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Scientific Name
Synonyms
Common Names
Family
Origin
Naturalised Distribution
Cultivation
Habitat
Distinguishing Features
Habit
Stems and Leaves
Flowers and Fruit
Reproduction and Dispersal
Impacts
Other Impacts
Control 
Similar Species
Legislation
Management
Sources
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Click on images to enlarge


habit (Photo: Land Protection, QDNRW)


habit in flower (Photo: Land Protection, QDNRW)


side-branches produced in pairs (Photo: Land Protection, QDNRW)


younger stem and paired leaves (Photo: Land Protection, QDNRW)


close-up of leaf (Photo: Land Protection, QDNRW)


leaves and young flower-heads (Photo: Chris Gardiner)


clusters of older flower-heads (Photo: Land Protection, QDNRW)


close-up of flower-heads (Photo: Chris Gardiner)


mature flower-heads (Photo: Land Protection, QDNRW)


close-up of seeds (Photo: Tracey Slotta at USDA PLANTS Database)


young plant (Photo: Land Protection, QDNRW)

Siam weed
Chromolaena odorata

Scientific Name

Chromolaena odorata (L.) R.M. King & H. Robinson

Synonyms

Eupatorium conyzoides Vahl
Eupatorium odoratum L.

Common Names

Armstrong's weed, baby tea, bitter bush, butterfly weed, Christmas bush, chromolaena, devil weed, eupatorium, Jack in the bush, Jack-in-the-bush, kingweed, paraffinbush, paraffinweed, Siam weed, turpentine weed, triffid weed

Family

Asteraceae (Queensland, New South Wales, the ACT, Victoria, Tasmania, Western Australia and the Northern Territory)
Compositae (South Australia)

Origin

This species is native to the warmer parts of south-eastern USA (i.e. southern Florida and Texas), Mexico, the Caribbean and tropical South America (i.e. French Guiana, Guyana, Surinam, Venezuela, Bolivia, Colombia, Ecuador, Peru, Paraguay, Brazil and northern Argentina).

Naturalised Distribution

Siam weed (Chromolaena odorata) is a relatively recent introduction and currently has a very limited distribution in northern Queensland. However, its potential distribution is much wider than at present. 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.

It is also very widely naturalised and extremely invasive in tropical and southern Africa, tropical Asia (e.g. India, China, Indonesia, East Timor and the Philippines), the Mascarenes and the Pacific (i.e. Micronesia, Melanesia, Polynesia and Hawaii).

Cultivation

An accidental introduction to Australia and not deliberately cultivated.

Habitat

A potential weed of tropical and sub-tropical regions that inhabits waterways (i.e. riparian areas), bushland, forest margins, roadsides, disturbed sites, waste areas, neglected pastures, crops and plantations.

Distinguishing Features

Habit

An upright (i.e. erect) or sprawling shrub forming thickets and usually growing 1.5 to 3 m tall in the open, however it may reach greater heights (6-20 m) when climbing over trees and other taller vegetation.

Stems and Leaves

The slender stems are generally yellowish-green and somewhat hairy (i.e. pubescent), but become woody towards the base of the plant. These stems grow up to 7 m or more in length and several are usually produced from the plants long-lived root-stock (i.e. crown). They are much-branched, with the side (i.e. lateral) branches usually being produced in pairs in the leaf forks (i.e. axils).

The oppositely arranged leaves (5-12 cm long and 3-7 cm wide) are triangular or egg-shaped in outline (i.e. ovate) and have a pointed tip (i.e. acute apex). They are hairy (i.e. pubescent) on both surfaces and have coarsely toothed (i.e. serrated) margins. These leaves are borne on stalks (i.e. petioles) up to 6 cm long (usually 10-15 mm), and give off a strong odour when crushed.

Flowers and Fruit

The small flower-heads (i.e. capitula) do not have any 'petals' (i.e. ray florets) and are borne in dense clusters at the ends of the branches (i.e. in terminal panicles). 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 (15-30) tiny flowers (i.e. tubular florets). These tiny flowers (10-12 mm long) are surrounded by several layers of overlapping slender bracts (i.e. an involucre) 8-9 mm long. Each flower-head (i.e. capitulum) is borne on a stalk (i.e. peduncle) 10-30 mm long. Flowering occurs from late summer through to early spring, but is most abundant during winter.

The black or dark brown 'seeds' (i.e. achenes) are 4-5 mm long and topped with a ring (i.e. pappus) of white to brownish coloured hairs (5-6 mm long).

Reproduction and Dispersal

This plant reproduces mainly by seed, which are easily blown and dispersed by the wind. Seeds may also be spread by machinery, water, vehicles, animals, in clothing, and in contaminated agricultural produce. Pieces of the crown of the plant can also take root and grow, and these may be spread about during cultivation and other soil moving activities (e.g. road maintenance).

Impacts

Though Siam weed (Chromolaena odorata) is not yet widespread in Australia, and is currently being eradicated, it is regarded as a potentially very serious environmental weed in northern Australia. It is thought that this species would thrive along much of the coastal and sub-coastal districts of northern Western Australia, the Northern Territory and Queensland to north-eastern New South Wales. For this reason it is listed on both the National Environmental Weed Alert List and the Northern Australia Quarantine Strategy list. It is also seen as a major invasive species worldwide, and the Global Invasive Species Database includes it in the top 100 of the world s worst invasive alien species.

Siam weed (Chromolaena odorata) forms dense stands that prevent the establishment of other species, both due to competition and allelopathic effects, and interferes with natural ecosystem processes in many countries. As well as altering the integrity and diversity of these natural plant communities, it can have an impact on animal populations by replacing food plants and making nesting habitats unsuitable.

