Short answer: It depends on the material, drainage, and plants. You do not always need a liner inside an urban planter box. In many urban planters, a liner can be helpful, but the wrong liner can cause more harm than good.
If your planter is made of untreated wood, a liner is usually a good idea. Wood in contact with constantly moist soil will rot over time. A liner separates the soil from the wood and extends the life of the box. Choose a breathable or perforated liner so water can still drain. Non-woven geotextile fabric is a common choice because it lets water pass while holding soil in. You can also use food-safe plastic or EPDM liner, but you must cut drainage holes and align them with the planter’s drainage holes. If you grow vegetables or herbs, avoid treated wood or unknown recycled plastics that may leach chemicals.
Metal planters can also benefit from a liner. Metal can rust when wet, and dark metal can heat up in summer, stressing roots. A liner or an inner fabric pot can create a buffer. Coco fiber liners, geotextile fabric, or a slightly smaller fabric grow bag can work well. Again, drainage is essential. If water sits between the metal and the liner, rust may still occur. Some metal planters are powder-coated or galvanized, but even these can eventually corrode. A liner is not a substitute for drainage holes.
Concrete and stone planters are durable but can be alkaline, especially new concrete. A liner can reduce direct contact with lime and protect roots of acid-loving plants. It can also make cleaning and repotting easier. For concrete, use a waterproof liner with drainage holes, or seal the inside with a masonry sealer. If the planter is already sealed and has proper drainage, a liner may be unnecessary.
If your planter is a fabric grow bag, self-watering container, or a plastic pot with drainage, you usually do not need an extra liner. These are designed to hold soil and drain. Adding an impermeable liner can block drainage and create soggy soil. A fabric pot is already breathable and air-prunes roots. A self-watering planter has a reservoir; adding a liner may interfere with its wicking system.
The key question is what problem you are trying to solve. Are you protecting the planter from rot or rust? Are you keeping soil from washing out of large drainage holes? Are you trying to retain moisture in a hot, windy urban balcony? Are you growing edibles and worried about toxins? If yes, a liner may help. If your planter is already durable, clean, and drains well, a liner may just add complexity.
When choosing a liner, avoid fully sealing the soil. Roots need oxygen, and soil life needs air. A liner that traps water at the bottom can lead to root rot, fungus gnats, and anaerobic odors. If you use plastic, punch many holes. If you use geotextile, make sure it is not so thick that it clogs. Some gardeners use a bottom layer of gravel, but this is usually not necessary and can actually raise the saturated zone. Better to rely on drainage holes and a permeable screen.
For urban planter boxes, a liner can also help with thermal insulation. On a rooftop or balcony that gets full sun, roots can overheat in metal or dark containers. A light-colored outer box or an insulating liner can reduce heat stress. In winter, a liner can offer slight protection from freezing, though it will not prevent a solid freeze in cold climates.
So, do you need a liner? Not always. Use one if you have untreated wood, rust-prone metal, new concrete, large drainage holes, or edible plants in questionable containers. Skip it if your planter is already non-toxic, waterproof, and drains freely. If you do use a liner, choose a food-safe, durable material, allow drainage, and check it annually. The best liner is the one that solves your specific problem without turning your planter into a swamp.