Yes—in many cases, you can leave an urban planter box outside during winter, but it is not a simple yes for every planter or every climate. The safest answer depends on three main things: the planter material, the plants inside it, and how cold or wet your winters become. A frost-proof resin, fiberglass, or thick wooden planter is usually much safer outdoors than thin ceramic, terracotta, or fragile plastic, which can crack when soil freezes and expands. If the planter is decorative or vulnerable to ice, move it into a shed, garage, or sheltered corner before the first hard freeze.
Drainage is just as important as cold tolerance. Make sure the planter has clear drainage holes and is raised slightly off the ground so water cannot pool underneath. Winter wetness is often more damaging than cold alone, especially for roots. If the planter sits directly on a patio or balcony, use pot feet or small blocks to improve airflow. You can also wrap the outer sides with burlap, bubble wrap, or a layer of mulch to insulate the root zone without blocking drainage. Snow can act as natural insulation, but heavy snow and ice should be brushed off gently to avoid cracking or bending the planter.
What you grow matters enormously. Hardy shrubs, evergreens, ornamental grasses, and cold-tolerant perennials may be fine outdoors in a protected planter. Tender plants, tropicals, and small young plants should be moved indoors or into a frost-free garage if possible. In very cold regions, even hardy plants in containers are more exposed than plants in the ground because their roots are above the soil line. In that case, group planters together, insulate them, and place them against a sheltered wall away from harsh wind. Check the soil occasionally but water very little; winter watering should be rare and only when the soil is dry and temperatures are above freezing.
So, can you leave your urban planter box outside during winter? Often yes, if it is durable, well drained, sheltered from the worst weather, and planted with suitable species. If it is fragile, poorly drained, or holds tender plants, bring it inside or protect it carefully. A little preparation before winter will go a long way toward keeping both the planter and its plants healthy until spring.