The short answer is: not always, but often yes—especially if the box is wood, metal, or sits on a surface you want to protect. A liner is mainly about planter moisture protection, not about making the box watertight. In an urban planter box, trapped moisture is usually a bigger risk than moisture alone. If water can drain freely and air can move, many plastic or composite boxes do not need a liner. Wooden planter boxes benefit from a liner because constant damp soil accelerates rot. Metal boxes benefit because soil and fertilizer salts can cause rust and staining. Concrete may benefit from a liner to reduce efflorescence and staining, but it often performs well without one if drainage is good. For balcony planter boxes, a liner or waterproof tray can protect tiles, decking, and neighbours below, but it must not block drainage.
If you use an urban planter box liner, choose the right kind. A flexible EPDM or HDPE liner can protect wood and metal, while a breathable waterproof membrane is useful where you want to reduce moisture without sealing the walls. Do not fully seal the bottom. Keep drainage holes open, add a drainage layer such as gravel or a drainage mat, and use geotextile fabric to stop soil from clogging it. The liner should cover the sides and base, but water must escape. A liner that creates a bathtub will cause waterlogging, root rot, and sometimes structural damage. Good planter box drainage matters more than a perfect waterproof seal.
For wooden planter liner projects, line the inside walls and bottom, leave the drainage holes clear, and avoid trapping a pool of water behind the liner. For metal planters, use a rubber liner to separate soil from the metal and allow drainage. For plastic or composite boxes, a liner is usually unnecessary unless the box is cracked, the drainage is poor, or you want extra protection for the surface underneath. In small balcony planters, a removable inner pot can be a simpler solution than lining the box itself. It lets you lift the plants out, clean the box, and check for moisture.
Also think about the planting mix and irrigation. Urban planters often dry out quickly, so people overwater to compensate. A liner will not fix poor drainage or overwatering. Use a free-draining potting mix, water until it runs from the drainage holes, and empty any saucer or tray after rain. Raise the planter on feet if it sits on a balcony floor so air can circulate underneath. If the box is against a wall, leave a small gap and check regularly for damp patches.
So, do you need a liner? If your urban planter box is wood or metal, or if it sits on a moisture-sensitive surface, a liner is a smart investment—provided it is installed with drainage. If the box is plastic and drains well, you can usually skip it. The goal is not to keep every drop of water inside the box. It is to keep soil moisture away from materials that can be damaged while letting excess water leave freely.