Using a liner inside your wooden urban planter box is highly recommended. It protects the wood from constant moisture and soil, which prevents rot and significantly extends the life of your planter. The key is choosing a liner that allows for proper drainage to avoid waterlogged soil, which harms plant roots.
For most situations, a durable plastic liner (like a pond liner or heavy-duty poly) is an excellent choice. Puncture several small drainage holes in its bottom before installation. Alternatively, landscape fabric is a breathable option that lets water drain while containing soil. It's less waterproof but good for moisture control.
Avoid lining the entire interior with non-porous materials without drainage holes. Always ensure excess water can escape. A good practice is to first install your chosen liner, then add a layer of gravel or pebbles at the bottom before filling with potting mix. This creates a drainage reservoir and keeps the liner's holes clear.
Ultimately, a proper liner safeguards your investment, keeping your wooden planter sturdy and your urban garden thriving for seasons to come.