The Collaborative International Dictionary of English v.0.48:

Stoneroot \Stone"root`\, n. (Bot.)
   A North American plant (Collinsonia Canadensis) having a
   very hard root; horse balm. See Horse balm, under Horse.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stoneroot
    n 1: erect perennial strong-scented with serrate pointed leaves
         and a loose panicle of yellowish flowers; the eastern
         United States [syn: horse balm, horseweed, stoneroot,
         stone-root, richweed, stone root, Collinsonia
         canadensis]