Mold on the bottom of an urban planter box is usually a moisture problem, not a sign that your whole garden is doomed. The bottom is dark, often shaded, and stays wet the longest, so it is the first place mold appears. To stop it, you need to dry out that zone and keep it from staying wet.
First, check drainage. A planter box needs clear drainage holes at the bottom and, ideally, along the lower side walls. If your box has no holes, drill several 1/4- to 1/2-inch holes. Keep them free of soil, roots, and debris. If the box sits flat on concrete or a balcony floor, the holes may be blocked even when they exist. Raise the planter on pot feet, bricks, wooden rails, or plastic risers. You want at least 1 to 2 inches of air space underneath. This lets water escape and air move across the bottom, which is one of the simplest ways to stop mold.
Second, stop overwatering. Most mold on planter bottoms comes from soil that stays constantly wet. Water only when the top 1 to 2 inches of soil feel dry. A moisture meter can help if you are unsure. When you water, do it slowly and thoroughly until water runs out of the drainage holes, then stop. Do not give small daily sips, because that keeps the bottom soggy while the top may still look dry. If you use drip irrigation or a timer, reduce the run time and check the soil after a few days. After heavy rain, empty any saucer or tray that holds water. Never let the planter sit in a puddle.
Third, use the right soil mix. Heavy garden soil compacts in containers and holds water like a sponge. Use a coarse, airy potting mix made for containers. You can improve it with perlite, pine bark, or coco coir. These materials create air pockets and help excess water drain. If your planter is very deep, a false bottom or a coarse drainage layer can help, but only if the drainage holes are clear and the soil above is not too fine. Otherwise, water can perch above the coarse layer and keep the bottom wet.
Fourth, improve airflow and light. Mold loves still, humid, dark conditions. Prune crowded plants so air can move around the box. Leave a gap between the planter and walls or railings. If the planter is in deep shade, move it to a sunnier spot if the plants can handle it. In a covered balcony or indoor space, a small fan or dehumidifier can help. Remove fallen leaves and dead plant material from the soil surface, because they can hold moisture and feed mold.
Fifth, clean the mold safely. Put on gloves and a mask if the mold is extensive. Scrub visible mold from the bottom and lower sides with a 1:4 solution of white vinegar and water, or with 3% hydrogen peroxide. Let the surface dry completely. For wooden planters, sand lightly if needed and treat the wood with a food-safe wood preservative or sealant that is safe for plants. For plastic or metal planters, wash with mild soapy water, rinse well, and dry. If the mold is in the soil or mulch, remove the affected layer and replace it with fresh, dry material. Do not compost moldy soil near your plants.
Finally, prevent it from coming back. Check drainage holes monthly, especially after autumn leaves or roots have grown. Reduce watering in cool or cloudy weather. Keep mulch thin and away from the planter edges, or use a light inorganic top dressing like pebbles in problem spots. If the bottom keeps molding, consider replacing the liner or adding a ventilated false floor. In some cases, switching from a solid plastic or metal box to a breathable wooden or fabric planter can make a big difference.
Mold on the bottom of an urban planter box is a signal that moisture is trapped. Fix the drainage, raise the box, water less, and let air reach the underside. Once the bottom can dry out between waterings, mold will have a much harder time growing.