Short answer: not always. Whether you need to line an urban planter box with plastic depends on what the box is made of, how it drains, and what you plan to grow. A plastic liner is a useful tool, not a strict rule. In many wooden planter boxes, a liner can help because it slows moisture damage and creates a barrier between the soil and treated timber. In metal, concrete, composite, or already sealed planters, a full plastic liner may be unnecessary and can even cause problems if it traps water.
The biggest risk with any liner is poor drainage. If you staple plastic around the entire inside of the box without holes, the planter becomes a bathtub. Roots need oxygen, and most plants dislike sitting in saturated soil. If you use plastic, make sure the existing drainage holes remain open. Line the box, then cut or punch holes in the plastic at every drainage point and along the lower sides. Leave the plastic slightly short of the top edge so water does not run behind it, or fold it neatly below the rim. Cover the bottom with landscape fabric if you want to keep soil from washing out while still letting water pass.
For wooden boxes, choose a durable, UV-resistant plastic such as HDPE or heavy-duty pond liner. If you are growing vegetables or herbs, avoid unknown recycled plastics and old treated wood. Food-grade HDPE or a liner designed for raised beds is a safer choice. You can also use landscape fabric or a breathable nonwoven liner. It will not waterproof the wood as well, but it allows more air movement and reduces the risk of soggy soil.
If the planter is metal, a plastic liner can help reduce soil contact and rust, but it must drain. If it is concrete, composite, or already waterproof, skip the plastic or use only a permeable fabric. In hot urban settings, black plastic can heat the soil, so consider a light-colored liner or add mulch to moderate temperature. In cold climates, a liner can also protect the box from freeze-thaw moisture, but only if water can escape.
In short, you do not always need to line an urban planter box with plastic. Use a liner if you want to protect a wooden box, separate soil from a questionable material, or extend the planter's life. But never create a sealed container. Prioritize drainage, choose safe materials, and check the liner regularly. If water drains freely and the soil is not waterlogged, your plants will be much happier than they would be in a perfectly dry but flooded box.