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Weeds: Siam weed
Siam weed is also known as chromolaena (Chromolaena odorata)
Siam weed is one of the world’s worst weeds, with a phenomenal growth rate and massive seed production. Dense infestations can smother tropical fruit crops, young forestry plantations and pastures. It is also a serious environmental weed, invading native woodlands and posing a major fire hazard in the dry season.
Keep a Top Watch
Be vigilant for any new and unfamiliar plants in your region. Siam weed could potentially invade much of Australia’s tropical and sub-tropical coastal fringe from the Kimberley region to northern NSW. Learn what it looks like and contact your quarantine officer if you think you’ve seen Siam weed plants in your region.
Profile: Siam weed forms impenetrable thickets to 3m tall in open sites like river banks and pastures. Plants can reach 10m by scrambling through adjacent vegetation. Dense infestations smother underlying plants by shading and competing for moisture and nutrients.
Identification: Branches and leaves grow in opposite pairs. Leaves are triangular with three prominent veins and toothed margins. From June to September it produces masses of fluffy white to pale pink flowers. The seeds are 4–5 mm long with a parachute of pale bristles that helps them travel in the wind and cling to clothing. The seeds are notorious for spreading with military personnel and equipment.
Photos below: A closeup of Siam weed flowers. The plant is usually 2-5 metres tall. Siam weed smothers native plants and forms dense infestations.
Distribution: Siam weed comes from South America but has become a serious weed in Africa, southeast Asia and New Guinea. A tiny outbreak near Tully, north Queensland, is under eradication.
Threat: If Siam weed established in Australia, control would cost millions of dollars a year. Tropical crops, grazing land and natural ecosystems would be threatened.
Quarantine: Siam weed is a key target of quarantine surveys in northern Australia. Rigorous inspection procedures ensure that no seeds or plant material return to Australia from high risk areas such as East Timor and New Guinea.