Apparently not the work of a prankster.
Gomphocarpus physocarpus, commonly known as hairy balls, balloonplant, balloon cotton-bush, bishop's balls, nailhead, or swan plant, is a species of dogbane. The plant is native to southeast Africa, but it has been widely naturalized. It is often used as an ornamental plant.
And there really isn't enough room on that sign to spell out gomphocarpus physocarpus.