To prepare your urban planter box for winter, start with a thorough cleanup. Pull out spent annuals, dead vegetables, weeds, and any diseased foliage. Harvest the last herbs, greens, or root crops before hard frost. If you grow perennials, remove only damaged or messy growth; leaving some stems can help catch snow and protect crowns, but trim anything that could trap moisture against the soil.
Next, check the container itself. Clear drainage holes so melting snow and rain can escape. Raise the box on pot feet, bricks, or a wooden pallet so it does not sit in standing water. If the planter is terracotta, thin plastic, or not frost-proof, move it to a sheltered spot or wrap the sides with burlap, bubble wrap, or old blankets. Group pots together and place them against a wall to reduce wind and temperature swings. On balconies and roofs, secure everything against strong winter gusts.
Improve the soil before the hardest freezes. Remove the top inch or two of old soil if it is crusted or diseased, then top up with compost. Do not feed with high-nitrogen fertilizer; plants are not actively growing. After the soil has frozen, add a 2- to 3-inch layer of winter mulch, such as straw, shredded leaves, pine bark, or evergreen boughs. Mulch helps keep root temperatures steady and prevents repeated freeze-thaw cycles. For tender perennials, an extra mound of mulch around the crown can add protection.
Water carefully through winter. In many urban climates, winter is wet, but containers under eaves or on windy roofs can dry out. Water only when the soil is dry and the temperature is above freezing, and do so earlier in the day. Never let the planter sit in a saucer of water. If you expect deep cold, insulate the outside of the container and avoid walking on or compacting the soil.
Finally, protect plants from salt, wind, and heavy snow. A burlap screen can block wind without sealing in moisture. Avoid piling salty snow from roads or sidewalks onto the planter. Brush off heavy snow if it threatens to break stems, but leave a light layer as insulation. Check the box once a month, and keep your tools clean and stored. With a little preparation, your urban planter box will come through winter ready for spring planting.