Siam weed (Chromolaena odorata)

Also known as: chromolaena, triffid weed, bitter bush, Jack-in-the-bush

Siam weed is a sprawling shrub that forms dense thickets. It is toxic to livestock, causes allergies in humans and outcompetes pastures, crops and native vegetation.

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How does this weed affect you?

Siam weed is a fast growing plant that forms dense thickets. It:

• is toxic to livestock
• can cause skin problems and asthma in allergy-prone people
• outcompetes pastures reducing productivity
• reduces yields of crops such as sugar cane and dryland rice
• competes with native vegetation
• can harbour feral animals and insect pests
• can host fungal diseases
• increases the dry season fire hazard.

Livestock poisoning

Siam weed contains pyrrolizidine alkaloids which are poisonous to livestock. It has killed cattle and caused cows to abort calves. It also contains high levels of nitrates which are toxic to ruminants (e.g. sheep, goats and cattle).

What does it look like?

Siam weed is an upright or sprawling perennial shrub which lives for up to 10 years. It is fast-growing and forms dense, tangled thickets around 2 m tall. It can grow up to 20 m tall if it climbs over other trees and shrubs. 

It dies back in the dry season but promptly reshoots and can grow back quickly after fire or slashing.

Leaves are: 

• diamond, teardrop or arrowhead-shaped
• 5–12 cm long and 3–7 cm wide
• pungent smelling when crushed
• lobed or toothed with shallow teeth on the margins
• prominently veined with 3 veins near the base which look like a pitchfork
• arranged in opposite pairs on the stem.

Flowerheads are:

• pale blue-lilac or pink-mauve (rarely white)
• 8–10 mm long and 3–4 mm wide
• in bunches of up to 70 at the end of branches
• topped with soft threads 
• on short stalks
• present from May to October. 

Seeds are:

• 4–5 mm long
• blackish with a tuft of white (or brown) hairs (5–6 mm long) on one end
• blow off the seed head by the wind.

Stems are:

• yellowish
• finely ribbed and slightly hairy
• branching in opposite pairs along the main stems
• soft when young, then hard and woody when mature. 

Roots are:

• fibrous
• shallow, mostly in the top 30 cm of soil
• enlarged where they join the stem (called a basal ball).

Similar looking plants:

There are a few other weeds present in NSW which look similar to Siam weed. The biggest difference is that Siam weed becomes a tall woody shrub, whereas the look-alikes are usually herbs less than 1 m tall. Similar plants include:

• Praxelis (Praxelis clematidea), which also has a pungent smell when the leaves are crushed. Its leaves are usually 2.5–6 cm long and 1–4 cm wide with deep teeth along the edge and it has slender flowerhead bases (i.e. are longer than they are wide).
• Billygoat weed (Ageratum houstonianum, A. conyzoides)which both have a pungent smell when the leaves are crushed. Their leaves are usually 2–7 cm long and 1.5–6 cm wide with shallow teeth along the edge. They have stout, barrel-shaped flowerhead bases (i.e. are roughly as long as they are wide).
• Crofton weed (Ageratina adenophora) and mist flower (Ageratina riparia) do not have a pungent smell when the leaves are crushed. They have leaves that are usually 2–12 cm long and 0.8–6 cm wide with shallow teeth along the edge and have stout, barrel-shaped flowerhead bases (i.e. are roughly as long as they are wide).

Indian weed (Sigesbeckia orientalis) is a native plant, which also looks similar to Siam weed when not in flower. It has yellow flowers.  

Where is it found?

Siam weed is not currently known to be growing in NSW. It was first found in Australia in 1994 in northern Queensland and is now found as far south as Rockhampton. 

Siam weed could spread across northern Australia and the eastern coastline, filling a similar range to lantana (Lantana camara).

It is a native of Central America and northern South America and is a weed in Oceania, Asia, India and Africa. 

What type of environment does it grow in?

Siam weed grows in tropical and subtropical regions.

