Whether your urban planter boxes need to come inside for the winter isn't a simple yes or no answer. It largely depends on three key factors: the hardiness of the plants you're growing, the material of your planter boxes, and the severity of your local winter climate.
For the plants themselves, the rule is straightforward. Tender perennials and tropical plants cannot survive freezing temperatures and must be brought indoors. This includes popular choices like geraniums, begonias, and citrus trees. However, many plants are surprisingly resilient. Ornamental kale, pansies, certain evergreen shrubs, and even some hardy herbs can withstand a frost and often thrive in cooler weather.
The planter box material is critical. Terracotta and ceramic pots are porous and can absorb water. When this water freezes, it expands, often cracking or shattering the container. These should be moved to a sheltered spot like a garage or shed. Conversely, planters made from wood, durable plastic, fiberglass, or metal (if treated to prevent rust) are less susceptible to freeze-thaw damage and can usually remain outside.
Your local winter conditions are the final deciding factor. In regions with mild winters where temperatures rarely dip below freezing, most well-drained planter boxes can stay outdoors with some protection for sensitive plants. In harsh climates with prolonged deep freezes, the roots of even cold-hardy plants are vulnerable because they lack the insulating buffer of the ground. In this case, providing winter protection is essential.
Instead of always bringing boxes inside, consider these practical strategies. You can group planter boxes together in a sheltered corner against a building wall for warmth. Insulating the containers by wrapping them in bubble wrap or burlap can protect the roots. Applying a thick layer of mulch on top of the soil also helps to regulate temperature. Ultimately, understanding the needs of your specific plants and planters will guide you to the right decision for a thriving garden come spring.