An upright, sprawling or scrambling shrub with stems produced from a long-lived root-stock. Its slender stems are much-branched, with the side-branches usually being produced in pairs. Its oppositely arranged leaves (5-12 cm long and 3-7 cm wide) are triangular-shaped and have toothed margins. Its small, fluffy, pale pink or pale mauve flower-heads are borne in dense clusters at the tips of the branches. Its black or dark brown seeds (4-5 mm long) are topped with numerous white to brownish hairs (5-6 mm long).
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).
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.
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.
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.
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.
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).
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).
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: Siam weed (Chromolaena odorata) is a large upright, terrestrial plant (1.5-5 m tall) with stems that are not hollow and relatively broad leaves with slightly toothed (i.e. serrated) margins. Its small flower-heads (about 3 mm across) are pale pink or mauve in colour and are borne in dense clusters at the tips of the branches.Senegal tea plant (Gymnocoronis spilanthoides) is a semi-aquatic plant (1-1.5 m tall) with somewhat hollow stems and relatively narrow leaves with finely toothed (i.e. serrated) margins. Its relatively large flower-heads (15-20 mm across) are white or pale purplish in colour and borne in few-flowered clusters at the tips of the branches.crofton weed (Ageratina adenophora) is an upright, terrestrial plant (1-2 m tall) with stems that are not hollow and relatively broad leaves with slightly toothed (i.e. crenated or serrated) margins. Its small flower-heads (5-7 mm across) are white in colour and borne in dense clusters at the tips of the branches.mistflower (Ageratina riparia) is a low-growing (i.e. decumbent) or scrambling terrestrial plant (40-60 cm tall) with stems that are not hollow and relatively narrow leaves with toothed (i.e. serrated) margins. Its small flower-heads (about 5 mm across) are white in colour and borne in dense clusters at the tips of the branches. 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).