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):

Oenanthe crocata
    n 1: European poisonous herb having tuberous roots, yellow juice
         that stains the skin, yellow flowers and foliage resembling
         celery; all parts extremely poisonous [syn: water
         dropwort, hemlock water dropwort, Oenanthe crocata]