Short answer: yes, a metal urban planter box can rust, but rust by itself usually will not kill your plants. The real risks are poor drainage, waterlogged soil, toxic metal coatings, and a box that rusts so badly it loses strength or creates sharp edges. In most cases, common iron or steel rust—iron oxide—is not highly toxic to plants. Iron is even a micronutrient they need in small amounts. A little surface rust is mainly an appearance and durability issue, not an immediate plant killer.
That said, rust is often a warning sign. It usually appears where water sits, paint is scratched, or drainage is poor. If the soil stays soggy, plant roots can rot, and that is far more likely to kill plants than rust itself. A rusty box may also have clogged drainage holes, a sealed bottom, or soil that holds too much water. In that situation, the problem is the wet environment, not the rust.
The metal type matters. Powder-coated steel, stainless steel, aluminum, and well-sealed corten steel behave differently. Galvanized metal can leach zinc, especially in acidic soil, and high zinc levels may harm some plants. Copper and lead should be avoided for edible planting because they can be toxic. Cheap painted metal may also contain coatings that are not plant-safe. If you are growing vegetables or herbs, use a food-safe liner or choose a planter designed for edible gardening.
To protect both the box and your plants, make sure there are proper drainage holes, use a free-draining potting mix, raise the box slightly off the ground so water can escape, and add a protective liner if the metal is already rusty or questionable. Inspect the inside each season, remove loose rust, and repair coatings with a plant-safe approach. Do not drill extra holes into a liner without allowing water to drain away from the metal. If the box is structurally weak, replace it rather than risking collapse.
So, will rust kill your plants? Usually no—not on its own. But rust can signal moisture problems, coating issues, or metal toxicity risks. Keep the soil well drained, choose a suitable metal and liner, and monitor the box. A slightly rusty metal planter can still grow healthy plants for years if water management and material safety are handled correctly.