Chromolaena odorata (L.) King & Robinson (Asteraceae: Eupatorieae), commonly called chromolaena, is a sprawling shrub that has become one of the worst invasive terrestrial weeds in the humid tropics and subtropics of the Old World (Figure 1). Until the early 1970s, chromolaena was known as Eupatorium odoratum (L.). Chromolaena is known by a large number of mostly local vernacular names, in both native and invasive ranges. In English, it is mostly commonly known as Siam weed, while in French, it is often called l’herbe du Laos.
Figure 1. Global distribution of Chromolaena odorata, on a country record basis. Map generated by Jimaima Le Grand (Queensland Department of Primary Industries and Fisheries). A version of this map was published in Zachariades et al. (2009) and is used here with permission from Cambridge University Press. [
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Chromolaena has a wide indigenous range in the neotropics, from the southern USA to northern Argentina. Here it is a pioneering, early succession plant, weedy in land that has been overgrazed or otherwise disturbed. However, it is soon replaced by other plant species. In contrast, in the Old World it is a transformer species, forming dense, smothering mats over other vegetation. In most habitats it is not progressively replaced by other plant species over time (although it can be replaced by, and replace, other invasive alien plants such as Lantana camara), but persists and changes the landscape it invades. It is a major weed of agriculture, affecting both pastoral and cropping, from subsistence and small-scale to large-scale commercial enterprises; of silviculture; and of conservation lands. It is also a direct threat to human settlements due to its high flammability when dry. Some of the attributes that contribute to chromolaena’s success as an invader are its rapid growth rate, its allelopathic properties and, in the dry season, its huge seed production (up to about 1 million seeds per plant per flowering season) and, due to high levels of essential oils, its high flammability. Although it readily invades disturbed areas, it can also invade natural habitats that are subject to minimal disturbance. It has a very plastic morphology: in open areas, it forms a compact shrub up to 3m tall, whereas under trees or on forest margins it can scramble up to 10m to emerge over the canopy. However, it does not tolerate complete shade. Chromolaena spreads rapidly due to its small, light seeds attached to a pappus, which are dispersed both by air currents and on animals and vehicles. |  C. odorata infestation, Homestead,
Florida, USA
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Chromolaena displays considerable morphological variation within its native range. Flower colour varies from white through pale lilac to blue, and other aspects of flower morphology can also vary (e.g. shape of bracts, diameter of capitula). Leaves and stems range from glabrous to hairy and as a consequence, their colour varies too. Plant architecture is variable (lax to upright), as is the odour of crushed leaves and stems. In some areas of the neotropics (e.g. the Caribbean and Central America), high local variability is evident, while in others (parts of the South American mainland), it is locally uniform in morphology.
C.
odorata infestation, East Timor.
Photo: Colin Wilson
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There appear to be two primary centres of invasion by chromolaena in the Old World. Each centre of invasion is characterised by plants which are morphologically homogeneous within that centre of invasion but morphologically distinct between the centres of invasion. In the first, more widespread centre of invasion, chromolaena probably spread throughout Asia and Oceania from the Calcutta Botanic Garden, where it had been planted in the mid-19th century. This form of chromolaena appears to have been taken from Sri Lanka to West Africa in the first half of the 20th century. From here, it spread and is now present from the Gambia to northern Angola and Tanzania. Because this constitutes a secondary introduction it is considered as part of the first centre of invasion. A second form of chromolaena appeared in south-eastern South Africa in the 1940s, from where it spread throughout climatically suitable areas of the subcontinent. Since the biology of plants in these two centres of invasion is apparently also distinct in some important characteristics, both these forms have been characterised as ‘biotypes’ and functionally, although not strictly, can be considered to be separate species. They are referred to from hereon as the Asian/West African (AWA) biotype and the southern African (SA) biotype. |  C. odorata bush scrambling up over vegetation, South
Africa |  C. odorata bush covered in mature seed. Photo: Colin
Wilson |
 Mature seed-head of C. odorata. Photo: Colin
Wilson |
Two morphological forms of C. odorata growing together in Jamaica.
The plant on the left is similar to the SA biotype
while the hairier plant on
the right is similar to the AWA biotype.
Control and management of chromolaena in its invasive range has
been achieved through various means. Various herbicides (foliar and cut stump)
have been used with good effect, but are expensive and therefore not viable in
the long term or over large areas. Young plants can be easily uprooted. Older
plants can be slashed but quickly coppice unless stumps are dug out or herbicide
is applied to them.
Regular burning has been shown to be effective in suppressing
infestations in grassland and savanna, although the AWA biotype does not appear
to be as easily killed by fire as the SA biotype. In West Africa especially, the
status of chromolaena has become controversial. While some farmers consider it a
serious weed, others view it as beneficial. It is said to enrich the soil,
suppress more noxious weeds, and shorten the fallow period that is necessary in
shifting agricultural systems.
Although individual chromolaena plants are easily killed using
mechanical and chemical means, the rapid rate of growth and recruitment of the
plant, together with the often low value or inaccessibility of the invaded land
and the fact that it is largely a weed of developing countries in the tropics
and subtropics (most of which lack the resources to mount large-scale control
programmes), have resulted in chromolaena becoming one of the most widespread,
abundant and damaging plants in the world. Classical biological control, using
host-specific arthropods and pathogens collected from the plant in its region of
origin, offers a crucial complementary tool in suppressing chromolaena to
manageable levels.
Key references:
Gautier, L. (1992). Taxonomy and
distribution of a tropical weed, Chromolaena odorata (L.) R. King and
H. Robinson. Candollea, 47, 645-662.
Goodall, J. M. and Erasmus, D. J. (1996). Review of the status
and integrated control of the invasive alien weed, Chromolaena odorata,
in South Africa. Agriculture, Ecosystems and Environment, 56,
151-164.
Holm, L. G, Plucknett, D.L., Pancho, J. V., and Herberger, P.
D. (1977). The World’s Worst Weeds. Distribution and Biology. Honolulu,
HI:University Press of Hawaii.
Kriticos, D. J., Yonow, T. and McFadyen, R. E. (2005). The
potential distribution of Chromolaena odorata (Siam weed) in relation
to climate. Weed Research, 45, 246-254.
Liggitt, B. (1983). The invasive plant Chromolaena
odorata, with regard to its status and control in Natal. Pietermaritzburg,
South Africa: Institute of Natural Resources, University of Natal, Rural
Studies Series, Monograph 2, 1-41.
McFadyen, R. E. C. (1989). Siam weed: a new threat to
Australia’s north. Plant Protection Quarterly, 4, 3-7.
Zachariades, C., Day, M., Muniappan, R. and Reddy, G.V.P.
(2009) Chromolaena odorata (L.) King and Robinson (Asteraceae). Chapter
8 in: Biological Control of Tropical Weeds using Arthropods. Muniappan,
R., Reddy, G.V.P. and Raman, A. (eds). Cambridge University Press. In press.
Zachariades, C., von Senger, I. and Barker, N. P. (2004)
Evidence for a northern Caribbean origin for the southern African biotype of
Chromolaena odorata. In Proceedings of the Sixth International
Workshop on Biological Control and Management of Chromolaena, ed. M. D. Day
and R. E. McFadyen. ACIAR Technical Reports, 55, 25-27
