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]