The Collaborative International Dictionary of English v.0.48:

Frostweed \Frost"weed`\, n. (Bot.)
   An American species of rockrose (Helianthemum Canadense),
   sometimes used in medicine as an astringent or aromatic
   tonic.
   [1913 Webster]

   Note: It has large yellow flowers which are often sterile,
         and later it has abundant but inconspicuous flowers
         which bear seed. It is so called because, late in
         autumn, crystals of ice shoot from the cracked bark at
         the root; -- called also frostwort.
         [1913 Webster]

WordNet (r) 3.0 (2006):

frostweed
    n 1: perennial of the eastern United States having early
         solitary yellow flowers followed by late petalless flowers;
         so-called because ice crystals form on it during first
         frosts [syn: frostweed, frost-weed, frostwort,
         Helianthemum canadense, Crocanthemum canadense]
    2: tall perennial herb having clusters of white flowers; the
       eastern United States [syn: Virginia crownbeard,
       frostweed, frost-weed, Verbesina virginica]