Siam weed (Chromolaena odorata)

PROHIBITED MATTER: If you see this plant report it. Call the NSW DPI Biosecurity Helpline 1800 680 244
Also known as: chromolaena, triffid weed, bitter bush, Jack-in-the-bush

Siam weed is considered one of the world’s worst tropical weeds. It invades and outcompetes pastures, crops and native vegetation.


How does this weed affect you?

Siam weed is considered one of the world’s worst tropical weeds. It invades and outcompetes pastures, crops and native vegetation.

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.

Where is it found?

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.

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).

Distribution map

How does it spread?

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.

What does it look like?

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.


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 WeedChromolaena odorata

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You must report this plant if you see it anywhere in NSW. Call the helpline listed above. Help will then be provided to remove and destroy it. This serious weed could spread if control efforts do not follow all protocols. Not reporting it is a breach of your legal biosecurity duty.

Herbicide options

Users of agricultural or veterinary chemical products must always read the label and any permit, before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this information. To view permits or product labels go to the Australian Pesticides and Veterinary Medicines Authority website

See Using herbicides for more information.

Fluroxypyr 333 g/L (Starane™ Advanced)
Rate: 210 ml in 100L of water
Comments: Handgun application
Withholding period: 7 days.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

Picloram 44.7 g/kg + Aminopyralid 4.47 g/L (Vigilant II ®)
Rate: Undiluted
Comments: Cut stump/stem injection application. Apply a 3–5 mm layer of gel for stems less than 20 mm. Apply 5 mm layer on stems above 20 mm .
Withholding period: Nil.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area Duty
All of NSW General Biosecurity Duty
All plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
All of NSW Prohibited Matter
A person who deals with prohibited matter or a carrier of prohibited matter is guilty of an offence. A person who becomes aware of or suspects the presence of prohibited matter must immediately notify the Department of Primary Industries

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For technical advice and assistance with identification please contact your local council weeds officer.
For further information call the NSW DPI Biosecurity Helpline on 1800 680 244 or send an email to

Reviewed 2017