The Collaborative International Dictionary of English v.0.48:

Cottonweed \Cot"ton*weed`\ (-w[=e]d`), n. (Bot.)
   See Cudweed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cottonweed
    n 1: an American everlasting having foliage with soft wooly
         hairs and corymbose heads with pearly white bracts [syn:
         pearly everlasting, cottonweed, Anaphalis
         margaritacea]
    2: any of various plants of the genus Froelichia found in sandy
       soils and on rocky slopes in warmer regions of America; grown
       for their spikes of woolly white flowers