This is a common concern for urban gardeners. The answer is: it depends heavily on the material of your planter box.
Not all materials are created equal. Concrete, fiberglass, and high-quality, thick polyethylene or polypropylene planters are generally frost-resistant and can withstand freeze-thaw cycles without cracking. They are designed for year-round outdoor use. Traditional terracotta or ceramic pots, however, are porous and can absorb water. When this water freezes, it expands and often causes the material to crack or shatter. Wooden planters can also be susceptible to rot and splitting in extreme cold and wet conditions.
Beyond material, proper drainage is non-negotiable. Even a frost-proof box needs excellent drainage holes to prevent water from pooling at the bottom and freezing around plant roots. For added plant protection in freezing temps, consider using bubble wrap or horticultural fleece to insulate the soil and roots. In very harsh climates, moving sensitive planters to a more sheltered spot or a garage during the coldest weeks is a wise precaution.
In summary, yes, many modern urban planter boxes are built to be left outside year-round, even in freezing temperatures. The key is to choose boxes made from durable, weather-resistant materials and ensure they have adequate drainage. Always check the manufacturer's specifications for "frost-proof" or "weather-resistant" guarantees to be sure.