Choosing the correct liner for your wooden urban planter box is crucial for its longevity and your plants' health. The primary purpose of a liner is to protect the wood from constant moisture and soil contact, which causes rot. It also helps retain soil and water.
Your best options are permeable landscape fabric or a rigid plastic liner with drilled drainage holes. Landscape fabric is excellent as it allows water and air to pass through while containing the soil, preventing rot and promoting healthy roots. For a more waterproof solution, use a rigid plastic sheet (like HDPE or pond liner) but you must drill several drainage holes in the bottom to prevent waterlogging. Avoid using non-permeable materials like standard plastic sheeting without holes, as this creates a swampy environment that drowns roots.
Always ensure your liner setup includes a drainage layer of gravel or pebbles at the bottom of the box, beneath the liner, to facilitate water flow. The liner should cover the entire interior, including the sides, and be secured just below the top rim. This simple step dramatically extends the life of your wooden planter and creates a thriving environment for your urban garden.