The Collaborative International Dictionary of English v.0.48:

Ragwort \Rag"wort`\ (r[a^]g"w[^u]rt`), n. (Bot.)
   A name given to several species of the composite genus
   Senecio.
   [1913 Webster]

   Note: Senecio aureus is the golden ragwort of the United
         States; Senecio elegans is the purple ragwort of
         South Africa.
         [1913 Webster]

WordNet (r) 3.0 (2006):

ragwort
    n 1: widespread European weed having yellow daisylike flowers;
         sometimes an obnoxious weed and toxic to cattle if consumed
         in quantity [syn: ragwort, tansy ragwort, ragweed,
         benweed, Senecio jacobaea]
    2: American ragwort with yellow flowers [syn: butterweed,
       ragwort, Senecio glabellus]