The Collaborative International Dictionary of English v.0.48:

Tarweed \Tar"weed`\, n. (Bot.)
   A name given to several resinous-glandular composite plants
   of California, esp. to the species of Grindelia,
   Hemizonia, and Madia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tarweed
    n 1: any of various resinous glandular plants of the genus
         Madia; of western North and South America
    2: any of various western American plants of the genus Grindelia
       having resinous leaves and stems formerly used medicinally;
       often poisonous to livestock [syn: gumweed, gum plant,
       tarweed, rosinweed]