Ageratum houstonianum Mill.
Blue billy goat weed (Ageratum houstonianum) is a short-lived, annual herb that is native to Mexico and Central and Tropical America. It was introduced to New Zealand as a garden ornamental and was first recorded in the wild (naturalised) in 1888. The plant typically grows 20–80 centimetres tall and has softly hairy, branched stems with triangular to egg-shaped leaves that have coarsely toothed margins. It flowers from spring to autumn, producing dense clusters of fluffy blue, mauve, pink or white flower heads, which develop into small black seeds. It does not tolerate frost or heavy shade and grows best in warm, open, disturbed habitats. It reproduces by seed. Seeds are dispersed by wind, water, animals, machinery and contaminated soil. The plant produces abundant seed and readily establishes in disturbed areas.
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