If you want the shortest answer: UV-stabilized, rotomolded HDPE, or high-density polyethylene, is the best all-around material for an urban planter box that won't crack in cold. In freezing weather, the real enemy is not just low temperature; it is the expansion of wet soil as it turns to ice. A rigid, brittle planter can split under that pressure. HDPE stays slightly flexible in low temperatures, resists impact, absorbs freeze-thaw movement, and does not rust or rot. Look for commercial-grade HDPE with UV inhibitors and thick walls, not thin decorative plastic.
Fiberglass-reinforced plastic, often called FRP, is a close second. High-quality FRP planters are strong, lightweight, and resistant to weather, but cheap fiberglass can crack or delaminate after years of UV exposure and freeze-thaw cycles. If you choose fiberglass, pick a reputable maker with a gelcoat finish and reinforced corners.
Other materials? Glazed ceramic, terracotta, and thin resin are among the worst choices in cold climates because they are brittle and can crack when water freezes. Concrete can last for decades, but only if it is air-entrained, properly reinforced, and allowed to drain; otherwise it cracks. Untreated wood can rot and check, while cedar or redwood performs better but still needs care. Metal, such as powder-coated aluminum or galvanized steel, does not crack easily, but it can dent and conduct cold. Use a liner and drainage to protect roots and reduce soil expansion damage.
The material matters, but design matters just as much. Choose a planter with drainage holes, a small reservoir or gravel layer, and feet to keep it off freezing pavement. Never let a planter sit in a saucer of water in winter. Use a lighter, well-draining potting mix rather than dense garden soil, and avoid packing it tightly. In extremely cold zones, insulate the sides with foam boards or move the planter to a sheltered spot.
So, for most urban gardeners, rotomolded HDPE is the best balance of cold resistance, durability, weight, and cost. For a premium look and extra rigidity, high-quality fiberglass is a strong alternative. Avoid brittle ceramics and cheap plastics, and add proper drainage: that is how you keep an urban planter box from cracking through winter.