Yes, using a liner is highly recommended to protect the wood in urban planter boxes. While not always an absolute requirement, a liner significantly extends the lifespan of your planter by creating a crucial barrier between the constantly moist soil and the wooden structure.
The primary threat to untreated or even treated wood in planter boxes is moisture. Soil retains water, and this persistent dampness leads to rot, warping, and eventual decay of the wood. A liner acts as a shield, preventing this direct contact. It also helps contain the soil and prevents staining from soil or fertilizer runoff.
However, proper drainage is non-negotiable. Never create a completely sealed environment. Your liner must have drainage holes aligned with holes in the planter box's bottom to allow excess water to escape. Without drainage, water will pool at the bottom, leading to root rot for your plants and creating a saturated pocket that can still damage the wood over time.
Suitable liner materials include heavy-duty polyethylene plastic, landscape fabric, or even pond liner. Plastic is an effective moisture barrier, while landscape fabric is water-permeable but helps contain soil. For an eco-friendly option, consider a natural burlap sack, though it will decompose and need replacement.
In summary, installing a liner with adequate drainage is a simple, cost-effective step. It protects your investment, ensures healthier plant roots, and keeps your urban garden looking vibrant for many more seasons.