Status: not present in WA
Native to tropical South America north to Mexico and to the Caribbean Islands, but now widespread throughout the tropics.
Perennial shrub up to seven metres tall when growing in the open or up to 20 metres as a climber; distinguished by the pungent odour of the leaves when crushed, its abundant white to pink flowers, and its soft green opposite leaves each with three prominent veins.
Stems: Stem with soft pith, soft at tip but soon becoming woody.
Flowers: Showy terminal flat-topped clusters of white to pale lilac heads of flowers. Flowers in winter.
Leaves: Triangular, 5–12 centimetres long, with forward facing hollow teeth on the margins, borne in opposite pairs, on stalks to six centimetres long.
Seeds: Brown to black, four to five millimetres long, with parachute-like hairs (pappus) four to five millimetres long at the top of the seed, white at first, but turning brown when mature.
Plant dispersal occurs primarily by seeds that are spread by wind and water. Seeds may be blown long distances and also moved in mud on machinery, clothing and recreational vehicles.
Agricultural and economic impacts
Siam weed is a serious weed of tropical and subtropical coastal areas where the rainfall exceeds 1000 millimetres/annum. It dies back to the crown in dry periods, re-shooting again after rain. It has caused death of cattle in other countries, and can cause allergic reactions in humans. The first outbreak in Australia was in 1994 along the Tully River, Queensland. It is also found on the Cocos (Keeling) Islands.
Declared pest category
The Western Australian Organism List (WAOL) contains information on the area(s) in which this pest is declared and the control and keeping categories to which it has been assigned in Western Australia (WA). Search for siam weed in the WAOL using the scientific name Chromolaena odorata.
Requirements for land owners/occupiers and other persons
Requirements for land owners/occupiers and other persons if this pest is found can be sourced through the declared plant requirements link.
Search > detect > report
Pest and Disease Information Service (PaDIS)
Control methods for this declared plant can be found through the siam weed control link.