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Ageratum houstonianum Mill. 1768
pronounced: ad-jer-RAH-tum hoos-toh-nee-AH-num
(Asteraceae – the daisy family)
common name: Blue Billygoat Weed
Ageratum comes from the Greek αγερατος (ageratos), not growing old, referring to the long-lasting flowers; houstonianum is named for Dr. William Houston, 18th century Scottish-born surgeon and botanist who collected plants in Central America and the West Indies.
Ageratum houstonianum is native to an area that stretches from southern Mexico to Guatemala on the Pacific coast and eastward to Belize on the Caribbean Sea. It is prone to becoming a rampant environmental weed when grown outside its natural range, and is now an invasive weed in Africa, south-east Asia, the USA, and Australia. It is a common roadside weed on Magnetic Island.
This is an erect, softly hairy annual plant that grows to a height of about 1 m. The stems are sometimes reddish, and have long white hairs.
The numerous blue, pale blue or whitish flowerheads are a little over 1 cm across, often forming dense domed to flat-topped clusters in leaf axils or at the ends of branches. It flowers for much of the year.
The dark seeds have scales, and end in a needle-like shape.
This plant and its close relative Ageratum conyzoides† are widely utilized in traditional medicine systems. In Brazil an infusion is prepared with the leaves or the whole plant, and used to treat colic, colds and fevers, diarrhoea, rheumatism, spasms, urinary infections, and as a tonic. It is also used there to treat burns and wounds. In other countries in Latin and South America the plant is widely used for its anti-bacterial properties. In parts of Africa, as well as for the above, it is used to treat headaches, pneumonia, eye diseases, sleeping sickness, venereal diseases, and burns. In Trinidad it is used as an abortifacient, decoagulant, and for treating diabetes. In some places it is also used as an insect repellent.
Plants of the Ageratum genus have an ingenious method of protecting themselves from insects. They contain a chemical which, when eaten by the insects, affects their juvenile hormone, making their larvae sterile.
† conyzoides is botanical Latin for ‘resembling Conyza (a type of fleabane)’. The main difference between the two species is that the flower bracts of A. conyzoides are hairy, while those of A. houstonianum are glabrous
Information about medicinal qualities of plants, or about their use as medicines, is for interest only, and is not intended to be used as a guide for the treatment of medical conditions.
Photographs taken 2009-2014, Picnic Bay
Page last updated 2nd October 2016