Start by finding exactly where the water is leaving your planter. Water stains usually come from drainage holes, overflow during watering, a cracked liner, or water sitting under the box and dripping through. Test with a small, controlled watering session and ask a neighbor or use a mirror or camera to watch underneath. Once you know the exit point, fix that path rather than just adding more soil or gravel.
The most reliable fix is a drip tray or saucer that is wider and deeper than the planter, with the planter lifted on pot feet or spacers so air can circulate. Empty the tray after rain and after watering. For a larger urban planter box, use a tray fitted with a drain nipple and a short hose that leads into a sealed jug or, if building rules allow, a balcony drain. Never let the hose end drip freely over the edge.
Line the inside of the box with a durable waterproof liner, such as pond liner or a heavy-duty planter liner. Bring it up the sides, slope it gently toward the drainage hole, and seal around the outlet with a proper bulkhead fitting. Do not seal the box completely unless it is a self-watering design; trapped water will eventually find a seam. If the planter is wood or metal, seal joints and check for rust or rot.
Reduce runoff by watering slowly and using a moisture meter or the finger test before watering. Use a free-draining mix with bark, perlite, or coarse compost, keep the soil level below the rim, and avoid overfertilizing, because fertilizer and soil particles are what leave dark stains. A self-watering planter with a reservoir can cut down on daily overflow, but empty and clean the reservoir regularly.
Add a waterproof mat or deflector under the tray, and route any overflow into a container rather than onto the balcony floor. Check the setup after heavy rain and before watering, empty trays, flush drainage holes, and replace clogged fabric or gravel. If you rent or live in a strata building, confirm that any hose or drain modification is allowed and make sure runoff does not enter a neighbor's space. With a tray, a liner, elevation, and smarter watering, you can keep the balcony below dry and stain-free.