The Collaborative International Dictionary of English v.0.48:

crown-of-the-field \crown-of-the-field\ n.
   a European annual (Agrostemma githago) having large
   trumpet-shaped reddish-purple flowers and poisonous seed; a
   common weed in grainfields and beside roadways; naturalized
   in America.

   Syn: corn cockle, corncockle, corn-cockle, corn campion,
        Agrostemma githago.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

crown-of-the-field
    n 1: European annual having large trumpet-shaped reddish-purple
         flowers and poisonous seed; a common weed in grainfields
         and beside roadways; naturalized in America [syn: corn
         cockle, corn campion, crown-of-the-field, Agrostemma
         githago]