For most metal urban planter boxes, the best liner is a custom-fitted HDPE sheet liner, with a 1–1.5 mm EPDM pond liner as the best flexible DIY alternative. Both create a continuous barrier that keeps soil, moisture, and fertilizer salts away from the metal, which is the main cause of rust and premature failure. HDPE is rigid, tough, chemically resistant, and can be heat-welded into a seamless box liner with sealed corners and a drainage sleeve. EPDM is rubber-like, easy to fold into corners, durable, and widely used in ponds and planting applications; choose a quality EPDM membrane rather than thin PVC or polyethylene sheeting.
The liner should cover the walls and the floor, not just the sides. Protect the liner from sharp metal edges with a geotextile underlay or rubber edging. Slope the base slightly toward a low point and install a proper overflow or drain fitting through the metal and liner, sealed with a bulkhead fitting and gasket. Do not simply punch holes through the liner, because that lets soil and water contact the metal. Cover the drain with washed gravel and filter fabric to stop soil from clogging it. Add a drainage layer of gravel at the bottom, then filter fabric, then soil. Leave the liner high enough to overlap above the soil line and finish it with a protective cap or trim.
For edible plants, choose HDPE or EPDM rated as inert and suitable for pond, water, or food-contact use where possible. Avoid thin builder’s plastic, bare landscape fabric, or bitumen products. If the planter is large, a fabricated HDPE liner is the most durable best solution; for a smaller box or lower budget, EPDM is the practical choice. The key is a tough, puncture-resistant, continuous membrane plus protected edges and reliable drainage. That combination prevents soil-to-metal contact and gives the planter a long service life.