Winterizing an urban planter box is mostly about protecting roots from freeze-thaw cycles, wind, and excess water. Container roots are far more exposed than roots in the ground, so even a hardy perennial may need extra help on a balcony, roof, or patio. Start by removing spent annuals, dead foliage, weeds, and fallen leaves from the soil surface. Cut back perennials only as recommended for the species, and leave ornamental grasses and seed heads if you want winter interest and wildlife value. Stop fertilizing well before autumn; late nitrogen encourages soft growth that winter damages.
Next, make sure the planter can drain. Clear drainage holes, remove saucers, and raise the box on pot feet, bricks, or a slatted stand so water cannot pool underneath. If the container is thin ceramic, terracotta, or not labeled frost-proof, move it under cover, nest it inside a larger plastic pot, or transfer the plants to a safer container before hard freezes. Never let an urban planter box sit in a puddle; wet soil expands as it freezes and can crack pots and suffocate roots.
If possible, move the planter to an unheated garage, shed, cold frame, or sheltered balcony corner. A cool, dark space that stays above freezing but below about 40°F (4°C) is ideal for many dormant perennials. Do not put dormant outdoor plants in a heated living room. If the planter must stay outside, group pots together against a wall or railing, out of the worst wind. Wrap the outside of the box with bubble wrap, burlap, blankets, or straw, but keep the material breathable and do not block drainage. For extra root insulation, use a layer of straw, leaves, or evergreen boughs over the soil after the first hard freeze; keep mulch a little away from plant crowns to avoid rot and rodents.
Water before the soil freezes and check monthly during thawing spells. Evergreens and newly planted perennials can suffer winter burn, so give them a deep drink on mild days if the soil feels dry. Protect plants from drying wind with a burlap screen, but avoid sealing them inside plastic. Brush heavy snow off gently so branches do not break, and never use rock salt around an urban planter box; use sand or a pet-safe de-icer instead.
In early spring, uncover pots gradually, remove excess mulch as growth resumes, and check for cracked containers, pests, and rotten roots. Water when new growth appears, but wait to fertilize until active growth is underway. With good drainage, insulation, shelter, and a little winter watering, your urban planter box and its plants have a much better chance of coming through until spring.