Whether an urban planter box requires a liner to protect it from rot isn't a simple yes or no answer. It primarily depends on the material of the planter box itself.
For wooden planter boxes, using a liner is highly recommended and often essential for longevity. Wood, even when pressure-treated or using rot-resistant species like cedar, is in constant contact with moist soil and water. A protective liner creates a crucial barrier, significantly slowing down the decay process. You can use heavy-duty plastic (with drainage holes punched in the bottom), pond liner, or even a commercial fabric pot placed inside the wooden frame. This protects the wood while allowing you to control drainage.
For planters made from naturally rot-resistant or non-porous materials like metal, composite lumber, plastic, or ceramic, a liner is generally not necessary for rot protection. These materials are designed to withstand moisture. However, a removable liner, like a fabric grow bag, can still be useful for easy soil changing and root management.
The critical factor that works in tandem with any liner decision is drainage. Stagnant water is the primary enemy, causing root rot and accelerating material decay. Ensure your planter has adequate drainage holes at the bottom. If using a solid liner like plastic, you must puncture several holes in it to align with the planter's drainage holes. A layer of drainage material, such as pebbles or broken pottery, at the very bottom can also help prevent clogging.
In summary, for wooden planters, a liner is a wise investment to prevent rot. For other materials, it's optional but can offer practical benefits. Always prioritize excellent drainage to ensure the health of your plants and the durability of your planter box.