Whether you can leave your urban planter boxes outside all winter depends on several key factors: your local climate, the plants you're growing, and the construction of the boxes themselves. Generally, hardy perennial plants, shrubs, or ornamental grasses in large, frost-resistant containers can often survive outdoors with proper preparation. The main threats are the plant's roots freezing in the limited soil volume and damage from freeze-thaw cycles.
To successfully overwinter your planter boxes, start by choosing plants rated for at least one USDA zone colder than your area. Ensure your containers are made of durable materials like fiberglass, metal, or thick wood that can expand without cracking. Terracotta and thin plastic often shatter. In late fall, stop fertilizing, water deeply before the ground freezes, and apply a thick layer of mulch on the soil surface. For extra protection, group containers together in a sheltered spot, wrap them in burlap or bubble wrap, and consider elevating them off cold pavement with pot feet. For very tender plants or in extremely cold regions, moving containers to an unheated garage or shed is the safest option. With the right preparation, you can enjoy your urban garden's structure and even some greenery throughout the winter months.