Yes, in many cases you can leave an urban planter box outside through winter, but it is not a simple yes for every planter, plant, or climate. The most important factors are the planter material, drainage, and the hardiness of the plants inside. Plastic, resin, fiberglass, and many metal planters are generally more winter-tolerant than terracotta, glazed ceramic, thin wood, or poorly sealed concrete, which can crack during freeze-thaw cycles. If your planter is decorative, antique, or not labeled frost-resistant, the safest choice is to empty it, clean it, and store it in a shed or garage.
For a frost-proof planter that will stay outside, make sure the drainage holes are open and never blocked. Raise the box on pot feet or bricks so water can escape and the base does not sit in ice or soggy ground. Remove saucers before winter, because trapped water can freeze and damage both the planter and the roots. In very cold climates, wrap the outside with burlap, bubble wrap, or a thick layer of straw, and mulch the soil surface to insulate roots. Group planters together in a sheltered spot away from harsh wind, but avoid placing them where snow from a roof will dump onto them.
Remember that plants in containers get much colder than plants in the ground. Even hardy perennials may struggle if the soil freezes solid for long periods. Choose compact, cold-hardy plants for urban planter boxes, or treat tender plants as seasonal and replant them each year. Check evergreens during dry, mild spells and water lightly when the soil is not frozen. Remove heavy snow from the planter to prevent crushing, but leave a light layer as insulation. If an extreme cold snap is coming, move the planter to a garage, porch, or unheated shed until conditions improve.
So, can you leave an urban planter box outside through winter? Yes, if it is frost-proof, well drained, and planted appropriately. If it is fragile, waterlogged, or holding tender plants, bring it inside or protect it carefully. A little preparation now can save both the planter and its plants when spring arrives.