Yes, you can use a liner in your urban planter boxes to help with drainage, but it must be done correctly to be beneficial. The primary purpose is to protect the planter material, especially wood, from constant moisture and soil contact, which can cause rot. However, the liner itself should not impede water flow.
For effective drainage, never line the very bottom of the box. Instead, place the liner along the sides and partially up the interior walls. At the base, create a drainage layer using materials like pebbles, gravel, or broken pottery shards before adding your soil. This layer prevents the drainage holes from becoming clogged with soil and allows excess water to escape freely.
Crucially, you must ensure your planter box has several adequate drainage holes at the bottom. Without these, water will pool at the base regardless of your liner or gravel layer, leading to root rot. Choose a permeable liner material, such as landscape fabric or burlap, which holds soil in while letting water pass through. Avoid non-porous plastics that can create a sealed, soggy environment for plant roots.
In summary, a properly installed, breathable liner combined with a dedicated drainage layer and sufficient holes can extend your planter's life and promote healthier plant growth by preventing waterlogging in your urban garden.