Yes, most well-constructed urban planter boxes do utilize some form of lining or protective barrier. This is a crucial feature for longevity, not an afterthought. The primary purpose of a liner is to create a shield between the constantly moist soil and the inner walls of the planter box itself.
Without a liner, soil and moisture directly contact the planter material. For wooden planters, this leads to rapid rot and decay. For metal planters, it can accelerate corrosion and rust. Even concrete and stone can degrade over time from constant moisture exposure and soil acids. A liner acts as a sacrificial layer, taking the brunt of this wear.
Common liner materials include landscape fabric (geotextile), which is breathable and allows for excellent drainage while containing soil. Plastic sheeting or pond liners are also used but require drainage holes punched in the bottom to prevent waterlogging. Some planters use a thin, recycled plastic insert. The best liners protect the structure while still facilitating proper water flow to prevent root rot.
In summary, adding a liner to your urban planter box is a simple step that significantly extends its life, protects your investment, and promotes healthier plant growth by managing moisture effectively. It is a standard and recommended practice for any permanent or semi-permanent planter installation.