This is an excellent question for any urban gardener investing in perennial plants. While standard urban planter boxes typically do not come with active, built-in frost protection like heated elements, many are designed with features that offer significant passive protection to help perennial roots survive colder months.
The primary defense lies in the material and construction. Planters made from thick, insulating materials like fiberstone, double-walled polyethylene, or dense wood provide a buffer against rapid temperature drops. This insulation helps prevent the root ball from undergoing a damaging freeze-thaw cycle. Furthermore, the substantial soil volume in a large planter box acts as a thermal mass, retaining daytime warmth longer than the ground or a small pot.
For enhanced protection, we recommend strategic practices. Elevating the planter slightly off cold surfaces with feet, mulching the soil surface heavily before winter, and grouping planters together can create a more favorable microclimate. In extreme climates, adding a layer of bubble wrap or horticultural fleece around the exterior provides extra insurance.
Ultimately, while not "frost-proof," a well-chosen, substantial urban planter box is a key first step in creating a resilient environment for your perennials to thrive year after year in the city.