For an urban planter box that gets both rain and direct sun, the best material is one that resists rot, UV damage, and repeated wet-dry cycling. In most city settings, powder-coated aluminum and UV-stabilized HDPE are the most reliable all-round choices. Powder-coated aluminum is lightweight, strong, rust-resistant if the coating is intact, and easy to specify in light colors that keep roots cooler. Choose a marine-grade powder coat and stainless-steel fasteners; avoid bare steel and untreated galvanized edges that can corrode.
HDPE is another excellent option: it will not rot, resists UV when stabilized, is easy to clean, and works well for food-growing. Double-walled HDPE adds insulation against hot sun. Composite planters are also practical—look for capped composite with UV inhibitors and a solid core—but cheaper composites can fade or sag, so check load ratings and warranty.
If you want wood, use rot-resistant hardwood such as cedar, teak, or iroko, or thermally treated ash. Wood looks warm and ages well, but it needs ventilation, a liner, and periodic oiling. Avoid untreated pine and low-grade pallet wood.
Fiberglass is lightweight and strong, with a UV-stable gelcoat; it is good for large rooftop planters but may chalk over time. Concrete and stone are durable and heavy—useful for stability on windy roofs—but they are hard to move, can raise soil pH, and should be sealed. Terracotta is breathable but can crack in freeze-thaw cycles and dries out quickly in sun.
Whatever material you choose, drainage matters more than the box itself. Use drainage holes, a sloped base or feet, a gravel or air layer if appropriate, and a liner that keeps soil off metal or wood. Raise the planter off paving so water can escape, and choose a light color or insulate the sunny side to protect roots. With good drainage, UV-stable finishes, and a suitable liner, aluminum, HDPE, composite, or rot-resistant wood will give the longest, lowest-maintenance service in rain and sun.