The question of whether to line a metal urban planter with plastic is common among city gardeners. The answer isn't a simple yes or no; it depends on your specific goals and the planter's construction.
Primarily, a plastic liner can act as a barrier against rust, especially for planters made from corten or untreated steel that are meant to develop a patina, not corrode through. It also helps retain moisture and soil, preventing stains on decks or balconies. For planters without pre-drilled drainage holes, a liner with punctured holes allows for controlled drainage while protecting the metal.
However, mandatory drainage is the critical rule. Any liner must have several holes punched in its bottom, aligned with the planter's drainage holes. Without this, water will pool at the bottom, creating a swampy environment that leads to root rot and eventually kills your plants. The plastic should not impede water escape.
Alternatively, skipping the plastic has benefits. It allows for superior drainage and air circulation to the roots, mimicking a more natural environment. For durable, powder-coated aluminum or galvanized steel planters, rust may not be a concern, making a liner unnecessary.
Final Verdict: Use a perforated plastic liner if your metal planter is prone to rust, lacks integrated drainage, or you need to protect surfaces. Ensure it doesn't block drainage holes. Opt for no liner if your planter has excellent drainage, is made of a non-corrosive material, or you prioritize maximum root aeration. Always prioritize proper drainage over any other consideration for thriving urban plants.