If your urban planter box sits on a deck, the short answer is yes: you should use a liner, but a breathable, drainage-friendly liner system rather than a sealed bucket. Decks are expensive, and moisture is their main enemy. Every time you water, excess water carries soil particles, fertilizer salts, and tannins out of the bottom of the planter. Without a liner, that runoff can soak into deck boards, stain composite surfaces, and create a continuously damp patch under the box. Over time, that moisture can lead to rot, mold, warping, and premature finish failure. A liner helps separate wet soil from the deck and directs excess water to a controlled drainage point.
But “liner” does not mean blocking all airflow. Urban planter boxes need drainage. If you fully seal the box and leave no way for water to escape, roots can drown and the planter can become heavy and sour. The best setup is a waterproof liner with pre-drilled drainage holes, a layer of gravel or drainage aggregate, and a geotextile fabric to keep soil from clogging the holes. If the planter is directly on the deck, add pot feet or a riser to allow air circulation underneath and prevent standing water. You can also place a drip tray or saucer under the planter, but empty it regularly so it does not become a mosquito nursery or a moisture trap.
Choose liner material carefully. EPDM pond liner, HDPE, or heavy-duty food-grade plastic are common choices. Avoid thin plastic sheeting that tears easily. Some gardeners use pond liner with holes punched at the lowest points, while others use commercial planter box liners designed for drainage. For wooden planter boxes, line the interior sides to keep soil moisture away from the wood, but never cover the drainage holes. For metal planters, a liner can reduce rust risk. For composite decks, a liner plus pot feet is especially smart because standing water can leave mineral stains and organic matter can create a slippery film.
Also consider weight. Wet soil is heavy. A liner will not reduce weight, but it can keep moisture from spreading. Place the planter where the deck structure can support it, and use a waterproof mat or deck-protection pad under the liner as an extra barrier. Check the underside of the deck and the area around the planter seasonally for dampness, stains, or soft spots. If water runs off the deck, direct it away from the house and foundation.
So yes, for an urban planter box on a deck, use a liner to protect the deck, but make it a drainage-smart liner. A waterproof barrier, drainage holes, gravel layer, fabric filter, and pot feet together give you the best protection. Skip the liner and you may save ten minutes now, but you risk much bigger repairs later.