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.