Yes, you can use an urban planter box year-round in a climate with harsh winters, but it requires more planning than using it in a mild climate. The planter itself should be sturdy, frost-resistant, and large enough to protect plant roots from rapid temperature swings. Wood, thick plastic, metal with insulation, and composite materials can all work, but thin containers are more likely to freeze through. Good drainage is essential: drill clear drainage holes, add a drainage layer if needed, and raise the box slightly off the ground so water can escape instead of freezing around the roots.
Insulation also makes a major difference. You can line the inside with bubble wrap, foam, or another insulated layer, and mulch the soil surface after planting. In very cold regions, choose hardy perennials, shrubs, dwarf evergreens, or winter-interest plants that match your climate zone. Avoid tender plants unless you can move the planter indoors or into a sheltered area. A location protected from strong wind but still able to catch winter sun is usually best.
Winter care matters as much as the setup. Snow can insulate plants, but heavy snow should be brushed off gently to prevent breakage. Do not let the soil stay waterlogged, and water sparingly during mild spells when the soil is not frozen. Avoid de-icing salt near the planter, and use frost blankets or burlap around sensitive plants when extreme cold is forecast. If the box is on wheels or casters, you can move it to a more sheltered spot during the worst weather.
In short, an urban planter box can absolutely be used year-round in harsh winters, but success depends on a durable container, reliable drainage, insulation, climate-appropriate plants, and consistent winter maintenance.