Yes, in many cases you can leave an urban planter box outside all year, but it is not automatically safe for every pot, plant, or climate. The decision depends on the planter material, drainage, plant hardiness, wind exposure, and how cold and wet your winters are. If the planter is frost-proof, has good drainage, and contains plants suited to your winter conditions, leaving it outside is often the simplest and healthiest option. If it is fragile, poorly drained, very small, or holds tropical plants, moving it inside or into a sheltered space is usually better.
First, check the planter itself. Frost-proof plastic, fiberglass, wood, and well-made metal containers are generally more tolerant of freezing weather. Terracotta, thin ceramic, and decorative glazed pots can crack when wet soil expands during freeze-thaw cycles. A planter box that is built in, very heavy, or fixed to a balcony or rooftop should be prepared for outdoor winter conditions rather than moved. Make sure it has drainage holes and that water cannot sit in a saucer or trapped liner. Raise the container slightly off the ground with pot feet or blocks so water can escape and the base does not freeze to the surface.
Next, consider the plants. Hardy shrubs, perennials, small trees, and many ornamental grasses can stay outside in a container if they are rated at least one hardiness zone colder than your local garden zone. Roots in containers are more exposed than roots in the ground, so a plant that survives in soil may still struggle in a pot. Tender annuals, tropical houseplants, citrus, herbs from warm climates, and other frost-sensitive plants should be brought inside or moved to a frost-free garage, sunroom, or sheltered balcony before the first hard freeze.
Winter care matters. Keep the soil moist but not soggy; evergreens in containers can suffer from dry winter wind even when the soil is cold. Apply a layer of mulch over the soil surface, wrap the planter with burlap or horticultural fleece if wind is severe, and group pots together for extra insulation. Avoid bringing a heavy, soil-filled planter indoors unless you can give the plants enough light and reasonable humidity. Indoor heating can dry out leaves, and pests or disease can spread in still air. If you do bring a planter inside, place it in a cool, bright room and reduce watering.
In short, leave an urban planter box outside all year if it is frost-resistant, drains freely, is protected from harsh wind, and contains hardy plants. Bring it inside, or move it to a sheltered spot, if the pot is fragile, the plants are tender, or your winters bring deep freezes and repeated freeze-thaw cycles. A little preparation in late autumn will protect both the planter and the plants through winter.