Each planter box comprises a base 40, at least two connecting walls 42, 43 each having an upper edge and a lower edge, and a front wall (45). The front wall has a height which is less than the height of the two connecting walls. Each connecting wall is provided with at least one upper key slot 48 extending vertically downwards from the upper edge of the connecting wall and at least one lower key slot 49 extending vertically upwards from the lower edge of the connecting wall. Each key slot is adapted to engage with a corresponding key member (70).The planter box may comprise one or more intermediate bracing walls extending between the front wall and a connecting wall, wherein the intermediate bracing wall may include a slot at its upper edge for accommodating an irrigation conduit. The boxes may be stacked to provide a green wall.
