Weeds in Australia


Chromolaena odorata


Siam Weed (Chromolaena odorata) is an upright shrub growing up to 5 m tall, forming dense spreading thickets, or sprawling or scrambling plants growing up to 20 m high when climbing up vegetation. Its stems are slender and woody towards the base, with many stems produced from a long-lived root stock. Side branches are usually produced in pairs. The leaves are arrowhead-shaped, 50-120 mm long and 30-70 mm wide, with three characteristic veins in a 'pitchfork' pattern. They grow in opposite pairs along the stems and branches. As the species name 'odorata' suggests, the leaves emit a pungent odour when crushed. Clusters of 10-35 pale pink-mauve tubular flowers, 10 mm long, are found at the ends of branches. The seeds are dark coloured, 4-5 mm long, narrow and oblong, with a parachute of white hairs which turn brown as the seed dries. The root system is fibrous and generally reaches a depth of 300 mm (Parsons & Cuthbertson 1992; Hill & Ostermeyer 2000; CRC 2003).

For further information and assistance with identification of Siam Weed contact the herbarium in your state or territory.


Currently the species has a very limited distribution in Australia and is confined to a small area in coastal northern Queensland (Navie 2004). Navie (2004) also claims that it has been recorded in the Northern Territory; however, the Northern Territory Department of Natural Resources, Environment and the Arts Weeds Branch could not substantiate this claim (Dixon 2007, pers. comm.).

Habit:Herb, Shrub
Key points:
  • Siam Weed (Chromolaena odorata) is one of the worlds worst weeds.
  • It is far more cost effective to prevent Siam Weed's spread than to try to control large infestations.
  • All known outbreaks of Siam Weed in Australia are under active management, with the aim of eradication.
  • Siam Weed flowers and seeds are conspicuous during June and July.
  • If you find Siam Weed report it to your state or territory weed management agency.
How it spreads:

Siam Weed produces numerous seeds which are dispersed by the wind. Seed can also become lodged in clothing, animal fur and machinery. Much of the spread in India has been attributed to the movement of people and equipment during World War II (CRC 2003). Water has also been implicated in its dispersal (GISD 2006). Other potential dispersal pathways are military movements, seed trade, timber imports, island trade, container cargo, cyclonic winds, stock, slashing, sand and gravel, bushwalkers and back packers (Maher & Funkhouser 2006).

Where it grows:

Siam Weed is an opportunistic species generally confined to forest edges and clearings and forms dense thickets in disturbed situations. It grows best in the tropics and subtropics in areas receiving 1200 mm of rainfall or more, and though not tolerant of frost it can be found at altitudes up to 1000 m. It grows on most soil types but prefers well drained soils in full sun (Hill & Ostermeyer 2000; CRC 2003). It has also been reported as occurring on stream banks, bushland, roadsides, waste areas, neglected pastures, crops and plantations (Navie 2004).

Flower colour:Purple, White, Pink
Distribution map: Weed Distribution Map

Siam Weed is on the Alert List for Environmental Weeds, a list of 28 non-native plants that threaten biodiversity and cause other environmental damage. Although only in the early stages of establishment, these weeds have the potential to seriously degrade Australia's ecosystems (CRC 2003).

Siam Weed poses great risks to Australia's environment and economy as it is recognised as one of the world's worst tropical weeds. Siam Weed out competes and smothers crops and native vegetation, is toxic to cattle and can cause death to stock (CRC 2003, Land Protection 2007).

Allelopathic effects that suppress other vegetation is listed as an impact by Hills and Ostermeyer (2000). Siam Weed can change the fire regime of infested areas, with fires becoming more frequent and intense. There are also some health issues to humans such as skin complaints and asthma in allergy-prone individuals (Land Protection 2007).


Siam Weed is native to the warmer parts of North America, Central America and South America, from south-eastern USA and Mexico to Brazil and northern Argentina, including the West Indies (Navie 2004).


Siam Weed was discovered in Australia in 1994 at Bingil Bay, North Queensland. Soon after infestations several kilometres long were found along the Tully River and Echo Creek. The seed is believed to have entered Australia as a contaminant of pasture seed used on a grazing property in the 1960s and 1970s (Csurhes & Edwards 1998).


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This database is designed to provide information, including biological and ecological, on invasive plant species that are on a national weed list, or are legislated against in a state or territory. While reasonable efforts have been made to ensure the accuracy of the information, no guarantee is given, nor responsibility taken, by the Commonwealth for its accuracy, currency or completeness. The Commonwealth does not accept any responsibility for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the information contained in this database. The information contained in this database does not necessarily represent the views of the Commonwealth. This database is not intended to be a complete source of information on the matters it deals with. For further information on the images contained in the database please contact the copyright owner. All images in the weed identification tool are managed by the Australian Plant Image Index (APII). Various copyright conditions apply for these images. For further information on the copyright conditions of images contained in the database please contact the APII at: photo@anbg.gov.au.