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Siam Weed : A Potential Weed in Widebay Burnett region

| by Dr. Lalith Gunasekera

Invasive Plants Specialist
Bargara Beach – Bundaberg – Queensland
Scientific name: Chromolaenaodorata

( July 19, 2013, Colombo, Sri Lanka Guardian) The worldwide common name for this plant species is “Siam Weed” as it was found and troublesome in different areas in Thailand (formerly Siam).

Siam weed originated from Mexico to Brazil including the West Indies. It has been introduced to India as an ornamental in the 1840s from where it spread initially through Bengal, Burma to Thailand. Further spread took place to Sri Lanka, China, the Pacific islands, Guam, Philippines and some other South Asian and African countries with the movement of people and materials during World War 11. Now this plant has been declared as one of the major invasive plants in the world.Siam weed was first identified in Australia in 1994 as several large infestations along the Tully River in Far North Queensland. It’s a Weeds of National Significance (WoNS), class 1 weed in Queensland and targeted species in Far North Queensland.

Siam weed growing under coconut in Sri Lanka
Siam wees is an erect or sprawling fast growing shrub, forming dense thickets 1.5 to 5 meter high or higher when supported on adjacent plants or other objects.

Stems: The stems are smooth, yellowish, erect and sprawling, slender, sparsely hairy, with fine longitudinal lines, much branched, lateral branches in pairs, succulent at the tips, woody at the base when old.

Leaves:The leaves of Siam weed are soft, green, opposite, hairy, more or less triangular, pointed towards the apex, with 3 prominent veins, 5 – 12 cm long, 3 to 7 cm wide. Leaf stalk is 6 cm long and leaves emit a pungent odour when crushed.

Roots: The root system is fibrous and shallow in most soils and develops an enlargement at the junction of the stem and root which is referred to as the basal ball.

Flowers: Produce large number of white flowers at the end of branches.

Seeds: Siam weed produces large numbers of windborne seeds within 8-10 weeks after flowering. One plant can produced up to 100000 seeds per year. Each seed has a tuft or white hairs that allow it to be transported by wind and water. Seeds will also attach to vehicles, machinery, clothing, footwear and animals. Light is essential for seed germination. Newly collected seeds show a greater percentage of germination when exposed to sunlight.

Most seeds germinate immediately after rain, though some appear to remain dormant for several years.


Siam weed poses great risks to natural environment, forestry, primary industries and the economy.
• Outcompetes and smothers crops and native vegetation. It is a weed of 13 crops in 23 countries.
• Siam weed contains very high nitrate levels. It has caused cattle deaths and abortions where animals have fed with Siam leaves contaminated fodder.
• The leaves also used as a fish poison.
• Dense thickets harbour for wild pigs.
• Difficult to control and costly.
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