Whether you need a liner for your urban planter box depends heavily on the box's material. The short answer is: for long-lasting, healthy plants, a liner is often recommended, but with crucial considerations.
Planting directly into a wooden planter box will cause the wood to rot over time from constant moisture and soil contact. For metal boxes, direct contact with soil can accelerate corrosion. In these cases, a liner is essential to protect the structure. You can plant directly into plastic, fiberglass, or concrete boxes, as they are non-porous and durable.
However, the primary purpose of a liner goes beyond protection. It is a key tool for managing drainage. A proper liner should have drainage holes that align with holes in the planter box itself. Never use a completely impermeable liner without drainage, as this will create a swampy environment and drown plant roots.
For a balanced approach, consider using a permeable landscape fabric liner. It allows water and air to pass through while containing the soil and protecting the box material. Always ensure a layer of drainage material, like pebbles or broken pottery, at the bottom before adding soil.
In summary, while you can plant directly in some materials, using a liner with adequate drainage holes is a best practice. It extends the life of your planter box and creates a healthier environment for your urban garden by preventing waterlogged soil and root rot.