Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {Spiked willow-herb}, a perennial herb ({Lythrum Salicaria})
      with willowy leaves and spiked purplish flowers.

Source : WordNet®

Lythrum salicaria
     n : marsh herb with a long spike of purple flowers; originally
         of Europe but now rampant in eastern United States [syn:
         {purple loosestrife}, {spiked loosestrife}]