Whether you should bring your urban planter boxes inside for winter depends on several key factors: the hardiness of your plants, your local climate, and the construction of the boxes themselves.
First, identify your plants. Tender perennials, tropicals, and many herbs cannot survive freezing temperatures and must be brought indoors. Hardy perennials, shrubs, and trees rated for your climate zone can often remain outside, though they need protection from extreme cold snaps.
Your local winter severity is crucial. In mild climates with rare, light frosts, leaving boxes outside with some protective mulch may suffice. In regions with sustained freezing temperatures, snow, and harsh winds, moving boxes to a sheltered spot like a garage, shed, or indoors is often necessary for plant survival.
The planter box material matters. Terracotta and ceramic pots can crack in freeze-thaw cycles. Wood, thick plastic, and fiberglass offer better insulation. Ensure all containers have excellent drainage to prevent waterlogged soil from freezing solid and damaging roots.
For boxes staying outside, group them together against a sheltered wall, wrap the containers with burlap or bubble wrap, and apply a thick layer of mulch over the soil. This insulates the root zone. Water plants moderately before a deep freeze, as dry soil gets colder faster than slightly moist soil.
Ultimately, the safest strategy for non-hardy plants is to move them indoors to a cool, bright location like a sunroom or under grow lights. For hardy plants in sturdy containers, proper outdoor winterization can be successful. Assess your specific conditions to make the best choice for your urban garden's health through the colder months.