The short answer is: not always, but in most urban planter boxes it is a good idea to line the box before adding dirt—especially if you want to protect the planter, keep the soil separate from the container material, or grow edible plants. The key is to use the right kind of lining and to make sure it does not block drainage.
If your planter is made of untreated wood, lining it can help slow down rot and keep moisture away from the wood. A food-safe waterproof liner, such as a pond liner or heavy-duty HDPE plastic, works well. Staple it neatly inside the box, but leave the drainage holes open. Without drainage, excess water will collect, and plant roots can suffocate or rot.
For metal planters, a liner can help prevent rust and reduce direct contact between soil and the metal. This is particularly useful if you are growing herbs or vegetables and want to avoid concerns about coatings or galvanized surfaces. Concrete planters may also benefit from a liner or sealant, depending on the material and whether you are growing edibles.
Do not use a solid plastic sheet as a “liner” unless you are deliberately creating a sealed reservoir or self-watering system. Most ordinary urban planter boxes need free drainage. A permeable geotextile fabric is often better over drainage holes: it lets water pass through while keeping soil from washing out. You can also use a geotextile layer between soil and a drainage layer to stop the drainage material from clogging.
So, do you need to line an urban planter box before adding dirt? If the box is rot-resistant, food-safe, and has good drainage, you can often skip a full liner and simply use a small piece of geotextile over the drainage holes. But if you want to extend the planter’s lifespan, protect the material, or create a cleaner separation between soil and container, lining is worth doing. Just choose a food-safe material if you are growing edibles, and never sacrifice drainage for a perfectly sealed box.