Chromolaena odorata (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

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

Protologue: Phytologia 20: 204 (1970).

Family: Compositae

Chromosome number: 2n= 40

• Eupatorium odoratum L. (1759),
• E. conyzoides Vahl (1794),
• Osmia odorata (L.) Schultz-Bip. (1866).

• Siam weed, Christmas bush, goat weed (En). Jack in the bush (Am).
• Herbe du Laos, fausse ramie, fleurit-Noël (Fr)
• Indonesia: ki rinyu (Sundanese)
• Malaysia: Siam weed, pokok kapal terbang
• Philippines: devil weed, gonoi, (hulo)hagonoy
• Burma (Myanmar) : bi-zat, tawbizat, curse of Caylan
• Cambodia: tontrien khaèt
• Laos: hnha:z fàlangx (general), hnha:z khi:lo:z (Paklay), nroj pawm tshis (hmong)
• Thailand: sapsua, ya-suamop
• Vietnam: có hoi, có Lao, yên-bach.

Siam weed originated in the humid tropics of the New World, where it occurs from southern Florida to northern Argentina. It has been introduced and is naturalized throughout humid, tropical Asia, from the Western Ghats in India throughout Indochina and Malesia to south-eastern Australia and the Mariana Islands in the east. It is found in the humid tropics of West and Central Africa and in south-eastern South Africa.

Although, under certain conditions, Siam weed is one of the most noxious weeds in agriculture and range management, it is also used as a green manure and mulch crop. In Cambodia it is used as a green manure in the production of lowland rice, cassava and black pepper. In Nigeria its use as mulch in yam, cassava and coffee is subject of research. It is recommended for the control of Imperata cylindrica (L.) Raeuschel. In Asia and Africa a natural fallow of Siam weed is becoming gradually more common in semi-permanent food crop production, and many small farmers in e.g. Indonesia and Laos consider it a most useful fallow crop.

A protein extract from the leaves was in use as a poultry feed during the civil war in Nigeria. Siam weed is not palatable to cattle and large tracts of extensively managed pasture land have been abandoned because of uncontrollable Siam weed infestation.

Leaves of Siam weed are reported to be useful in controlling the weevil Cylas formicarius and the butterfly Phtorimae operculella in sweet potato, the nematode Heterodera marioni in black pepper, as well as nematodes in sugar cane and tomato.

Siam weed is used as a medicine for intestinal pains, colds and cough in the Caribbean region; in Ivory coast, for healing wounds, as a purgative, a remedy against cough, malaria, smallpox and yellow fever. In Thailand, C. odorata is traditionally used to stop bleeding.

At the end of the dry season a Siam weed fallow contains per 100 g dry matter:

• leaves: N 2.1-3.0 g, P 0.17-0.21 g, K 1.4-1.6 g, Ca 1.6-1.9 g, Mg 0.4-0.5 g;
• branches: N 0.2-0.4 g, P 0.02-0.03 g, K 0.6-0.8 g, Ca 0.3 g, Mg 0.2 g;
• litter: N 0.5-0.7 g, P 0.03-0.04 g, K 0.3-0.5 g, Ca 0.7-0.9 g, Mg 0.2 g.

Leaves and petioles have glandular dots emitting a strong pungent smell when crushed. Phenols and alkaloids in the plant, in particular in the leaves, have an allelopathic effect, inhibiting the germination of its own seeds and seedling development of other plants. Siam weed contains essential oils having an anti-bacterial activity on Staphylococcus aureus and Escherichia coli. Applied as a green manure in rice paddies, it may kill fish. It contains 4',5,6,7-tetramethoxyflavone, which enhances blood coagulation.

The weight of 1000 fresh achenes is about 0.25 g.

• A spreading, much-branched, tangled, thicket-forming, perennial shrub, up to 3 m tall, scrambling up to 7 m. Root system rather superficial, upper part of the root growing horizontally and swollen, but taproot growing deep and massive. Stem profusely branched, herbaceous when young, tough and semi-woody when older, cylindrical, finely striate, yellowish, shortly hairy or nearly glabrous; branches slightly ridged longitudinally, pubescent.
• Leaves simple, opposite; petiole 1-3 cm long or more, glabrous or sparingly pubescent; blade ovate-triangular, conspicuously 3(-5)-veined, 5-14 cm × 2-8 cm, acuminate, margins toothed, dotted with glands, sparsely hispid-hairy to glabrous, often purple when young.
• Inflorescence a homogamous, 10-35-flowered head, arranged in corymbose clusters arising from the axils of upper leaves; peduncle 1-2 cm long; involucre cylindrical, 8-10 mm × 3-4 mm, bracts in 5 or 6 rows, closely overlapping, oblong, increasing in size upwards, up to 10 mm × 3 mm, straw-coloured to greenish.
• Corolla tubular, 5 mm long, 5-lobed, pale mauve, pale blue or whitish, protruding from the involucre; stigma with a long, exserted arm.
• Fruit a narrow achene, linear, angular, 3-5 mm long, brown or black, with short, white, stiff hairs along the edges; pappus white, consisting of rough bristles, 4-5 mm long.
• Seed minute.

