Whether an urban planter box requires a liner to prevent rotting isn't a simple yes or no answer. It primarily depends on the material of your planter box and your desired longevity.
For wooden planter boxes, using a liner is highly recommended. Wood, even cedar or redwood, will eventually decay when in constant contact with moist soil. A durable plastic liner (with drainage holes punched in the bottom) creates a crucial barrier, significantly extending the life of your wooden box by preventing direct soil contact and retaining moisture that causes rot. Avoid lining the very bottom with impermeable materials like solid plastic sheets, as this can trap water and cause root rot.
Metal and concrete planters are less susceptible to rot but can corrode or leach minerals. A liner here protects the material and insulates plant roots from extreme temperatures. Modern composite or plastic planters are often designed to be rot-resistant and may not need an additional liner.
The universal rule, regardless of liner use, is ensuring excellent drainage. Always have adequate holes in the bottom of your planter. A layer of gravel or pebbles at the base beneath the liner can further improve drainage and air circulation, creating a healthier environment for plant roots and the planter itself.
In summary, while not always mandatory, a liner is a wise investment for wooden planters to prevent rotting. It acts as a protective shield, allowing you to enjoy your urban garden for many more seasons. Focus on combining a protective barrier with impeccable drainage for the best results.