Yes—in most urban balcony settings, a liner is strongly recommended, but a liner alone is not always enough. A liner protects the planter box itself from soil moisture, fertilizers, and rot, while a separate waterproof drip tray or saucer protects your balcony floor. If you place a planter box directly on tile, concrete, wood, or composite decking, water, mud, and rust can leave permanent stains, promote mold, or weaken the surface over time.
Think about what happens after you water. Excess water drains through the soil and out of the planter. Without a barrier, that water can spread across the floor, carrying dissolved fertilizer and fine soil. Even small amounts can seep into grout, wood grain, or seams. Metal planter boxes may drip rust stains. Wooden boxes can rot and stain the floor. Concrete can develop mineral deposits or efflorescence.
A liner inside the planter box is useful because it separates soil from the planter material and can extend the life of wood or metal boxes. Choose a durable, waterproof material such as EPDM, pond liner, or a custom planter liner. Do not block drainage holes. If you seal the box completely, water will collect around roots and cause rot. Instead, create proper drainage: a drainage layer, drainage holes, and a drip tray underneath.
For balcony floor protection, the most important item is often a waterproof tray or saucer that sits under the planter. It should be wider than the planter base so drips and overflow are captured. Empty it after heavy rain or watering. If the planter is large, use a custom tray or a shallow waterproof pan. Pair it with pot feet or a raised stand to allow air circulation and make leaks easier to spot.
If your planter box is already a sealed, self-watering model with an integrated reservoir, you may not need an internal liner. You still need to protect the floor from overflow and condensation. Check condo or HOA rules too, because some buildings require drip trays on balconies.
In short, yes: urban planter boxes usually need a liner and a floor-protection tray. The liner protects the box; the tray protects your balcony. Use both, keep drainage working, and clean up spills promptly. That combination prevents most water stains, mold, rust marks, and expensive floor damage.