The Collaborative International Dictionary of English v.0.48:

Water hemlock \Wa"ter hem"lock\ (Bot)
   (a) A poisonous umbelliferous plant (Cicuta virosa) of
       Europe; also, any one of several plants of that genus.
   (b) A poisonous plant ([OE]nanthe crocata) resembling the
       above.
       [1913 Webster]

WordNet (r) 3.0 (2006):

water hemlock
    n 1: tall erect highly poisonous Eurasiatic perennial herb
         locally abundant in marshy areas [syn: water hemlock,
         Cicuta verosa]