Other Impacts

This species may cause skin complaints and asthma in allergy-prone people and its leaves are known to be toxic to livestock. It is also a major weed in forestry plantations and crops in tropical regions, including rubber, oil palm and coffee plantations. during the dry season, Siam weed (Chromolaena odorata) plants dry out and may become a fuel source that promotes bushfires.

Control

Biosecurity Queensland Control Fact Sheet

Similar Species

Siam weed (Chromolaena odorata) may occasionally be confused with Senegal tea plant (Gymnocoronis spilanthoides ), crofton weed (Ageratina adenophora ) and mistflower (Ageratina riparia ). These species can be distinguished by the following differences:

Billygoat weed (Ageratum conyzoides subsp. conyzoides), blue billygoat weed (Ageratum houstonianum ), praxelis (Praxelis clematidea ) and vernonia (Cyanthillium cinereum) have similar flowers to Siam weed (Chromolaena odorata), but they are usually much darker pink or bluish in colour. These species are also much smaller annual plants (usually less than 1 m tall).

Another species of chromolaena (i.e. Chromolaena squalida) has also become locally naturalised in northern Queensland. It has similar leaves and flowers to Siam weed (Chromolaena odorata), but it is usually not very branched and much smaller in stature (less than 1.5 m tall).

Legislation

This species is declared under legislation in the following states and territories:

Management

For information on the management of this species see the following resources:

Sources

Anonymous (2001). Siam weed. Northern Pastoral Memo, July 2001. Department of Agriculture, Western Australia.

Anonymous (2002). A Global Compendium of Weeds. http://www.hear.org/gcw. Hawaiian Ecosystems at Risk Project and Department of Agriculture - Western Australia.

Anonymous (2003). Department of Agriculture, Western Australia. http://www.agric.wa.gov.au. The State of Western Australia, Perth, Western Australia.

Anonymous (2003). Siam weed or chromolaena (Chromolaena odorata). Weed Management Guide. CRC for Australian Weed Management and the Commonwealth Department of Environment and Heritage.

Anonymous (2004). Siam weed. Fact sheet No. 56. http://www.affa.gov.au. Department of Agriculture, Fisheries and Forestry, Commonwealth of Australia.

Anonymous (2006). Australia's Virtual Herbarium. http://www.anbg.gov.au/avh. Australian National Botanic Gardens, Environment Australia, Canberra, ACT.

Anonymous (2006). Chromolaena odorata (L.) R.M.King & H.Robinson, Asteraceae. Pacific Island Ecosystems at Risk (PIER): plant threats to Pacific ecosystems. http://www.hear.org/Pier/species/chromolaena_odorata.htm. Institute of Pacific Islands Forestry, Hawaii, USA.

Anonymous (2006). Declared Plants of Queensland. Natural Resources and Water Facts - pest series, PP1. The State of Queensland (Department of Natural Resources and Water), Brisbane, Queensland.

Anonymous (2006). Germplasm Resources Information Network (GRIN). http://www.ars-grin.gov/cgi-bin/npgs/html/index.pl. National Germplasm Resources Laboratory, National Genetic Resources Program, Agricultural Research Service (ARS), United States Department of Agriculture (USDA), Beltsville, Maryland, USA.

Anonymous (2006). Siam weed. Chromolaena odorata. Natural Resources and Water Facts - pest series, PP49. The State of Queensland (Department of Natural Resources and Water), Brisbane, Queensland.

Anonymous (2007). NSW Department of Primary Industries. http://www.dpi.nsw.gov.au. NSW Department of Primary Industries, Orange, New South Wales.

Anonymous (2007). Weeds Australia. http://www.weeds.org.au. National Weeds Strategy Executive Committee, Launceston, Tasmania.

Binggeli, P. (1997). Chromolaena odorata (L.) King & Robinson (Asteraceae). http://members.lycos.co.uk/WoodyPlantEcology. Woody Plant Ecology - Invasive Woody Plants. Pierre Binggeli.

Bostock, P.D. and Holland, A.E. (2007). Census of the Queensland Flora 2007. Queensland Herbarium, Environmental Protection Agency (EPA), Brisbane, Queensland.

Francis, J.K. (2003). Chromolaena odorata (L.) King & H.E. Robins. Christmas bush. Asteraceae. US Department of Agriculture, Forest Service, San Juan, Puerto Rico, USA.

Henderson, L. (2001). Alien Weeds and Invasive Plants. Plant Protection Research Institute, Agricultural Research Council, South Africa.

Hills, L.A. and Ostermeyer, N. (2000). Siam Weed or Christmas Bush (Chromolaena odorata). Agnote. Weeds Branch, Primary Industry and Fisheries, Northern Territory of Australia, Darwin, Northern Territory.

Lazarides, M., Cowley, K. and Hohnen, P. (1997). CSIRO Handbook of Australian Weeds. CSIRO Publishing, Collingwood, Victoria.

Navie, S.C. (2004). Declared Plants of Australia. CD-ROM. The University of Queensland, St. Lucia, Queensland.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious Weeds of Australia. Inkata Press, Melbourne, Victoria.

Smith, N.M. (2002). Weeds of the Wet/Dry Tropics of Australia - A Field Guide. Environment Centre Northern Territory, Darwin, Northern Territory.

Waterhouse, B.M. (2003). Know your enemy: recent records of potentially serious weeds in northern Australia, Papua New Guinea and Papua (Indonesia). Telopea 10: 477-485.