An urban planter box in January is exposed to cold air, wind, and often cold concrete or balcony surfaces, so the goal is not usually to keep every bit of soil unfrozen. In most climates, a hard freeze is unavoidable. The practical goal is to stop the box from freezing into a solid, damaging block, protect roots, and prevent cracks or waterlogged soil from making things worse.
Start with drainage. Before the coldest weather, check that drainage holes are open and that the box is not sitting in a saucer of frozen water. Raise the planter slightly on bricks, pot feet, or wooden blocks so it is not in direct contact with cold concrete. Clear snow away from drainage holes so meltwater can escape. If water cannot drain, it can freeze, expand, and crack the planter or suffocate roots.
Insulate the sides, not just the top. Wrap the outside of the box with bubble wrap, burlap, old blankets, straw, or cardboard, especially if it is metal or thin plastic. Wood insulates better, but it still benefits from extra protection. Leave the drainage holes unobstructed. For a raised urban planter, you can also pack straw or leaves around the outside in a temporary frame, but keep the material dry if possible.
Mulch the soil surface. Add 2 to 4 inches of straw, shredded leaves, bark chips, or evergreen boughs over the soil. This slows heat loss from the soil and reduces repeated freezing and thawing. Keep mulch a little away from plant stems and crowns so they do not stay wet and rot.
Group and shelter the box if you can. Move smaller containers close together, place them against a wall, and use a temporary windbreak such as a piece of plywood or a row of sturdy bins. In an urban setting, wind and reflected cold from concrete can be more damaging than air temperature alone. Avoid putting the planter where road salt or de-icing spray will reach it.
Water carefully before a freeze. Dry soil freezes faster and can be more damaging than slightly moist soil, but soggy soil is worse. Water before a hard freeze only if the soil is dry, and never water frozen soil. Do not let snowmelt collect in the box. If the planter is under a roof or balcony, check it during mild spells and water lightly only if it is dry.
Cover plants and the box during extreme cold. A frost blanket, old sheet, or double layer of burlap over a simple hoop or frame can keep the worst cold off. Vent the cover on sunny days so condensation does not build up, and remove it when temperatures rise above freezing. For small boxes, a cloche or low mini-tunnel works well. Light snow can actually insulate the box, so do not brush it all away unless it is heavy enough to break plants or the planter.
Finally, accept that hardy plants are supposed to slow down in January. Choose frost-tolerant plants for urban boxes, and bring tender plants indoors if possible. If the box does freeze solid, do not pry at it or water it. Let it thaw gradually and check drainage afterward. With better drainage, side insulation, mulch, and a little shelter, an urban planter box can come through January without cracking or losing its root system.