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Erect, many-stemmed herb to subshrub to 1-2 m with a perennial fibrous rootstock. Stems often die back in winter, are densely covered in stalked sticky hairs, are usually purple, become woody with age, have branches in opposite pairs, and often have galls which are formed by a parasitic fly. Diamond-shaped leaves (55-80 x 35-70 mm) with irregularly round-toothed edges are in opposite pairs along the stems. From August to December dense clusters of small, white flowers (5-7 mm diameter) are produced, followed by 5-angled black seeds (1.8 mm long).
Long-lived, quick maturing, producing many highly viable, well dispersed seed. Drooping stems can take root where they touch the ground. Dense-growing, this species overtops groundcovers and smaller native species. It can also block waterways or at least impede waterflow. Tolerant of moderate shade, damage, grazing, salt, most soils, drought and damp; thus this species will easily invade a variety of environments, including rough pastureland, strips of land along waterways, wetlands and roadsides.