The key is to keep the root zone from sitting in water that freezes and thaws repeatedly. Start with a container that has large drainage holes and use a coarse, well-draining potting mix, not garden soil. Raise the planter box on feet, bricks, or a pallet so it is not sitting directly on cold concrete or pavement. In a rooftop or balcony setting, wind and reflected cold make this even more important.
Insulate the sides of the planter box, not just the soil surface. When you insulate planter box sides with bubble wrap, burlap, blankets, straw, or rigid foam board, leave drainage holes and the base open so water can escape. A thick layer of straw, leaves, or bark mulch on top—about 3 to 4 inches—will protect plant roots and slow heat loss. Keep mulch a little away from stems to avoid rot. For container gardening in winter, group pots together and place them against a sheltered wall or windbreak; this reduces wind chill and shared warmth helps.
Water carefully. Before a hard freeze, check the soil: if it is dry, water deeply so roots are not stressed, but never leave the box waterlogged. In winter, water less often and only when the soil is thawed and slightly dry. Do not water a frozen planter or let water pool on the surface. If snow falls, it can insulate, but brush heavy snow off brittle branches.
Choose plants that are cold-hardy enough for your area, remembering that roots in containers are more exposed than roots in the ground. In many climates, choose plants rated one or two USDA zones colder than your garden zone. Move smaller or movable planters into an unheated garage, shed, or sheltered porch during the coldest spells. Avoid bringing hardy plants into a warm room, which can break dormancy too early.
To prevent planter freeze damage, do not fertilize in late autumn and avoid encouraging soft new growth. A temporary cold frame, wind screen, or extra blanket during extreme cold can help, but make sure air can still circulate. Check drainage after freezing rain. The goal is not to keep the box warm all winter; it is to keep it from freezing solid, cracking, or holding ice around the roots. With good drainage, insulation, shelter, and suitable plants, good urban planter box winter care can help your planter box come through winter safely.