Source : Webster's Revised Unabridged Dictionary (1913)

Willow-weed \Wil"low-weed`\, n. (Bot.)
   (a) A European species of loosestrife ({Lysimachia
       vulgaris}).
   (b) Any kind of Polygonum with willowlike foliage.

Source : WordNet®

Lysimachia vulgaris
     n : frequently considered a weed; Europe and Asia [syn: {yellow
         loosestrife}, {garden loosestrife}]