A rotten-egg smell after heavy rain almost always means one thing: your planter soil has gone anaerobic. When water fills the box and the drainage holes are blocked or too small, oxygen disappears. Anaerobic bacteria break down organic matter and release hydrogen sulfide, which smells like rotten eggs. The fix is to get air and drainage back into the root zone, then prevent the same problem next storm.
Start right after the rain. Tip out any water in the saucer or tray. Clear leaves and debris from the top of the soil and around the drainage holes. Use a screwdriver, wire, or bamboo stake to gently open clogged holes. If water still sits, carefully lift the planter or tilt it to drain. Do not let it stand in a puddle.
Check the box design. Most urban planters need several drainage holes, not just one. If the bottom is solid, drill additional holes about 1/4 to 3/8 inch wide. Cover them with a fine mesh or coffee filter so soil does not wash out and block them. Raise the planter on pot feet, bricks, or wooden blocks so water can flow freely underneath. If the planter sits flat on concrete, water often has nowhere to go.
Improve the soil. Heavy garden soil compacts and holds water like a sponge. Use a light potting mix with perlite, pine bark, coir, or compost. These materials create air pockets and let excess water drain. Avoid pressing the soil down hard when planting. If the mix has been waterlogged for days, gently fork the top few inches to add oxygen. Remove any rotten roots, dead plants, or slimy organic matter.
Water smarter. Before a forecast of heavy rain, skip watering. After a storm, wait until the top inch or two of soil feels dry before watering again. A thin layer of mulch can reduce splashing, but do not pile it deeply around stems, because that can keep the crown wet and cause rot. If the planter has a saucer, empty it after every rain.
If the smell is very strong, like sewage or rotten eggs, and the soil is black and slimy, the anaerobic problem is advanced. Repot the plants with fresh, well-draining mix, wash the empty planter with soap and water, and check whether the smell is coming from a nearby drain, downspout, or sewer backup. In that case, keep the planter raised and consider a location away from runoff.
Finally, think about the plant. Most herbs and flowering annuals hate soggy roots. Choose plants that match your climate, and do not overcrowd them. Good drainage, airy soil, and regular checks after storms will keep your urban planter box fresh, healthy, and free of that rotten-egg smell.