Weed Alert: Siam weed

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Siam Weed (Chromolaena odorata)

World Status | Identification | Propagation | Control | Legislation

Other common names — triffid weed, bitter bush, jack in the bush

World Status

Siam weed is a native of Central America and is considered to be one of the world’s worst weeds.

Siam weed was first identified in Australia in 1994 with infestations along the Tully River and near Mission Beach in north Queensland. It has the potential to spread across northern Australia and down the eastern coastline, displacing native vegetation, invading pastures and crops, and increasing the dry season fire hazard.

Siam weed was introduced into India as an ornamental plant in the 1840s where it spread to Malaya, Burma and Indonesia. This spread increased with the movement of people and materials during World War II.

Siam weed is now a serious weed in Mauritius, India, Sri Lanka, south-east Asia, China, the Philippines and Guam.

It was first reported in Africa in the 1940s. Today, it is a major weed in Nigeria, Ghana, Cameroon, Zaire and South Africa.

The Problem

Siam weed is considered one of the world’s worst tropical weeds due to its quick invasion and establishment. It invades and out-competes pastures, crops and native vegetation.

Siam weed is also a prolific seeder, producing up to 87,000 seeds per plant. The plant is toxic to livestock, killing more than 3000 cattle annually in the Philippines. The toxin also causes abortions incattle and is suspected of being a fish poison.

In some countries, dense thickets harbour wild pigs and rodents.

Identification

Habit

Siam weed is an erect or sprawling fast-growing perennial shrub, forming dense tangled thickets from 1.5–5 metres high. The growth is soft when young, but becomes hard and woody when mature.

Key identification features

• Stems have fine longitudinal lines. Branches occur in pairs (opposite) along the main stem.
• Leaves are almost triangular with a few coarse teeth and three prominent veins. Glands (dots) can be seen when held up to the light. The leaves emit a pungent odour when crushed.
• Flowers are pale blue-lilac with protruding two-branched stigmas. Siam weed flowers in May and October.
• Seed is blackish with 4–5 pale, roughened ribs.

Propagation

The seed is very light and equipped with a fine pappus which allows widespread dispersal by wind and water. The seed also contains fine barbs which means it readily sticks to clothing, equipment and animals. Most seed germinates immediately after rain though seeds can remain dormant for five years or more. Disturbance or removal of existing vegetation encourages the establishment of Siam weed.

Control

Mechanical removal of isolated plants, although effective, is impractical for dense infestations.

Slashing and burning offer temporary control but are usually followed by rapid regrowth. Herbicide application can be successful, however, repeated applications are required to kill the perennial roots.

Biological control has been investigated since 1966 with a leaf-feeding arctiid moth (Pareuchaetes pseudoinsulata) giving excellent control in the Marianas Islands but the moth is less-effective elsewhere.

Legislation

Siam weed is a prohibited import and cannot be brought into Australia. It is a Class 1 noxious weed throughout NSW under the NSW Noxious Weeds Act 1993. As such, the weed must be eradicated from the land and the land kept free of the plant.

As a notifiable weed, all outbreaks must be reported to the local council within three days.

Acknowledgements

Author: Jeff Burton, former Extension Officer (Weeds), NSW Agriculture

References

• Parsons WT, Cuthbertson EG 2001. Noxious Weeds of Australia. CSIRO Publishing. Collingwood Victoria.
• Queensland Government — Natural Resources and Mines 2001. NRM Facts — Siam Weed Chromolaena odorata.