The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
thimbleweed n 1: a common North American anemone with cylindrical fruit clusters resembling thimbles [syn: thimbleweed, Anemone cylindrica]