Viability of fresh seed ranges from 33-66%. After 2 years up to 40% of the seed still germinates. A small proportion of the seed germinates when mature, but most remains dormant. Seeds are photoblastic, but they may emerge when buried up to 3 cm deep. Emergence takes 4-12 days. During the first 3 months the seedlings stay rather small and mainly form leaves. Later, the length and biomass of the stem increase rapidly. Before growing downwards, the primary root forms a small, horizontal part, from which many secondary roots develop. During further growth it swells progressively and serves as a storage organ from which new shoots may sprout.

Plants start branching after reaching a height of about 120 cm. The shoots of Siam weed bend over due to their increasing weight. Consequently, apical dominance is broken and new shoots develop. The bent shoots die and form a thick, sagging mat in the vegetation which absorbs the light of plants in the understorey and hinders their vertical development by mechanical pressure.

Siam weed is a pioneer species, suppressing grasses in the succession from open space to forest, making it a noxious weed in rangelands. Its lifetime depends on the presence of woody species in the vegetation. In India and Ivory Coast Siam weed diminishes from the fifth year onwards, as tree species start shading it out. When shrubs and trees are absent the lifetime of Siam weed may exceed 15 years.

Siam weed flowers annually from the first year. Time of flower initiation and inflorescence development may differ among plants and even among branches of one plant. The central cyme of the inflorescence flowers slightly earlier than the peripheral ones. Pollination is by insects. Apomictic fruit development also occurs. The period from flower initiation to fertilization is about 1.5 months, from fertilization to seed dispersal takes another month. Flower initiation is rather complex: shorter daylengths, diminishing rainfall and falling temperatures seem decisive factors.

The number of seeds produced is often extremely high. Production of 100 000-180 000 seeds/m²has been recorded in natural stands of Siam weed. Seeds are dispersed mainly by wind, but dispersal by animals and humans are important as well. Vegetative growth stagnates during flowering and seed production. After flowering most leaves wither and fall. New leaves and shoots grow from the old leaf axils, while the dead terminal part of the stems with the old inflorescences drop off.

C. odorata has been excluded from the genus Eupatorium L.; it can be distinguished by its rather consistent pattern of many rows of bracts, giving a cylindrical appearance to the head, by the 3 prominent veins of the leaves, and by the pungent smell of crushed leaves. In Hawaii, the name Eupatorium odoratum is often used for Ageratina adenophora (Spreng.) R.M. King & H. Robertson. The name Eupatorium conyzoides Miller is a synonym of Vernonia arborescens Swartz, but is sometimes used erroneously for C. odorata.

The natural habitat of Siam weed includes forest clearings, river banks and borders between savanna and closed forest. It can be found from sea level up to 1000(-1500) m altitude. Minimum annual rainfall required is 1100 mm, with a dry season of up to 5 months, although it can be found sporadically at 700 mm annual rainfall. It is not found in temperate regions and a mean daily temperature of 25-30 °C is optimal. Siam weed is heliophile. It needs light to germinate and is suppressed when shaded by other plants. It grows on a wide range of soils, but not on inundated sites. Through symbiosis with vesicular-arbuscular mycorrhizae it can grow well on soils poor in P. Under a Siam weed fallow the soil structure improves and the pH and biological activity of the soil increase.

It tolerates mechanical injuries caused by slashing and burning, as it is able to form new shoots on the swollen part of the root. However, frequent injuries deplete the plant's regenerative capacity.

To date Siam weed has not been planted as a crop. It normally grows from seed, but can be propagated vegetatively, as the nodes of branches readily root when in contact with moist soil.

Under humid tropical lowland conditions in Ivory Coast biomass production of a natural stand of Siam weed at the end of the dry season varied from 14.4 t/ha after 2 years to 21.7 t/ha after a 4 years fallow, supplying 70-140 kg N, 5-8 kg P, 109-125 kg K, 70-160 kg Ca and 30-50 kg Mg per ha. Regrowth of Siam weed after cropping is best when the impact of cultural practices during cropping is modest and the cropping period is limited to 1 or 2 seasons. Under humid tropical conditions a mulch of 8.5 t dry matter/ha, half of it composed of leaves and young shoots, will be decomposed in 38 days.

In trials in Cambodia a mulch of 20 t/ha of Siam weed increased rice yields from 1.5 t/ha to 2.8 t/ha. This increase was similar to that resulting from an application per ha of 30 kg N, 30 kg P and 30 kg K applied as chemical fertilizer. The combination of green manure with chemical fertilizer increased rice yield to 4.3 t/ha. Ploughing in the green manure or leaving it as mulch on the soil surface were equally effective. The Siam weed green manure repelled crabs from the rice field, but also killed the fish in the paddies.

