Scientific name

Chromolaena odorata (L.) R.M. King & H. Robinson

Synonyms

Eupatorium conyzoides Vahl; Eupatorium odoratumL.

Common names

Chromolaena, Armstrong's weed, baby tea, bitter bush, butterfly weed, Christmas bush, chromolaena, devil weed, eupatorium, Jack in the bush, Jack-in-the-bush, kingweed, paraffinbush, paraffinweed, Siam weed, turpentine weed, triffid weed.

Family

Asteraceae (Compositae)

Origin

This species is native to the warmer parts of south-eastern USA, Mexico, the Caribbean and tropical South America.

Naturalised distribution (global)

Locations within which Chromolaena odorata is naturalised include tropical and southern Africa, tropical Asia and some oceanic islands with warm climates.

Introduced, naturalised or invasive in East Africa

Chromolaena odorata is invasive in parts of Kenya, Tanzania and Uganda (A.B.R. Witt pers. obs.). The arrival of C. odorata in East Africa is relatively recent and the species has been recorded in parts of western Kenya, western Uganda and north-western Tanzania. It is spreading and has the potential to transform landscapes.

Habitat

A potential weed of tropical and subtropical regions that invades riparian zones (banks of watercourses), bushland, forest margins, roadsides, disturbed sites, waste areas, neglected pastures, crops and plantations.

Description

An upright (erect) or sprawling shrub forming thickets and usually growing 1.5 to 3 m tall in the open. However it may reach greater heights (6-20 m) when climbing over trees and other taller vegetation.

The slender stems are generally yellowish-green and somewhat hairy (pubescent), but become woody towards the base of the plant. These stems grow up to 7 m or more in length and several are usually produced from the plants long-lived root-stock (crown). They are much-branched, with the side (lateral) branches usually being produced in pairs in the leaf forks (axils).

The oppositely arranged leaves (5-12 cm long and 3-7 cm wide) are triangular or egg-shaped in outline with broad end at base (ovate) and have a pointed tip (acute apex). They are hairy (pubescent) on both surfaces and have coarsely toothed (serrated) margins. These leaves are borne on stalks (petioles) up to 6 cm long (usually 10-15 mm), and give off a strong odour when crushed.

The small flower-heads (capitula) do not have any 'petals' (ray florets) and are borne in dense clusters at the ends of the branches (in terminal panicles). These flower-heads (about 10 mm long and 3 mm wide) are pale pink or pale mauve in colour (sometimes appearing whitish when older) and consist of numerous (15-30) tiny flowers (tubular florets). These tiny flowers (10-12 mm long) are surrounded by several layers of overlapping slender bracts (an involucre) 8-9 mm long. Each flower-head (capitulum) is borne on a stalk (peduncle) 10-30 mm long.

The black or dark brown 'seeds' (achenes) are 4-5 mm long and topped with a ring (pappus) of white to brownish coloured hairs (5-6 mm long).

There are two different biotypes invasive in Africa. The western African form has purplish flowers and while the southern African form has white flowers.

Reproduction and dispersal

This plant reproduces  by seeds, which are easily blown and dispersed by wind. Seeds may also be spread by machinery, water, vehicles, animals, in clothing, and in contaminated agricultural produce. Pieces of the crown of the plant can also take root and grow, and these may be spread about during cultivation and other soil moving activities (e.g. road maintenance).

Similar species

Chromolaena odorata may occasionally be confused with Gymnocoronis spilanthoides (Senegal tea plant) and Ageratina adenophora (crofton weed). These species can be distinguished by the following differences:

• C. odorata is a large upright, terrestrial plant (1.5-5 m tall) with stems that are not hollow and relatively broad leaves with slightly toothed (serrated) margins. Its small flower-heads (about 3 mm across) are pale pink or mauve in colour and are borne in dense clusters at the tips of the branches.
• G. spilanthoides is a semi-aquatic plant (1-1.5 m tall) with somewhat hollow 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.
• A. adenophora 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 in colour and borne in dense 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).

Economic and other uses

Chromolaena odorata is an ornamental plant. It can be used as a green manure and it may possess insecticidal properties. In Ghana it is used for embalming dead bodies. However, these uses cannot compensate for this plant's overall negative impacts.

Environmental and other impacts

Chromolaena odorata is regarded as an environmental weed in many parts of the world. It has been nominated as among 100 of the "World's Worst" invaders by the IUCN Invasive Species Specialist Group and it has been listed as a noxious weed in South Africa (prohibited plants that must be controlled. They serve no economic purpose and possess characteristics that are harmful to humans, animals or the environment.

