The Collaborative International Dictionary of English v.0.48:

Willow-herb \Wil"low-herb`\, n. (Bot.)
   A perennial herb (Epilobium spicatum) with narrow
   willowlike leaves and showy rose-purple flowers. The name is
   sometimes made to include other species of the same genus.
   [1913 Webster]

   Spiked willow-herb, a perennial herb (Lythrum Salicaria)
      with willowy leaves and spiked purplish flowers.
      [1913 Webster]

WordNet (r) 3.0 (2006):

willowherb
    n 1: a plant of the genus Epilobium having pink or yellow
         flowers and seeds with silky hairs