​WILLOW-HERB, in botany, the popular name for the species of Epilobium, a genus of often tail herbaceous plants, several of which are natives of Britain. The slender stems bear narrow leaves and pink or purple flowers, which in the rose-bay (E. angustifolium), found by moist river-sides and in copses, are 1 in. in diameter and form showy spikes. E. hirsutum, found by sides of ditches and rivers, a tall plant with many large rose-purple flowers, is known popularly as codlins-and-cream.