The Collaborative International Dictionary of English v.0.48:

Silkweed \Silk"weed`\, n. (Bot.)
   Any plant of the genera Asclepias and Acerates whose seed
   vessels contain a long, silky down; milkweed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

silkweed
    n 1: any of numerous plants of the genus Asclepias having milky
         juice and pods that split open releasing seeds with downy
         tufts [syn: milkweed, silkweed]