Chromolaena or Siam weed

Chromolaena odorata (L.) King and Robinson

Synonyms - Eupatorium conyzoides, Eupatorium odoratum.

Family: - Asteraceae


Chromolaena is from the Greek “chroma” meaning colour and the Latin “leana” meaning cloak referring to the colourful appearance of the flowering plant.
Odorata refers to the odour of the leaves when crushed.
Siam weed recognizes its occurrence in Siam which is now Thailand.

Other Names:

Bitter bush, Chromolaena, Christmas bush, devil weed, hagonoy, saap suea, sam-solokh, Siam weed, triffid weed


An erect sprawling, fast growing shrub forming dense tangled thickets, 1.5-2 m high or up to 6 m high when climbing on vegetation in Queensland. It has annual stems with a perennial rootstock. The leaves are opposite, large and triangular with teeth and have a pungent odour when crushed. The flowers are small and pale mauve to white in showy clusters at the top of the plant.




First leaves:


Opposite, triangular with three prominent veins. Emits a pungent odour when crushed.
Stipules - None.
Petiole - to 15-60 mm long and densely hairy
Blade - 50-120 mm long by 30-70 mm wide. Egg shaped (ovate to ovate-rhomboid). Tip pointed. Sides with a few coarse teeth especially near the base. Base tapering (cuneate). Hairy on both side with slender soft hairs and glandular hairs on the lower surface.


Yellow, round, erect and sprawling, slender with fine longitudinal lines.
To 7 m long, succulent at the tips and woody at the base.
Many branched with the lateral branches in pairs.
Sparsely hairy.

Flower head:

Up to 70 florets grouped into cylindrical to bell shaped heads in terminal clusters about 100 mm across.
Flower head corymbose, homogamous with 30-70 tubular flowers, involucral bracts numerous, many serrate and outer shorter. Cypsela is five-angled. (A cypsela is a dry single-seeded fruit formed from a double ovary of which only one develops into a seed).


Tubular florets 3 mm diameter by 10 mm long surrounded by 5-6 distinct whorls of overlapping bracts, the inner ones whitish with 3 distinct green lines.,
Ovary - 2 branched stigmas.
Perianth - Pale bluish mauve, rarely white.
Stamens -
Anthers - Anther bases conspicuously sagitate.


Cypsela (a dry single seeded fruit).


Blackish with 4-5 pale roughened ribs. Five angled.
Small 3-5 mm long ~1mm wide and weigh about 2.5 mg per seed.
Oblong, angular, slender.
Pappus hairs to 5 mm long, initially white turning brown with when dry.
Produces up to 87,000 seeds per plant.
The seeds bear minute hooks, and cling to animal hair, vehicles and machinery.


Mostly fibrous roots in the top 300 mm soil. Some plants may form a stout taproot.

Key Characters:

Leaves opposite and emit a pungent odour when crushed.
Stems solid.
Style arms long, linear or apically clavate, covered in short papillae beginning conspicuously above base of style arms (florets isomorphic, all hermaphrodite, tubular; corollas never yellow)
Anther bases conspicuously sagitate, thecae long-acute or acuminate
Style arms lacking ring of hairs and glabrous or variously papillate or pubescent
Capitula lacking bilabiate florets, corollas all tubular or outer florets distinctly rayed, ray limb not bilabiate
Style arms bifurcate or bilobed, rarely connate, not pilose outside; upper part of style glabrous or sometimes with a ring of hairs just beneath style arm division. Phyllaries imbricate in 2 or more series, free or connate, if uniseriate then not cohering or pappus absent or capitula unisexual
Corollas not ligulate, or if strap-shaped then limb with 4 or less apical teeth, or teeth unequal.
Stems without spines.
Receptacle flat to slightly convex.
Phyllaries all deciduous leaving a naked receptacle, remaining appressed until lost, not spreading with age (subtribe Praxelinae)
Capitula without 4 sub equal phyllaries and 4 florets or with clavate style arms apices or with a defective pappus
Phyllaries distant to imbricate and bases sclerified or articulated; receptacle sclerified between areolae or paleaceous
Pappus present.
Adapted from Kew Keys


Allohexaploid with 60 chromosomes.

Life cycle:

Seeds germinate at the start of the rainy season and growth is produced from the crown and auxiliary buds at the same time. Plants grow rapidly and flower from December to July in the tropics. The fruits are dispersed soon after maturation. Most of the leaves fall after flowering and the plant remains dormant during the dry season.


