The Collaborative International Dictionary of English v.0.48:

Frostwort \Frost`wort"\, n. (Bot.)
   Same as Frostweed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

frostwort
    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]