Source : Webster's Revised Unabridged Dictionary (1913)

Thimbleweed \Thim"ble*weed`\, n. (Bot.)
   Any plant of the composite genus {Rudbeckia}, coarse herbs
   somewhat resembling the sunflower; -- so called from their
   conical receptacles.

Source : WordNet®

thimbleweed
     n : a common North American anemone with cylindrical fruit
         clusters resembling thimbles [syn: {Anemone cylindrica}]