Siam Weed

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Chromolaena odorata


Siam weed is a perennial that grows as a dense tangling bush up to 3 metres; however it can climb up trees to 20 metres. This plant looks very similar to lantana when not flowering. It often occurs amongst thickets of lantana.

Stem and roots several smooth, round and brittle stems develop from the crown. The root system is shallow and there is an enlargement at the junction of the stem and root.

Leaves are triangular, soft, green and hairy, with a distinctive three vein pitchfork pattern. New leaf growth can be purple.

Flowers are pale lilac or white which turn a darker pink when mature. It flowers in autumn, winter and spring.

Seeds are numerous with white hairs and tiny sticky barbs.

Habitat it is suited to highly productive land types and invades rainforests, as well as creek and riverbank vegetation.

Weed characteristics it can out-compete other crops and vegetation because of its growth rate. It is poisonous to cattle and may also cause skin complaints and asthma in allergy prone people.

Dispersal Seeds can be transported by wind and water or by sticking to clothing, foot wear, animals, vehicles and machinery.

Declaration Details

This species is a Class 1 declared plant under Queensland legislation.

How to act

New infestations must be reported to Biosecurity Queensland on 13 25 23. The plants must be identified and removed by approved officers and have their locations recorded prior to removal. All control methods should target plants before they set seed. Physical removal of basal/root ball is effective for isolated and small infestations with removal of all plant material important, or chemical control with a broad leaf selective herbicide. It is essential to mark the site for follow up monitoring and control of emergent growth.

Refer to Weed Control Methods.


Related information

Siam Weed © NQ Dry Tropics 2011
Siam Weed © NQ Dry Tropics 2011

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