It grows best in:

• rich well drained soils
• full sun or partial shade
• areas with more than 600 mm annual rainfall
• frost free regions.

Siam weed can tolerate dry periods but it does not grow well on waterlogged soils, saline soils or in full shade.

It can grow in bushland, grazing land, crops and along watercourses and roadsides.

Maps and records

• Recorded presence of Siam weed during property inspections (Map: Biosecurity Information System - Weeds, 2017-2023)
  These records are made by authorised officers during property inspections under the Biosecurity Act 2015. Officers record the presence of priority weeds in their council area and provide this to the NSW Department of Primary Industries. Records reflect the presence of the weed on the date of inspection.
  
  

How does it spread?

By seed

Siam weed produces up to 87,000 seeds per plant which are easily spread by the wind. Fine barbs on the seed also help them stick to clothing, machinery, equipment and animals. Siam weed could also spread in pasture seed.  In Asia and Africa seeds have even spread via contaminated ballast.

Most seeds germinate immediately after rain but they can remain viable for more than 6 years. Disturbance or removal of existing vegetation encourages the establishment of siam weed.

Plant parts

In cropping areas the basal root ball could be broken and spread during cultivation. Plants can reshoot if dug up but left in contact with the soil.

References

CRC Weed Management (2003) Weed Management Guide: Siam weed Chromolaena odorata.  Retrieved July 2020 from: https://www.environment.gov.au/biodiversity/invasive/weeds/publications/guidelines/alert/pubs/c-odorata.pdf

DAF (2020) Queensland Department of Agriculture and Fisheries (DAF) Siam weed Chromolaena odorata and Chromolaena squalida Fact Sheet. Retrieved July 2020 from: https://www.daf.qld.gov.au/__data/assets/pdf_file/0015/50028/siam-weed.pdf 

Koutika, L. S., & Rainey, H. J. (2010). Chromolaena odorata in different ecosystems: weed or fallow plant?. Applied Ecology and Environmental Research, 8(2), 131-142.

McKenzie, R. (2012). Australia's poisonous plants, fungi and cyanobacteria: a guide to species of medical and veterinary importance. CSIRO.

Parsons, W.T., & Cuthbertson, E. G. (2001). Noxious weeds of Australia. CSIRO publishing.

Pasiecznik, N. (2007). CABI Invasive Species Compendium Data Sheet: Chromolaena odorata (Siam weed). Retrieved July 2020 from: https://www.cabi.org/isc/datasheet/23248#toimpactSummary

Queensland Government: Natural Resources & Mines (2001) NRM Facts — Siam Weed Chromolaena odorata

Queensland Government (2016) Restricted Invasive Plants: Siam Weed. Retrieved July 2020 from:  https://www.business.qld.gov.au/industries/farms-fishing-forestry/agriculture/land-management/health-pests-weeds-diseases/weeds-diseases/invasive-plants/restricted/siam-weed 

More information

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Control

Please do not attempt to treat or dispose of this weed yourself. Report this plant if you see it anywhere in NSW by calling the helpline listed at the top of this page immediately. 

NSW DPI will lead an initial response for the treatment and disposal of the plant to stop it from spreading.

Herbicide options

See Using herbicides for more information.

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Fluroxypyr 333 g/L (Starane™ Advanced)
Rate: 210 ml in 100L of water
Comments: Handgun application
Withholding period: Do not graze failed crops and treated pastures or cut for stock food for 7 days after application. See label for more information.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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Picloram 44.7 g/L + Aminopyralid 4.47 g/L (Vigilant II ®)
Rate: Undiluted
Comments: Cut stump/stem injection application. Apply a 3–5 mm layer of gel for stems less than 20 mm. Apply 5 mm layer on stems above 20 mm .
Withholding period: Nil.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All pest plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
All of NSW	Prohibited Matter A person who deals with prohibited matter or a carrier of prohibited matter is guilty of an offence. A person who becomes aware of or suspects the presence of prohibited matter must immediately notify the Department of Primary Industries

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Reviewed 2020