The Collaborative International Dictionary of English v.0.48:

Sneezeweed \Sneeze"weed`\, n. (Bot.)
   A yellow-flowered composite plant (Helenium autumnale) the
   odor of which is said to cause sneezing.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sneezeweed
    n 1: any of various plants of the genus Helenium
         characteristically causing sneezing