The short answer is: not always, but in many urban settings, lining a planter box before adding soil is a smart idea. Whether you need to line it depends on the material of the box, what you plan to grow, and how long you want the planter to last.
If your planter is made of untreated wood, lining can help slow rot and keep soil from constantly touching damp wood. If the wood is pressure-treated or painted with an unknown finish, a food-safe liner can create a barrier between the soil and chemicals you may not want near herbs or vegetables. Metal planters often benefit from a liner too, especially if they are not powder-coated or rust-resistant; soil and water can cause rust over time. Concrete or cement planters can be lined to reduce lime leaching, which may raise soil pH, and to make the container easier to maintain. Plastic, glazed ceramic, and fiberglass planters usually do not need a liner if they are already non-toxic, stable, and have drainage holes.
That said, lining is not the same as sealing. The biggest mistake is turning the planter into a watertight bucket. Most plants need oxygen around their roots, and soggy soil leads to root rot. If you add a liner, make sure water can still drain freely. Use a liner that lets excess water escape, or punch holes in it at the bottom and along the lower sides. Keep the planter’s existing drainage holes clear. If the liner blocks them, your soil will stay too wet.
Good liner options include landscape fabric or geotextile, which allows water through while keeping soil contained; food-safe plastic sheeting with drainage holes; pond liner with holes added; or a custom-fit fabric liner. Avoid using loose plastic that traps water, and avoid liners made from treated materials unless they are safe for edible plants. If you are growing food, choose food-safe, non-toxic materials. If the planter is decorative and holds a separate inner pot, you may not need to line the outer box at all—just make sure the inner pot drains properly.
Do you always need a liner? No. If your urban planter box is made from rot-resistant, food-safe material, has adequate drainage, and will not be exposed to harsh weather for years, you can usually add soil directly. But if you want to protect the box, separate soil from questionable materials, reduce moisture loss, or extend the planter’s life, lining is a practical step. In short: line when it protects the planter, the soil, or your plants—but never at the cost of drainage.