For an urban planter box that must survive weather, pollution, irrigation, and occasional neglect, the best all-around material is usually UV-stabilized HDPE, or high-density polyethylene. It cannot rust because it contains no iron, and it resists cracking far better than concrete, terracotta, wood, or thin composite plastics. Quality HDPE is flexible enough to handle freeze-thaw movement, impact, and root pressure without splitting. It also will not rot, corrode, or attract insects. Look for thick walls, UV inhibitors, and molded drainage channels or drainage holes. Cheaper recycled plastic can become brittle after years of sun exposure, so UV stabilization matters.
If you want a more architectural or premium look, powder-coated marine-grade aluminum is the best metal option. Aluminum does not rust like steel. Instead, it forms a protective oxide layer, and the powder coating adds color, scratch resistance, and protection from urban pollution. It is rigid, lightweight, and will not crack like concrete or ceramic. The main risk is that chips in the coating can lead to pitting in salty or alkaline environments, especially near coastal roads. Choose 5052 or 6061 aluminum and a high-quality powder coat if you go this route.
Stainless steel 304 or 316 also will not rust under normal urban conditions, and 316 is better for coastal areas. However, it is expensive, can become very hot in full sun, and may show scratches. It is durable, but it is not the best value for most residential or commercial planter projects.
Materials to avoid if rust and cracking are your main concerns include mild steel, galvanized steel, corten steel, untreated wood, terracotta, and standard concrete. Mild steel and galvanized steel can rust once the coating fails. Corten steel is designed to rust. Wood can crack, rot, and warp. Terracotta and concrete can crack with freeze-thaw cycles, settling, or impact. Fiberglass and composite planters can work well, but low-quality versions may crack or chalk over time.
For the lowest-risk choice, use UV-stabilized HDPE with good wall thickness and drainage. For a sleek, modern metal planter that will not rust or crack under normal use, choose powder-coated aluminum. No material is completely indestructible, but these two offer the best balance of durability, weather resistance, and low maintenance for urban environments.