For coastal urban settings, the planter box that generally holds up best against salt air is a high-quality fiberglass-reinforced plastic planter with a UV-stable gelcoat and marine-grade 316 stainless steel fasteners. Fiberglass does not rust, resists chloride-laden air, is lightweight enough for rooftops and streetscapes, and needs little maintenance. It also tolerates repeated damp-dry cycles better than many metals. The key is to choose a reputable manufacturer, because cheap fiberglass can chalk, fade, or crack if the resin and gelcoat are not designed for outdoor UV exposure.
A close second is powder-coated marine-grade aluminum, especially when all cut edges are sealed and fittings are 316 stainless. It is strong, relatively light, and corrosion-resistant, but chips in the coating can create vulnerable spots if not repaired. Powder-coated steel can perform in coastal areas if it is galvanized, properly primed, and regularly maintained, but it is less forgiving. Corten steel is usually a poor choice close to the sea because salt accelerates corrosion and staining. Concrete is durable but heavy and can raise pH or leach over time. Timber needs frequent sealing or should be specified as rot-resistant and marine-grade.
For the best long-term result, choose a planter with raised feet, good drainage, a sealed liner, and drainage holes that keep roots from sitting in salty water. Avoid mixing dissimilar metals, and specify 316 stainless hardware rather than 304 where possible. Rinse planters occasionally, inspect coatings, and repair scratches promptly. In short, if you want the lowest-maintenance urban planter box for salt air near the coast, start with UV-stable fiberglass and 316 stainless hardware. Choose marine-grade aluminum when a metal finish is required.