The Collaborative International Dictionary of English v.0.48:

Hogweed \Hog"weed`\, n. (Bot.)
      (a) A common weed (Ambrosia artemisi[ae]ge). See
          Ambrosia, 3.
      (b) In England, the Heracleum Sphondylium.
          [1913 Webster]

WordNet (r) 3.0 (2006):

hogweed
    n 1: tall coarse plant having thick stems and cluster of white
         to purple flowers [syn: cow parsnip, hogweed,
         Heracleum sphondylium]