Start by thinking about where the planter sits. Urban planters often stand on balconies, rooftops, or paved courtyards, where cold comes from both the air above and the hard surface below. If the planter is movable, move it into a sheltered spot against a wall, under an overhang, or into an unheated garage or shed during the coldest spells. Group several planters together so they share warmth and block wind from one another. Raise pots slightly off cold concrete or stone using wooden blocks or pot feet, because a frozen surface can pull heat out of the root zone quickly.
Check drainage before you insulate anything. A planter that traps water is far more likely to freeze into a solid block and damage roots or crack the container. Clear drainage holes, add a drainage layer if needed, and make sure the planter is not sitting in a puddle. Soil should be moist going into winter, not soaking wet. Water deeply before a hard freeze if the soil is dry, because moist soil holds and releases heat more slowly than dry soil, but never leave water standing in the saucer or base.
Insulate the sides of the planter rather than sealing the top. For a fixed raised planter, line the inside walls with foam board or a few layers of bubble wrap, leaving drainage holes open. On the outside, wrap the sides with bubble wrap, burlap, or old blankets, and tie them securely without covering the drainage holes. For smaller pots, place them inside a larger pot and fill the gap with straw, dry leaves, or shredded paper. Do not use airtight plastic around the whole planter; it traps moisture and can encourage rot. Breathable insulation is better.
Add a thick layer of mulch over the soil surface after the soil has cooled. Straw, dry leaves, bark chips, or evergreen boughs help buffer the root zone from repeated freeze-thaw cycles. In snowy climates, let clean snow pile up around the planter as natural insulation, but avoid piling salty street snow near plants. A windbreak made from burlap, a low screen, or a row of sturdy pots can also reduce wind chill without blocking sunlight completely.
Adjust your care through winter. Avoid late feeding, because soft new growth is more vulnerable to cold. Do not keep the soil bone dry, but water sparingly on mild days if the planter is under an overhang and has not received rain or snow. Keep an eye on cold frames or covered planters on sunny days and vent them to prevent overheating and condensation. If extreme cold is forecast, wrap the planter sides, add extra mulch, and move small containers to a protected spot. With good drainage, side insulation, wind protection, and a breathable mulch layer, an urban planter box is much less likely to freeze solid and much more likely to bring its plants through winter safely.