If you want the least plastic-looking urban planter box, the best answer is usually a cast stone or concrete planter with a matte, sandblasted or acid-etched finish. It has the weight, mineral texture, and subtle color variation of real stone, so it reads as natural rather than molded. A weathered Corten steel planter is a close second: its rusted patina ages organically, though it feels more earthy-industrial than garden-natural. Solid timber planters—cedar, teak, or oak—also feel authentic because of the grain and silvering, but they need a liner and more maintenance in wet urban conditions. Unglazed terracotta looks very natural but can crack in freezing weather.
For the most natural feel with urban durability, choose cast stone first, Corten steel second, and real wood third. Avoid glossy fiberglass, polyresin, PVC, high-gloss powder-coated metal, and composite "wood" with printed grain; those are the finishes that most often look plastic. Look for matte surfaces, visible texture, slight tonal variation, and simple forms. A planter with a rough limestone finish, thick walls, and good drainage will feel like a natural part of the street or patio, not a plastic tub.