Considerable increases in yield were also found in cassava. Mulching black pepper with Siam weed reduced the nematode infestation (Heterodera marioni) and secondary infection of Pythium spp. All the pepper vines in the untreated plots died within 3 years, but nearly all survived in the mulched plots.

In low input agriculture slashing and burning the fallow crop before planting and an early weeding proved most appropriate to reduce the development of Chromolaena as a weed. In perennial cropping and forest plantations it can be controlled by repeated slashing and will eventually be shaded out when the canopy closes over. However, it hampers the establishment of a leguminous ground cover. In Malaysia it has been shown to depress the growth of rubber trees.

Siam weed is known to harbour parasites and pathogens injurious to crops, like grasshoppers (Zonocerus variegatus), weevils (Aphis spp.), nematodes (Scutellonema bradys) and microorganisms Cercospora spp. causing leaf spot disease, Fusarium oxysporum and Pseudomonas solanacearum).

Although many pathogens and insects have been found on Siam weed, they rarely do serious harm. Only the arctiid moth Pareuchaetes pseudoinsulata and the seed-feeding weevil Apion brunneonigrum, both oligophage insects originating from tropical America, are known to cause considerable damage to Siam weed. They have been introduced into several African and Asian countries for the biological control of this weedy plant, with varying degrees of success.

Due to its fast growth and nutrient accumulation and its copious litter production, Siam weed may play an important role as a fallow crop for restoration of soil fertility in cropping systems where shortening of the fallow period is inevitable. Used as a mulch it may contribute to the increase or maintenance of soil fertility and the improvement of the physical condition of the soil. Due to its rapid lateral spreading and superficial rooting, Siam weed also has good prospects for the control of soil erosion and Imperata cylindrica. However, more research is needed to incorporate Siam weed into semi-permanent cropping systems and to develop adequate measures to control its weediness in extensively managed crop production and rangeland systems.

• Audru, J., Berekoutou, M., Deat, M., de Wispelaere, G., Dufour, F., Kintz, D., le Masson, A. & Menozzi, Ph., 1988. L'herbe du Laos - Synthèse des connaissances actuelles sur la plante et sur les moyens de lutte [Siam weed - survey of actual knowledge of the plant and of methods of its control]. Etudes et synthèses de l'IEMVT No 28. Institut d'Elevage et de Médecine Vétérinaire des Pays Tropicaux, Centre de Coopération Internationale en Recherche Agronomique pour le Développement, Maisons-Alfort, France. 186 pp.
• Dove, M.R., 1986. The practical reason of weeds in Indonesia: peasant vs. state views of Imperata and Chromolaena. Human Ecology 14(2): 163-190.
• Gautier, L., 1992. Contact forêt-savane en Côte d'Ivoire centrale: Rôle de Chromolaena odorata (L.) R. King & H. Robinson dans la dynamique de la végétation [Contact forest-savanna in central Ivory Coast: role of Chromolaena odorata (L.) R. King & H. Robinson in the dynamics of the vegetation]. Conservatoire et Jardins Botaniques de la ville de Genève, Genève, Switzerland. 258 pp.
• Holm, L.G., Plucknett, D.L., Pancho, J.V. & Herberger, P.D., 1977. The world's worst weeds - distribution and biology. University Press of Hawaii, Honolulu, United States. pp. 212-216.
• Litzenberger, S. & Lip, Ho Tong, 1961. Utilizing Eupatorium odoratum L. to improve crop yields in Cambodia. Agronomy Journal 53: 321-324.
• Lucas, E.O., 1989. Siam weed (Chromolaena odorata) and crop production in Nigeria. Outlook on Agriculture 18(3): 133-138.
• Muniappan, R. & Ferrar, P. (Editors), 1991. Ecology and management of Chromolaena odorata. BIOTROP Special Publication No 44. ORSTOM and SEAMEO BIOTROP, Bogor, Indonesia. 167 pp.
• Muniappan, R. & Marutani, M., 1988. Ecology and distribution of Chromolaena odorata in Asia and Pacific. In: Muniappan, R. (Editor): Proceedings of the First International Workshop on biological control of Chromolaena odorata, Feb. 29-Mar. 4, 1988, Bangkok, Thailand. Agricultural Experiment Station, Mangilao, Guam, United States. pp. 21-24.
• Roder, W., Phengchanh, S., Keoboualapha, B. & Maniphone, S., 1995. Chromolaena odorata in slash-and-burn rice systems of Northern Laos. Agroforestry Systems 31: 79-92.
• Slaats, J.J.P., 1995. Chromolaena odorata fallow in food cropping systems. An agronomic assessment in South-West Ivory Coast. PhD thesis. Wageningen Agricultural University, Wageningen, the Netherlands. 177 pp.

J.J.P. Slaats