The Collaborative International Dictionary of English v.0.48:

Pinkroot \Pink"root`\, n.
   1. (Med.) The root of Spigelia Marilandica, used as a
      powerful vermifuge; also, that of Spigelia Anthelmia.
      See definition 2 (below).
      [1913 Webster]

   2. (Bot.)
      (a) A perennial North American herb (Spigelia
          Marilandica), sometimes cultivated for its showy red
          blossoms. Called also Carolina pink, Maryland
          pinkroot, and worm grass.
      (b) An annual South American and West Indian plant
          (Spigelia Anthelmia).
          [1913 Webster]

WordNet (r) 3.0 (2006):

pinkroot
    n 1: a fungal disease of onions