In most cases, yes. If your urban planter box is wooden and the soil will sit directly against the timber, a liner is a practical way to protect the wood and extend the planter’s life. Soil stays constantly moist after watering or rain, and in an urban setting—balconies, rooftops, courtyards—planters often face wind, heat, and limited drainage. That combination encourages rot, warping, and staining, even in rot-resistant woods such as cedar or pressure-treated timber.
A liner creates a barrier between wet soil and wood. It also reduces soil loss through drainage holes, keeps roots away from seams, and makes it easier to refresh the soil. For most wooden urban planter boxes, use a durable waterproof liner such as EPDM, HDPE, or a heavy-duty pond liner. If you plan to grow edibles, choose a liner and sealant labeled food-safe and avoid materials that can leach harmful chemicals.
But a liner only helps if the planter still drains well. Before lining, drill or clear weep holes and create a slight slope toward them. Line the box, press the liner into corners, and cut it so it sits above the final soil line rather than trapping water behind the wood. Add a drainage layer of gravel or lightweight aggregate, cover the drainage holes with geotextile to stop clogging, and leave clear channels for water to escape. Never create a sealed, watertight box with no outlet; trapped water can cause more damage than bare wood.
You may not need a separate liner if the wooden box is only a decorative sleeve around a plastic or metal pot, or if it already has a built-in waterproof insert with drainage. You also do not need a liner if the planter is designed as a sealed, self-watering system—but check that its overflow works. For direct-soil wooden planters, however, a liner is usually worth the small extra cost.
In short: line your urban wooden planter box when soil touches the wood, but pair the liner with generous drainage, air circulation, and a raised base. That combination protects the timber and keeps your plants healthier.