The Collaborative International Dictionary of English v.0.48:
Willow-weed \Wil"low-weed`\, n. (Bot.) (a) A European species of loosestrife (Lysimachia vulgaris). (b) Any kind of Polygonum with willowlike foliage. [1913 Webster]WordNet (r) 3.0 (2006):
Lysimachia vulgaris n 1: frequently considered a weed; Europe and Asia [syn: yellow loosestrife, garden loosestrife, Lysimachia vulgaris]