Whether your urban planter boxes need to come inside for the winter isn't a simple yes or no answer. It depends on a combination of crucial factors: what you're growing, what the box is made of, and how severe your winters are.
First, consider your plants. Tender perennials and most annuals cannot survive freezing roots and must be brought indoors. Hardy perennials, shrubs, and trees often can stay out, but their survival hinges on the planter's size and insulation. A large, deep box provides more root protection than a small, shallow one.
Next, evaluate your planter material. Porous materials like terracotta and concrete can crack when wet soil inside freezes and expands. These often require extra protection or moving to a sheltered spot. Durable materials like fiberglass, thick plastic, or treated wood are more frost-resistant.
Your local climate is the final deciding factor. In mild winters, many plants in robust containers will be fine. In areas with deep, prolonged freezes, even hardy plants in large pots risk root death. The key is preventing the root ball from freezing solid.
Instead of automatically moving heavy boxes, try these winterizing strategies. Group containers together in a sheltered, sunny spot against a building wall for warmth. Insulate the sides with bubble wrap or burlap and mulch the soil surface heavily. For valuable plants, moving them to an unheated garage or shed is an excellent middle ground.
Ultimately, listen to your plants and your weather. By understanding these dynamics, you can make an informed decision to ensure your urban garden thrives again in spring.