Algae on the outside of an urban planter box usually means the surface is staying damp long enough for spores to settle and grow. It is rarely a serious problem, but it looks untidy and can make the planter slippery. The key is to remove the conditions algae like: moisture, light, and a thin layer of dust, soil, or fertilizer on the surface.
Start by checking drainage. Clogged drainage holes, overflowing saucers, or water running down the outside of the planter will keep the surface wet. Clear the holes, empty saucers after rain, and make sure the planter is slightly raised so air can move underneath. If you use drip irrigation, aim the emitters at the soil rather than the rim or the wall, and avoid overwatering. Water that escapes from the top or drains too slowly is one of the most common causes of exterior algae.
Improve airflow around the planter. Leave a small gap between the planter and walls or railings, trim dense plants that trap humidity, and avoid packing several planters tightly together. Better airflow helps the exterior dry faster after rain or watering. You can also add mulch to the soil surface to reduce splashing, but keep mulch away from the planter rim so it does not hold moisture against the outside surface.
Clean the exterior regularly before algae becomes established. Use a soft brush with mild soapy water, then rinse and dry the surface. For light algae, a diluted vinegar solution can help, but test it on an inconspicuous area first, especially on metal, painted, or natural stone surfaces. Avoid harsh abrasive cleaners that can scratch the finish; scratches trap dirt and make future cleaning harder.
Choose or maintain the right material. Non-porous planters such as glazed ceramic, resin, or sealed metal dry faster and are easier to wipe down. Porous materials like terracotta, concrete, and untreated wood can hold moisture, so use them in well-ventilated spots and consider a suitable sealant if the manufacturer allows it. Reapply sealant as recommended, and repair cracks or chipped coatings where water can collect.
Finally, keep fertilizer and soil runoff off the exterior. Wipe up spills promptly, direct drainage away from the planter wall, and give the outside a quick dry brush every few weeks. With better drainage, less splashing, good airflow, and routine cleaning, algae will have far fewer chances to settle and grow.