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]

The Collaborative International Dictionary of English v.0.48:

Rockrose \Rock"rose`\, n. (Bot.)
   A name given to any species of the genus Helianthemum, low
   shrubs or herbs with yellow flowers, especially the European
   Helianthemum vulgare and the American frostweed,
   Helianthemum Canadense.
   [1913 Webster]

   Cretan rockrose, a related shrub (Cistus Creticus), one
      of the plants yielding the fragrant gum called ladanum.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Helianthemum canadense
    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]