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Giant hogweed is a perennial herb that can grow up to 6m tall and has large serrated leaves up to 50cm long. It has stout dark reddish-purple stems, and spotted leaf stalks with sturdy bristles. The stems are 5-10cm in diameter, hollow, grooved and covered in fine bristles and red-purple spots. When the plant is two to three years old it produces large, umbrella-like clusters of greenish-white flowers. Giant hogweed usually grows on the banks of rivers or creeks or in rich, moist soils.
Giant hogweed is poisonous to humans, causing skin irritation and swelling. It forms dense colonies that compete with and exclude native vegetation that grows along the banks of rivers or streams. One plant can produce up to 50,000 viable seeds which are spread by water. When it dies down in winter it leaves infested banks bare of vegetation and susceptible to erosion or invasion by weeds.