The Collaborative International Dictionary of English v.0.48:

Bitterroot \Bit"ter*root`\, n. (Bot.)
   A plant (Lewisia rediviva) allied to the purslane, but with
   fleshy, farinaceous roots, growing in the mountains of Idaho,
   Montana, etc. It gives the name to the Bitter Root mountains
   and river. The Indians call both the plant and the river
   Sp[ae]t'lum.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Lewisia rediviva
    n 1: showy succulent ground-hugging plant of Rocky Mountains
         regions having deep to pale pink flowers and fleshy
         farinaceous roots; the Montana state flower [syn:
         bitterroot, Lewisia rediviva]