C. odorata forms dense stands that prevent the establishment of other species, both due to competition and allelopathic effects, and interferes with natural ecosystem processes. as well as altering the integrity and diversity of these natural plant communities. C. odorata rapidly becomes dominant in native vegetation significantly increasing fire intensities as a result of its dry stems and leaves which are rich in oils. C. odorata plants dry out during the dry season and may become a fuel source that promotes bushfires. It is also a major weed in forestry plantations and crops in tropical regions, including rubber, oil palm and coffee plantations.

C. odorata  can have an impact on animal populations by replacing food plants and making nesting habitats unsuitable. It is unpalatable to vertebrate herbivores. In South Africa it reduced pasture carrying capacities from approximately 6 ha. per livestock unit (LSU) to more than 15 ha. per LSU. The high nitrate levels in young foliage could be the cause of livestock death (Sajise et al. 1974) while alkaloids in the flowers killed goats which ate the flowers (McFadyen 2004). 

This species may cause skin complaints and asthma in allergy-prone people and its leaves are known to be toxic to livestock.

Management

The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.

The best form of invasive species management is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Controlling the weed before it seeds will reduce future problems. Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.

When manually or mechanically controlling Chromolaena odorata it is essential to dig out the roots, otherwise the plant will coppice. Growing competitive species has been a proposed control method and fire has been used control C. odorata in grasslands in South Africa. The vine is hard to kill with chemicals due to its numerous tubers, waxy leaves, and numerous roots. Tubers can be physically removed, and a foliar spray of a suitable herbicide applied on plants and tubers as soon as green sprouts have two or four leaves on each sprout. If left too long, new underground tubers will form, jeopardising successful control.. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.

Biological control releases involving a stem gall fly (Cecidochares connexa [Procecidochares connexa]) in various countries have been extremely successful in pure stands though less successful on scattered individual (GISD 2006). A moth (Pareuchaetes pseudoinsulata) and several other agents have been released  in some countries with variable success.

Legislation

Not listed as a noxious weed by the state or governments in Kenya, Tanzania and Uganda.

References

Binggeli, P. (1997). Chromolaena odorata (L.) King & Robinson (Asteraceae). http://members.lycos.co.uk/WoodyPlantEcology. Woody Plant Ecology - Invasive Woody Plants. Pierre Binggeli.

CABI Invasive Species Compendium online data sheet. Chromolaena odorata (siam weed). CABI Publishing 2011. www.cabi.org/ISC. Accessed March 2011.

Germplasm Resources Information Network (GRIN). www.ars-grin.gov/npgs/index.html. National Germplasm Resources Laboratory, National Genetic Resources Program, Agricultural Research Service (ARS), United States Department of Agriculture (USDA), Beltsville, Maryland, USA. Accessed March 2011.

Global Compendium of Weeds. www.hear.org/gcw. Hawaiian Ecosystems at Risk Project. Accessed March 2011.

GISD (2006). Global Invasive Species Database online data sheet. Chromolaena odorata (herb). www.issg.org/database. Accessed March 2011.

Henderson, L. (2001).  Alien weeds and invasive plants. A complete guide to declared weeds and invaders in South Africa.  Plant Protection Research Institute Handbook No. 12, 300pp. PPR, ARC South Africa.

Lazarides, M., Cowley, K. and Hohnen, P. (1997). CSIRO Handbook of Australian Weeds. CSIRO Publishing, Collingwood, Victoria.

McFadyen, R.E.C. (2004) Chromolaena in East Timor: history, extent and control. In: Day, M.D. and McFadyen, R.E. (eds). Proceedings of the Sixth International Workshop on Biological Control and  Management of Chromolaena odorata, pp. 8-10. ACIAR Technical Reports 55. Canberra, Australia: ACIAR.

Pacific Island Ecosystems at Risk (PIER). Chromolaena odorata (L.) R.M. King & H. Robinson, Asteraceae: plant threats to Pacific ecosystems. www.hear.org/Pier/species/chromolaena_odorata.htm. Institute of Pacific Islands Forestry, Hawaii, USA. Accessed March 2011.

Sajise, P.E., Palis, R.K., Norcio, N.V. and Lales, J.S. (1974). The biology of Chromolaena odorata (L.) R. M. King and H. Robinson. I. Flowering behavior, pattern of growth and nitrate metabolism. Philippine Weed Science Bulletin 1, 17-24.

Editors

Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.

Acknowledgments

This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).

Contact

BioNET-EAFRINET Regional Coordinator: [email protected]