Drought tolerant, frost intolerant.


By seed and buds at the crown and along the stems.
Apomictic (Seeds are clones of the mother plant).

Flowering times:

December to January in the tropics.

Seed Biology and Germination:

Produces up to 87,000 seeds per plant.
The seed viability was low (39%) for C odorata in India.
Seeds do not germinate in dark conditions.
Edwards (1974) and Auld and Martin (1975) have shown that the seeds require light for germination.
Optimal temperature for germination is around 25 degrees C and higher germination levels occur under alternating temperatures.
Most of the seeds were present in the top 2 cm soil layer. The viability and germination of seeds recovered from soil is quite low. The viable population of the seeds decreased exponentially after their burial in soil. A large proportion of seeds remained under enforced dormancy during their burial in soil with only a small fraction exhibiting induced dormancy. The longevity of buried seeds increased on account of the dormancy imposed on them. The loss of seeds through degeneration and/or decomposition was low.

Vegetative Propagules:

Buds are formed at the crown and along the stems.


Several edaphic ecotypes.


It is allelopathic, reducing the growth of companion plants.
Aqueous extracts reduced the germination of wheat.
Leaf leachates, root exudates and the rhizosphere soil of E. odoratum (C odorata) had allelochemicals that inhibited crop growth.
The presence of allelochemicals such as phenols, alkaloids and amino acids in the plant and decomposing residues inhibited crop growth up to six months. Allelochemicals also rendered the plants more resistant to pathogens and water stress (Ambika and Jayachandra, 1980, 1981, 1992).

Population Dynamics and Dispersal:

Spread by wind dispersed seed and vegetative buds on stems assisted by cultivation or earthworks.
The seeds have minute hooks and cling to animal hair, clothes, vehicles and machinery.
Troop movements during Second World War were responsible for much of the spread through South-East Asia and the Pacific.
The shortening of the slash and burn agriculture cycle from 20-30 years to 4-
5 years due to increased population pressure has also resulted in the spread of C. odorata to large areas of the north-eastern region of India (Kushwaha et al.
It is a pioneer, early successional species and performs very well as long as light does not become a limiting factor.
Seedling population experienced heavy juvenile mortality with only 1.4 % of seedlings surviving over one year period. Adult plants, on the contrary, exhibited
high survivorship in India.
Light may be one of the major factors in regulating the population size during secondary succession.
Plants colonize new areas through seeds but in the established communities the seedling survivorship is almost negligible.
The longevity of the adult plants is enhanced due to human disturbance.

Origin and History:

Native to tropical America from Mexico to Brazil and in the West Indies.
It was under an eradication program until 2012.
It first reported in Australia after 1992.



Courtesy Australia's Virtual Herbarium.


Open to partly shaded areas. Doesn't tolerate heavy shade.


Tropics and subtropics with an annual rainfall greater than 1200 mm.
Tolerates severe dry periods.
Does not tolerate frost.


Occurs on many well drained soil types. Doesn't tolerate waterlogged or saline soils

Plant Associations:

Often associated with blady grass (Imperata cylindrica).


It is not a serious weed in its native habitat and is a transient coloniser of cleared areas.


Harvested as a mulch and organic fertiliser.
Used as a green manure and may suppress some pathogens and nematodes.
The high concentration of mercury in the leaves indicate that it could have potential for phytoremediation of inorganic mercury affected sites.
Used in traditional medicine.
It has high crude protein, low fibre and low extractable phenolics content but needs treatment to reduce toxicity and improve palatability for feed uses.


Very competitive in plantations.
Infests tea, teak and rubber plantations and vegetable crops.
Serious weed of dryland rice.
Competes with native vegetation and is serious fire hazard.
Often forms pure stands reducing biodiversity.
Dense canopies reduce the photosynthetically active radiation to 7 percent of the
ambient level.
Alternative host for fungal diseases.
Harbour a number of insects and mites injurious to other crops in Asia.
Thickets are harbours for feral animals such as wild pigs and rodents.


