Siam Weed

Chromolaena odorata

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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, 5–12 cm long, with forward facing serrations on the margins. Stem with soft pith (see photo). Seeds brown to black, 4–5 mm long, with white parachute-like white hairs (pappus) at the top of the seed, 4–5 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.