The short answer is powder-coated aluminum. For most urban planter boxes, aluminum is the lightest material that still feels genuinely sturdy, especially when it is well designed with folded edges, internal ribs, a reinforced base, and a quality powder-coated finish. It is much lighter than steel, concrete, or timber, it will not rust like ordinary steel, and it can feel solid rather than tinny if the wall thickness and framing are right. For very large planters, hidden steel inserts can be added at mounting points or corners to increase rigidity without adding much overall weight.
If you want to go even lighter, commercial fiberglass-reinforced plastic, often called FRP, is the next best option. High-quality FRP planters are rigid, weather-resistant, and widely used in street, plaza, and rooftop projects because they can be molded into shapes that resemble stone or concrete while weighing far less. The trade-off is that cheap fiberglass can feel thin and may chalk, crack, or delaminate after impact or long sun exposure, so the quality of the composite matters a lot.
HDPE, or high-density polyethylene, is technically lighter than both aluminum and FRP, but it only feels sturdy when it is double-walled, ribbed, UV-stabilized, and properly weighted at the base. Thin single-wall plastic planters tend to flex, rattle, or blow over in windy urban settings, so they are not the best choice when a solid feel is important.
In practice, choose powder-coated aluminum for a premium, long-lasting lightweight planter; choose high-quality FRP for large, shaped, or roof-terrace planters where every kilogram matters; and choose heavy-wall HDPE for budget-friendly, movable planters in sheltered areas. Also remember that wet soil is usually the heaviest part of any planter, so drainage, a lightweight soil mix, and casters or lifting points can matter as much as the container material itself.