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Giant knotweed is a large, thicket-forming herb that can grow up to 4m tall. It has large (up to 30cm long), heart-shaped leaves that are smooth on top and sometimes covered in fine hairs underneath. The stems are thick, hollow, often green to reddish-brown and die back to the root base in winter. It has a thick root mass with creeping underground stems. The flowers are greenish-white in dense drooping clusters near the end of stems.
Giant knotweed can grow in gardens, river and stream edges, forest margins and any waste areas. It tolerates wet to moderately dry conditions and warm to cold temperatures, but is intolerant of shade.
Giant knotweed forms dense long-lived thickets, which shade and crowd out other species. Fortunately, Giant knotweed does not produce seed in New Zealand, however care must be taken with its removal, as even a small piece of the plant’s root or stem can grow into a new plant.