The best way to clean an urban planter box before replanting is to treat it as both a physical cleaning job and a basic sanitation job. Urban planter boxes collect more than old soil: they can gather soot, road dust, cigarette butts, pet waste, salt, mold, and plant pathogens. Start by putting on gardening gloves and, if the box is very dirty or has been exposed to bird droppings or waste, a simple face mask. Remove all old plants, roots, and as much old soil as possible. Do not dump old soil into a storm drain or onto the street; bag it and dispose of it according to local rules, especially if you suspect contamination.
Next, scrape away compacted soil, mineral crust, and dried grime from the sides and bottom with a sturdy trowel or scraper. Check the drainage holes and clear them with a small brush or wire. Wash the inside and outside with warm water and a mild unscented dish soap or castile soap, using a stiff scrub brush. Avoid harsh household cleaners, because residues can harm new plants. If the planter has had diseased plants, mold, or animal waste, disinfect it with a dilute bleach solution of one part household bleach to nine parts water, or use a plant-safe disinfectant. Let the solution sit for about ten minutes, then rinse the box thoroughly and repeatedly. Bleach must be completely rinsed away before replanting, and the box should be left to dry in the sun if possible.
After cleaning, inspect the planter for cracks, rust, loose liners, or poor drainage. Repair or add drainage material if needed, but avoid blocking the holes. When the box is fully dry, refill it with fresh, high-quality potting mix rather than reusing the old soil. Add compost if appropriate, plant your new plants, and water gently. Cleaning your tools and gloves afterward is also part of good planter sanitation. This process takes a little extra time, but it gives replanted urban planter boxes a cleaner, healthier start.