For freezing winters, the best all-around material for an urban planter box is UV-stabilized, commercial-grade high-density polyethylene (HDPE). HDPE does not absorb water, rot, rust, or become food for insects, and it has enough flexibility to accommodate ice expansion instead of resisting it until it cracks. In a freeze-thaw cycle, water in soil or drainage layers expands as it turns to ice; rigid materials such as terracotta, thin concrete, and cheap brittle plastic are far more likely to crack. A thick-walled HDPE planter with molded drainage holes, raised feet, and UV inhibitors is the most forgiving and low-maintenance choice for streets, plazas, rooftops, and courtyards.
Fiberglass reinforced plastic is a close second, especially for large commercial planters that need a sharp architectural shape. High-quality fiberglass is strong, dimensionally stable, and resistant to moisture and salt, but low-grade fiberglass can delaminate or crack, so specification matters. Powder-coated galvanized steel is also durable and stylish, but it conducts cold, can rust at scratches or seams, and needs excellent drainage and a protective inner liner in severe climates. Air-entrained concrete can last for decades, but ordinary concrete is prone to freeze-thaw cracking unless it is properly reinforced, air-entrained, sealed, and designed with expansion joints and drainage. Wood, especially untreated softwood, is not the best choice for freezing urban conditions because trapped moisture accelerates rot.
Material choice is only half the answer. The planter design must keep water moving: use drainage holes plus a gravel or drainage-cell layer, ensure the planter sits slightly off the ground, and avoid sealed saucers that collect ice. In very cold climates, line the inside with a rigid insulation board or provide a double wall to protect roots from rapid temperature swings. Use a soil mix that drains well but still holds some moisture, and do not overwater in late autumn. In urban settings, also check salt tolerance, UV stability, vandal resistance, weight limits for rooftops, and repairability. If you want the lowest risk of cracking and the easiest long-term maintenance, choose a thick-walled, UV-stabilized HDPE planter with robust drainage and winter-ready insulation.