Quick Facts

Quick facts

  • Siam Weed (Chromolaena odorata), a native plant of North America, Central America and South America, 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.

What Does It Look Like?

What is it?

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.

The small flower-heads do not have any 'petals' and are borne in dense clusters of 10–35 tubular flowers, at the ends of the branches. These flower-heads (about 10 mm long and 3 mm wide) are pale pink or pale mauve in colour (sometimes appearing whitish when older) and consist of numerous tiny flowers (tubular florets). The flowers are surrounded by several layers of overlapping slender bracts 8–9 mm long. Flowering occurs from late summer through to early spring, but is most abundant during winter (Weeds of Australia 2016).

The seeds are achenes (fruit) and 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.

Flower colour

Purple, White, Pink

Growth form (weed type/habit)

Herb, Shrub

Where it currently grows? Preferred habitat

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).

Are there similar species?

Siam Weed may occasionally be confused with Gymnocoronis spilanthoides (Senegal Tea Plant), Ageratina adenophora (Crofton Weed) and Ageratina riparia (Mistflower). Navie (2004) provided the following guide for distinguishing between these species:

Siam Weed is a large upright, terrestrial plant 1.5-5 m tall with stems that are not hollow. Leaves are relatively broad with slightly toothed margins. Its small flower heads are 15-20 mm in diameter, pale pink or mauve in colour and borne in dense clusters at the tips of the branches.

Senegal Tea Plant is an aquatic plant with somewhat hollow stems and narrowly egg shaped (ovate) or lance-shaped (lanceolate) leaves with finely toothed (serrated) margins. Flower heads are 15-20 mm in diameter and are white or purplish white and borne in clusters at the tips of the stems.

Crofton Weed is an erect terrestrial plant 1-2 m tall with stems that are not hollow. Leaves are relatively broad with slightly toothed (crenate or serrate) margins. Flower heads are relatively small 5-7 mm in diameter, and are white in colour and borne in dense clusters at the tips of the branches.

Mistflower is a creeping or scrambling terrestrial plant 40 -60 cm high. Stems are not hollow. Leaves are relatively narrow with toothed margins. Flower heads are relatively small, about 5 mm in diameter, are white in colour and borne in dense clusters at the tips of the branches.

Billygoat Weed (Ageratum conyzoides), Blue Billygoat Weed (Ageratum houstonianum) and Vernonia (Cyanthillium cinereum) have similar flowers to Siam Weed, but these are much smaller annual plants (less than 1 m) and usually have alternately arranged upper leaves.

Why Is It A Weed?

What are its impacts?

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).

Agriculture: 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).

Native ecosystems: 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).

Human: Siam Weed may cause skin complaints and asthma in allergy-prone people (Weeds of Australia 2016).

How does it spread?

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).

What is its history in Australia?

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).

How To Manage It?

Best practice management

Siam Weed is a species targeted for national eradication under the National Resource Management Ministerial Council's National Cost-Sharing Eradication Programmes. These programmes map and monitor the full distribution of the species, and coordinate or undertake activities to eradicate that species from Australia.

Do not try to control Siam Weed without expert assistance from your state or territory weed management agency or local council. Control effort that is poorly performed or not followed up can actually help spread the weed and worsen the problem (CRC 2003).

Chemical control: Currently herbicides are used to control Siam Weed, however the potential also exists to treat it with biological control agents. A leaf feeding moth and a gall fly have shown some success in controlling Siam Weed in Indonesia (CRC 2003).

Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au.

Non-chemical control: Some other management strategies include preventing the use of unsterilised sand; maintaining strict hygiene standards for machinery; preventing stock movement and sale of pasture seed from infested areas; regulating camping and bushwalking in infested areas; and continued monitoring by landowners and governance agencies of affected jurisdictions (Maher & Funkhouser 2006).

Biological control: A stem-galling fly, Cecidochares connexa, was released in certain areas of Far North Queensland as a biological control from Siam Weed (see New aerial biocontrol weapon released to help fight notorious tropical Siam weed).

Does it have a biological control agent?


When does it grow? (lifecycle/growth calendar)

Siam Weed is a perennial species that can live for up to 10 years. Seed of Siam Weed germinates during the wet season and seedling growth is prolific with seedlings that have germinated in the early wet season sometimes flowering the following season (June-July). Flowering is triggered by day length and thus all plants flower at much the same time. The ripe fruits are dispersed several months after flower. It is very well adapted to the wet-dry tropical climate of Australia's north because its above ground foliage can die off during the dry season (May-October) when virtually no rain falls. However, the roots remain alive and the vegetation grows back vigorously during the wet season (November-March) (CRC 2003).

Where Is It Found?

Which states and territories is it found?


What areas within states and territories is it found?

Currently the species has a limited distribution in Queensland (Navie 2004). It was first found in Australia in 1994, when several large infestations were spotted along the Tully River and at Bingil Bay near Mission Beach in far northern Queensland. More recent surveys have identified infestations in the Townsville-Thuringowa, Mossman and Mt Garnet areas (Weeds of Australia 2016).

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.).

Where does it originate?

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).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?


Where is it a declared weed?


Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?


Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?


Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Chromolaena odorata

Other scientific names (synonyms)?

  • Eupatorium affine Hook & Arn.
  • Eupatorium brachiatum Wikstrom
  • Eupatorium clematis DC.
  • Eupatorium conyzoides Vahl
  • Eupatorium conyzoides Mill.
  • Eupatorium divergens Less
  • Eupatorium floribundum Kunth
  • Eupatorium graciliflorum DC.
  • Eupatorium odoratum L.
  • Eupatorium sabeanum Buckley
  • Eupatorium stigmatosum Meyen & Walp.
  • Osmia conyzoides (Vahl) Sch.-Bip.
  • Osmia divergens (Less.) Sch.-Bip.
  • Osmia floribunda (Kunth) Sch.-Bip.
  • Osmia graciliflora (DC.) Sch.-Bip.
  • Osmia odorata (L.) Sch.-Bip.

Does it have other known common name(s)?

Chromolaena, Eupatorium, Bitter Bush, Christmas Bush, Devil Weed, Hagonoy, Jack in the Bush, Triffid Weed, Turpentine Weed, Armstrong's Weed, King Weed, Paraffin Weed, Paraffin Bush, Baby Tea, Agonoi, Siam-kraut

Other Management Resources

Blackberry – a community-driven approach in Victoria

Blackberry the weed (Rubus fruticosus aggregate) was first introduced to Australia by European settlers in the mid-1800s as a fruit. It was recognised as a weed by mid-1880s. Blackberry is a serious issue across Australia. It is estimated that blackberry infests approximately 8.8 million hectares of land at an estimated cost of $103 million in annual control and production losses.

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