From 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]
From 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]