Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

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}, {rosinweed}]