Weed Alert: Siam weed
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- Declared in NSW under the Noxious Weeds Act 1993 (Current Status)
- National Environmental Alert List Weed (definition)
Contacts and Further InformationIf you find this weed please help to prevent its further spread by contacting your local Council Weeds Officer or the nearest NSW Department of Primary Industries office immediately for positive identification and further assistance.
Alternatively call the NSW Weeds Hotline on
1800 680 244 or send an email to email@example.com
Siam weed (Chromolaena odorata)
Siam weed is considered one of the world’s worst tropical weeds. It invades and outcompetes pastures, crops and native vegetation. Siam weed is also known as chromolaena, triffid weed, bitter bush, and Jack-in-the-bush.
Siam weed is a prolific seeder, producing up to 87 000 seeds per plant. The plant is toxic to livestock. In the Philippines it is reported to kill more than 3000 cattle annually. The toxin also causes abortions in cattle and is suspected of being a fish poison. It can cause skin problems and asthma in allergy-prone people. Dense thickets of Siam weed also harbour wild pigs and rodents.
Siam weed was first identified in Australia in 1994 with infestations along the Tully River and near Mission Beach in north Queensland. It is now found in the Townsville, Charters Towers, Cassowary Coast, Cairns and Tablelands Regional Council areas. It has the potential to spread across northern Australia and down the eastern coastline filling a similar range to lantana (Lantana camara), displacing native vegetation, invading pastures and crops, and increasing the dry season fire hazard.
Siam weed is not known to occur in New South Wales (NSW).
Siam weed is a native of Central America. Siam weed was introduced into India as an ornamental plant in the 1840s, from where it spread to Malaya, Burma and Indonesia. This spread increased with the movement of people and materials during World War II. Siam weed is now a serious weed in Mauritius, India, Sri Lanka, south-east Asia, China, the Philippines and Guam.
In Africa Siam weed was first reported in the 1940s, and is now a major weed in Nigeria, Ghana, Cameroon, Zaire and South Africa.
The seeds are very light and equipped with a fine pappus which allows widespread dispersal by wind and water. Seeds also contain fine barbs allowing them to stick easily to clothing, equipment and animals. Most seeds germinate immediately after rain and can remain dormant for 5 years or more. Disturbance or removal of existing vegetation encourages the establishment of Siam weed.
Siam weed is an erect or sprawling fast-growing perennial shrub, forming dense tangled thickets 1.5–5 metres high. The growth is soft when young, but becomes hard and woody when mature.
Key identification features
- Stems have fine longitudinal lines. Branches occur in opposite pairs along the main stems.
- Leaves occur in opposite pairs along the stems, are almost triangular, are 5-12 cm long, have a few coarse teeth on the margins, and display three prominent veins. Glands (dots) can be seen when held up to the light. The leaves emit a pungent odour when crushed.
- Flowers are pale blue-lilac with protruding two-branched stigmas. Siam weed flowers from May to October.
- Seeds are blackish with 4–5 pale, roughened ribs.
Contact your local council weeds officer for assistance with identification and control. Mechanical and chemical treatments may be used but ongoing and repeated treatments must be applied. To achieve eradication any treatments and monitoring should be conducted in association with the local council.
Siam weed is a Class 1 State Prohibited Weed across NSW under the Noxious Weeds Act 1993. It must be eradicated and land must be kept free of the plant. As a notifiable weed, all outbreaks must be reported to the local council within 24 hours, and the plant is prohibited from sale in NSW.
Written by: Jeff Burton 2003; 2012 Edition reviewed by: Rod Ensbey, John Hosking; Edited and prepared by: Elissa van Oosterhout, Birgitte Verbeek
- Parsons WT & Cuthbertson EG (2001) Noxious Weeds of Australia, CSIRO Publishing, Collingwood
- Queensland Government — Natural Resources & Mines (2001) NRM Facts — Siam Weed Chromolaena odorata