Not always. In most urban planter boxes, landscape fabric is optional rather than essential, but it can be helpful in certain situations. The key is to use it correctly so it supports drainage and soil retention instead of creating problems.
A landscape fabric liner is most useful when your planter box has large drainage holes, loose potting mix, or wooden sides. In these cases, a breathable woven fabric can keep soil from washing out through the drainage holes, separate moist soil from wood, and reduce mess on balconies, patios, and rooftops. It can also make future soil changes a little cleaner.
However, you do not need landscape fabric if your planter already has a built-in geotextile liner, a well-designed drainage system, or drainage holes that are small enough to hold soil in place. In some containers, fabric can become unnecessary clutter, and poor-quality fabric may clog with fine soil particles, slow drainage, or break down over time. Avoid using solid plastic sheeting as a substitute, because it can trap water and cause root rot.
If you decide to line your urban planter box, choose a woven geotextile or landscape fabric that allows water and air to pass through. Line the bottom and lower sides if needed, but do not seal the drainage holes. Let the fabric sit loosely over the drainage area, and add a small amount of gravel or coarse material only if your planter design requires it. Make sure excess water can still escape freely.
For most urban gardeners, the practical answer is this: line the planter only when it solves a clear problem, such as soil loss, wooden side protection, or large drainage openings. Use breathable fabric, keep drainage unobstructed, and monitor the soil after watering. If water drains well and the soil stays in place, landscape fabric is not mandatory.