo and ask your local extension service or a qualified mycologist. In short: remove them safely, reduce excess moisture, improve drainage and airflow, and refresh the soil if the problem persists.