Chromolaena odorata contains tumorigenic pyrrolizidine alkaloids.
Toxic to stock due to high nitrate levels which can be 5-6 times the normal toxic concentrations.
It contains high levels of pyrerolizidine alkaloids which render it unpalatable for
grazing (McFadyen 2003). If cattle and goats graze on it, the alkaloids progressively destroy their liver and the animals die. The poisoning
of horses by the closely related species E. adenophorum causes Telle-budgera horse disease or Numinbach disease which begins as an acute oedema of lungs followed by haemorrhage (Jones 1954).
Probably toxic to fish.


Cause abortions in cattle.


Remove stock from infestations


Noxious weed.
New infestations should be reported immediately to allow local eradication.

Management and Control:

Mechanical removal of isolated plants is effective but impractical for dense infestations.
Slashing and burning give temporary control followed by strong regrowth.
2,4-D controls seedlings and 2,4-D plus picloram controls actively growing large plants. Repeat applications are required to kill the perennial root system.
Blady grass appears to be more competitive than Chromolaena and this may assist control.
Burning caused increased population density and longevity of individuals on the burnt sites.



Eradication strategies:

Use repeated applications of picloram plus 2,4-D herbicides.

Herbicide resistance:

Unlikely to develop herbicide resistance as it is apomictic.

Biological Control:

Gall fly Cecidochares connexa.
Cecidochares connexa forms stem galls resulting in reductions to stem growth, seed production and carbohydrate storage, often leading to reduced plant growth and even plant death (McFadyen et al. 2003).
A leaf feeding arctiid moth Pareuchaetes pseudoinsulata has given good control in the Marianas but was less effective elsewhere.
Aureobasidium pullulans, a facultative parasite, which causes sterile and malformed seeds, brings about a decrease in seed production and its spread to virgin lands.
Releases of Apion brunneonigrum a seed eating beetle have occurred.
Other biological agents used elsewhere are Calycomyza eupatorivora and
the stem boring fly, Lixus demulus.
Pathogens causing stem rot disease cause seedling mortality. Auld
(1969) also reported that Cercospora eupatorii Peck attacks the related E. adenophorum.

Related plants:

Chromolaena squalida occurs in Queensland
Praxelis clematidea is a minor weed of Queensland. It has a conical rather than flat to slightly convex receptacle.

Plants of similar appearance:

Ageratina adenophora (Crofton weed) is an upright, terrestrial plant (1-2 m tall) with stems that are not hollow and relatively broad leaves with slightly toothed (crenated or serrated) margins. Its small flower-heads (5-7 mm across) are white rather than mauve in colour and borne in dense clusters at the tips of the branches.

Gymnocoronis spilanthoides (Senegal tea plant) is a semi-aquatic plant (1-1.5 m tall) with somewhat hollow stems (Chromolaena has solid stems) and relatively narrow leaves with finely toothed (serrated) margins. Its relatively large flower-heads (15-20 mm across) are white or pale purplish in colour and borne in few-flowered clusters at the tips of the branches.

Billygoat weed (Ageratum conyzoides subsp. conyzoides), blue billygoat weed (Ageratum houstonianum), praxelis (Praxelis clematidea) and vernonia (Cyanthillium cinereum) have similar flowers to C. odorata, but they are usually much darker pink or bluish in colour. These species are also much smaller annual plants (usually less than 1 m tall).


Bodkin, F. (1986). Encyclopaedia Botanica. (Angus and Robertson, Australia).

Everist, S.L. (1974). Poisonous Plants of Australia. (Angus and Robertson, Sydney).

Hussey, B.M.J., Keighery, G.J., Cousens, R.D., Dodd, J. and Lloyd, S.G. (2007). Western Weeds. A guide to the weeds of Western Australia. (Second Edition). Plant Protection Society of Western Australia, Perth, Western Australia. P. Photo.

Lazarides, M. and Cowley, K. and Hohnen, P. (1997). CSIRO handbook of Australian Weeds. (CSIRO, Melbourne). 242.1

Parsons, W.T. and Cuthbertson, E.G. (1992) Noxious weeds of Australia. (Inkata Press, Melbourne). P 270-272. Photos.

Randall, J.M. and Marinelli, J. (1996) Invasive Plants. (Brooklyn Botanic Gardens Inc. Brooklyn). P. Photo.

McFadyen, REC, Desmier de Chenon, R & Sipayung, A 2003, 'Biology and host specificity of the chromolaena stem gall fly, Cecidochares connexa (Macquart) (Diptera: Tephritidae)', Australian Journal of Entomology, vol. 42, pp. 294-297.


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