The Collaborative International Dictionary of English v.0.48:

Richweed \Rich"weed`\, n. (Bot.)
   An herb (Pilea pumila) of the Nettle family, having a
   smooth, juicy, pellucid stem; -- called also clearweed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

richweed
    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]
    2: a plants of the genus Pilea having drooping green flower
       clusters and smooth translucent stems and leaves [syn:
       richweed, clearweed, dead nettle, Pilea pumilla]