Whether an urban planter box requires a liner depends primarily on the box's construction material and your goal for plant health and longevity. For wooden planter boxes, a liner is highly recommended. Untreated wood will quickly rot from constant soil moisture, while pressure-treated or chemically treated woods may leach harmful substances (like copper, chromium, or arsenic in older treatments) into the soil, which can be absorbed by edible plants. A durable plastic liner creates a crucial barrier, protecting the wood and isolating the soil from potential toxins. Ensure the liner has drainage holes punched in the bottom to match the box's drainage system.
For planters made from inert, non-porous materials like fiberglass, metal (with a rust-proof coating), or high-quality plastic, a liner is often unnecessary as these materials are inherently safe for direct soil contact and won't degrade quickly. The primary consideration shifts to drainage. Regardless of material, excellent drainage is non-negotiable. Without proper drainage holes and a layer of coarse material like pebbles or pot shards at the bottom, water will accumulate, leading to root rot and killing your plants.
In summary, use a perforated liner for wooden boxes to preserve the structure and ensure soil safety. For safe, non-porous materials, you can plant directly, but always prioritize a robust drainage system. This approach protects your investment and creates a healthy root environment for thriving urban plants.