Weed Identification

Australia > > Siam Weed

Siam Weed

Chromolaena odorata


IMPORTANT: IF YOU SUSPECT THE PRESENCE OF THIS PLANT, PLEASE REPORT IT BY CALLING 1800 084 881 (local call cost anywhere in Australia).

Alternative Name(s): Chromolaena.

Family: Asteraceae.

Form: Shrub

Origin: Native to tropical South America north to Mexico and to the Caribbean Islands.

Flowers/Seedhead: Flowers: Terminal flat-topped clusters of white to pale lilac heads of flowers. Flowers winter.

Description: Perennial bush to 7 m tall when growing in the open or to 20 m as a climber. Leaves triangular, 512 cm long, with forward facing serrations on the margins. Stem with soft pith (see photo). Seeds brown to black, 45 mm long, with white parachute-like white hairs (pappus) at the top of the seed, 45 mm long.

Distinguishing features: Distinguished by the pungent odour of the leaves when crushed, climbing habit, and soft green opposite leaves with three prominent veins.

Dispersal: Seeds by wind and water, and by movement of stem and root fragments. Seeds may be blown long distances and also moved in mud on machinery and recreational vehicles.

Scrambling shrub with white flowers
& buds, Indonesia, February
photo C. Wilson

Notes: Potentially a serious weed of tropical and subtropical coastal areas where the rainfall exceeds 1,000 mm/annum. Dies back to the crown in dry periods, re-shooting again after rain. Has caused death of cattle in other countries. Can cause allergic reactions in humans. First outbreak in Australia found in 1994 along the Tully River, Queensland.


    Department of Natural Resources and Mines, Queensland. NR&M Fact Sheet series No. PP49. World's Worst Weeds. L. Holm et al., 1977, pages 212216. Noxious Weeds of Australia. W. Parsons and E. Cuthbertson, 1992, pages 270272.

Three-veined leaves
photo C. Wilson

Top: seeds & pappus
Middle: seedling
Bottom: bark & bark

This weed has been included in the WEEDeck field guide as card S20

More information about WEEDeck is available from Sainty & Associates Pty. Ltd.


Australia > > Siam Weed


National Weeds Strategy
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Fax: (03) 6343 1877
Email: jthorp@weeds.org.au

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