Whether you need to line your wooden urban planter boxes with plastic is a common gardening dilemma. The short answer is: it's not strictly necessary and can sometimes cause more harm than good. While a plastic liner can create a barrier between moist soil and the wood, it often traps excess moisture against the interior boards, accelerating rot from the inside out. This creates a sealed, soggy environment that wood cannot breathe in.
The key to longevity for wooden planters is not a waterproof seal, but excellent drainage and using the right wood. Always use rot-resistant lumber like cedar, redwood, or pressure-treated wood rated for ground contact. The most critical step is to ensure multiple drainage holes in the bottom of the box. A better alternative to a full plastic liner is to use a permeable landscape fabric along the interior sides and bottom. This allows water to drain freely while keeping soil from washing out and slightly reducing direct soil-to-wood contact.
For ultimate protection, consider applying a non-toxic, water-sealant designed for garden beds to the *interior* of the planter before assembly or filling. Focus on the corners and bottom. Ultimately, proper construction, durable materials, and unrestricted drainage will do more to prevent rot than a plastic liner ever could.