Cannonroot is harvested from a plant originally native to the dragonborn home world. Its secret has been carried with them in their conquest of new worlds. The plant itself grows near regions of geological turmoil. It is common near volcanoes and other hot spots.
Like many technologies of the dragonborn empire, cannonroot and its uses was learned from a defeated culture. Since the dragonborn have ever defined the history of their empire, the name and race of the originators is most likely lost forever.