Whether your urban planter box needs a liner isn't a simple yes or no. The answer depends primarily on the planter's construction material and your goal to protect the plants, the planter itself, or both.
The primary purpose of a liner is to create a barrier. For plant safety, this barrier prevents potentially harmful substances in certain materials (like chemicals in treated wood or rust from metals) from leaching into the soil and harming roots. For planter safety, especially with porous materials like wood or terracotta, a liner shields the structure from constant moisture, drastically slowing rot and decay.
Let's examine common materials:
* Wood (Especially Pressure-Treated): A liner is highly recommended. Modern pressure-treated wood uses safer chemicals, but a plastic or landscape fabric liner provides an extra safety margin for edible plants and prevents soil moisture from rotting the wood.
* Metal: Liners are crucial for thin metals prone to rust. They prevent corrosion from soil and fertilizers. Some metal planters come with a baked-on coating, but an added liner offers extra insurance.
* Concrete/Cement: New concrete can leach lime, raising soil pH. Sealing the interior or using a liner for the first season protects sensitive plants.
* Plastic, Fiberglass, Glazed Ceramic: These are inherently water-resistant and non-porous. A liner is usually unnecessary unless for easy cleanup or if the drainage holes are excessively large.
Regardless of material, drainage is non-negotiable. Any liner used must allow excess water to escape. Never create a completely watertight seal. Use materials like landscape fabric or drill holes in a rigid plastic liner. A layer of coarse gravel or pebbles at the bottom before adding soil can also aid drainage.
Final Verdict: While not always mandatory for plant survival, using a liner is generally a wise practice. It extends the lifespan of your planter box, provides a buffer against material-specific risks, and offers greater control over your garden's soil environment. For edible gardens or planters made of wood or metal, installing a proper, drainable liner is the best practice for long-term success.