For a planter box that sits outside all year in an urban setting, the most durable practical material is powder-coated galvanized steel. If budget is truly unlimited, 316 stainless steel is the ultimate choice for corrosion resistance, but it is rarely the best value for most streetscape, plaza, or rooftop projects. For the best balance of strength, weather resistance, weight, and cost, powder-coated galvanized steel is the material I would specify.
Powder-coated galvanized steel works well because the galvanized core resists rust while the powder coating protects the surface from UV exposure, scratches, and urban grime. It holds its shape through freeze-thaw cycles, resists impact and vandalism better than many lightweight materials, and is easy to clean. It also accepts a wide range of colors, which matters when a durable urban planter box needs to fit a design scheme. To maximize planter box durability, choose a sufficiently thick steel gauge, specify a high-quality powder coat, and make sure the design includes drainage holes, a slightly sloped internal base, and raised feet so water never sits against the bottom.
Other materials can be good, but each has trade-offs. Fiberglass is lightweight and weather-resistant when made with a quality gelcoat, though it can crack under heavy impact. Concrete is extremely sturdy and can last for decades, but it is very heavy and may crack in freeze-thaw conditions unless properly reinforced. HDPE plastic will not rot or rust, but it can bow or fade over time unless it is UV-stabilized and well supported. Wood looks warm and natural, but even rot-resistant species need regular maintenance outdoors. Corten steel is durable, but its rust runoff can stain paving and nearby surfaces. Aluminum is light and corrosion-resistant, but it can dent more easily than steel.
For a year-round planter, the best outdoor planter material is not just about the raw substance; it is also about construction details. Good drainage, a breathable or removable liner where needed, sturdy corners, and a repaintable finish will extend service life more than a premium material with poor detailing. In most urban environments, powder-coated galvanized steel gives the longest reliable service life for the money, making it the most durable practical choice for a planter box that stays outside all year.