That smell is usually a sign that the soil has stayed wet for too long and has started to go anaerobic. In an urban planter box, trapped water, blocked drainage holes, dense potting mix, and buried organic matter can all create the odor of rotting roots. The solution is not just to cover the smell; you need to dry out the root zone, restore airflow, and remove anything that is already decomposing.
First, check the drainage. Lift or tilt the planter box and make sure every drainage hole is clear. If the box has no holes, drill some if possible, or repot into a container that does. Empty any saucer or tray that holds standing water, and raise the box slightly so water can escape freely.
Next, stop watering for a while. Let the top two to three inches of soil dry out before you water again. Use your finger or a moisture meter to check below the surface. When you do water, water deeply until it drains, then pour away any excess. In urban planters, it is easy to overwater because the box may look dry on top while the lower soil is still soaked.
Aerate the soil gently. Poke a few holes into the top layer with a chopstick or thin stake, being careful not to damage healthy roots. This helps air reach the root zone and speeds drying. Remove dead leaves, fallen flowers, and any uncomposted food scraps or thick layers of mulch that may be trapping moisture and rotting.
If the smell is strong or the soil stays wet, inspect the roots. Slide the plant out and look for brown, mushy, foul-smelling roots. Trim those away with clean, sterilized scissors, then repot in fresh container mix. Choose a light, airy mix amended with perlite or coir rather than heavy garden soil, and make sure the new pot or box drains well.
Improve airflow around the planter too. Thin crowded plants, move the box to a breezy spot if possible, and avoid sealing it against a wall where air cannot move. If you use a self-watering planter, flush the reservoir and adjust it so the soil is never constantly saturated.
To prevent the problem from returning, water based on soil moisture rather than a strict schedule, keep drainage holes clear, and refresh the top layer of soil each season. If the odor persists after drying and repotting, empty the box, scrub it, disinfect it with a diluted bleach solution, rinse thoroughly, and start again with fresh mix. A healthy urban planter should smell of damp earth, not rot.