Gomphocarpus physocarpus, commonly known as balloonplant, balloon cotton-bush, bishop's balls, nailhead, or swan plant, is a species of milkweed. The plant is native to southeast Africa, but it has been widely naturalized. It is often used as an ornamental plant. The name "balloonplant" is an allusion to the swelling bladder-like follicles which are full of seeds.