Yes, lining your urban planter boxes is generally recommended, but the "what" and "why" depend on your box's material and your gardening goals. The primary purpose of a liner is not to hold the soil in, but to protect the planter itself from moisture damage and to aid in proper drainage.
For wooden planter boxes, a liner is crucial. Untreated wood will quickly rot from constant contact with damp soil. Use a durable, permeable landscape fabric. It allows water to drain out while keeping soil in, creating a barrier that prolongs the life of the wood. Avoid non-porous materials like plastic sheets, as they can trap water and lead to root rot by creating soggy soil conditions.
For plastic, metal, or fiberglass boxes, a liner is often optional for protection, but still beneficial for drainage control. A landscape fabric layer at the bottom can prevent soil from washing out of the drainage holes.
Regardless of material, ensure your planter has adequate drainage holes. Before adding any liner or soil, elevate your planter slightly using pot feet or bricks to prevent bottom saturation. The best practice is to add a layer of coarse gravel or broken pottery shards at the bottom, then cover it with your landscape fabric liner. This creates a reservoir for excess water away from plant roots. Finally, fill with a high-quality potting mix suited for containers.
In short, lining with a breathable material like landscape fabric is a simple step that protects your investment and promotes healthier plant growth by managing moisture effectively.