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Plantwise Technical Factsheet

Siam weed (Chromolaena odorata)

Host plants / species affected
Ananas comosus (pineapple)
Camellia sinensis (tea)
Cocos nucifera (coconut)
Coffea (coffee)
Elaeis guineensis (African oil palm)
Gossypium hirsutum (Bourbon cotton)
Hevea brasiliensis (rubber)
Musa textilis (manila hemp)
Nicotiana tabacum (tobacco)
Oryza sativa (rice)
Saccharum officinarum (sugarcane)
Tectona grandis (teak)
Theobroma cacao (cocoa)
Zea mays (maize)
Description

C. odorata is a herbaceous to woody perennial with a bushy habit which forms a very dense thicket about 2 m high, in almost pure stands. This many-branched plant becomes lianescent when it has the opportunity to climb on a support. Isolated individuals start to branch when they reach a height of about 120 cm. After the first year of growth, the plant develops a strong, woody underground storage organ, which can reach a diameter of 20 cm. Stems are terete and become woody. Twigs are slightly striolate longitudinally, pubescent, opposite-decussate. Leaves are simple, opposite-decussate and without stipules. They are rhomboid-ovate to ovate with an acute apex and a cuneate base. The blades are trinerved a few millimetres after the base, roughly crenate-serrate beyond their maximum breadth, slightly pubescent above and pubescent with numerous small yellow dots below (a lens is needed to see this). The petiole is 1-3 cm long, and the blade 5-14 cm long and 2.5-8 cm broad. Leaves and twigs produce a characteristic smell when crushed (Gautier, 1992a). Capitula are grouped in one, three or five convex trichotomic corymbs 5-10 cm in diameter, at the end of the twigs. The involucre is cylindrical, 8-10 mm long by 3-4 mm broad. It is made of a series of four or five oblong bracts, the external being the shorter. These bracts are obtuse, chartaceous, pale in colour with three or five nerves. The receptacle is convex, without scales. There are 15-35 florets per capitulum. The corolla is 5 mm long and has five lobes. Its colour ranges from pale-lilac to white. Styles are of the same colour, exserted and flexuous. Cypselae are composed of a 3- to 4-mm-long fusiform blackish achene, with five beige barbelate ribs, overtopped by a pappus of about 30 barbelate beige capillary bristles which are 4-5 mm long (Gautier, 1992a).

Prevention and control
Control

Cultural control

Slashing C. odorata stems manually or mechanically is not a solution, because the plants coppice profusely from the roots. The root itself must be dug out, which can sometimes be done by hand if the soil is moist, or by mechanized equipment. This type of control must be conducted at least twice during the growing season (Liggit, 1983; Audru et al., 1988; Muniappan and Marutani, 1991).

Cultural practices involving other competitive species have been proposed: using Leucaena leucocephala in pastures in the Philippines (Castillo et al., 1977); Tephrosia purpurea in cocoa plantations in Sri Lanka (Salgado, 1972); and signalgrass (Brachiaria decumbens) in Yunnan, China (Wu and Xu, 1991). For perennial plantations, a ground-cover crop such as Pueraria phaseoloides, Calopogonium mucunoides, Centrosema pubescens or Vigna unguiculata can control C. odorata in the first years, before the crop canopy is closed (Liggit, 1983; Audru et al., 1988; Muniappan and Marutani, 1991).

In South Africa, an annual burning regime can effectively control the plant in grassland situations by killing mature plants and preventing new seedlings from establishing (Goodall and Zachariades, 2002).

Mechanical control

Seedlings and young plants can be removed by hand-pulling, but follow-up clearance every 2-3 months is necessary because of rapid regrowth (Zachariades and Goodall, 2002).

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

Impact

C. odorata can be considered as a very serious weed in all types of perennial crops in the humid areas of the Palaeotropics. In low-growing plantations such as coffee and cocoa, C. odorata can completely smother the crop, whereas in taller plantations such as rubber or teak, as soon as the canopy is closed the weed is no longer a problem.

In shifting cultivation, C. odorata replaces the natural secondary succession and becomes the dominant fallow species (De Foresta and Schwartz, 1991; De Rouw, 1991; Gautier, 1992a; Roder et al., 1995; Slaats, 1995). It is often considered as beneficial by local farmers (Baxter, 1995) because it is easier to clear than the secondary vegetation, helps to suppress Imperata cylindrica, and is believed to accelerate the recovery of soil fertility. This last aspect is still a matter of debate among scientists, but there is an increasing requirement for further studies on the influence of the C. odorata fallow system before any biological control programme is launched (Herren-Gemmil, 1991; De Foresta, 1996).

C. odorata causes severe problems in pastures in the Central African Republic (Audru et al., 1988), Java (Indonesia) and the Philippines because it invades overgrazed areas. Due to the high nitrate content of the leaves, C. odorata is poisonous to cattle and generally not grazed (Sajise et al., 1974).

C. odorata can also transmit pathogenic fungi (Vayssière, 1957; Esuruoso, 1971; Oritsejafor, 1986), and act as a host for insect pests including Zonocerus variegatus (Modder, 1984; Chapman et al., 1986), whose nymphs feed on leaves, flowers and fruits in Africa; Aphis citricola and Rhopalosiphum maidis in India; as well as various other polyphagous insects.

In regions where there are dry seasons C. odorata can be a fire hazard (Englberger, 2009).

Summary of invasiveness

C. odorata is a very widely distributed tropical shrub that is still expanding its range, and is considered one of the world’s worst weeds. It continues to spread due to its effective short- and long-distance dispersal. It can form pure stands where established, often in disturbed areas, grasslands, fallow areas and forestry plantations, and is highly competitive. It is viewed as a major environmental weed, but is appreciated by some agriculturalists as it shortens fallow time in shifting cultivation.

Related treatment support
Plantwise Factsheets for Farmers
Kudra, A.; Mgonja, F.; CABI, 2016, English language
 
Pest Management Decision Guides
Kudra, A.; Mgonja, F.; CABI, 2016, English language
CABI; CABI, 2017, English language
Spencer, J. D.; Johnson, R. A. B.; Swaray, J. M.; Kpana, F.; CABI, 2012, English language
Kanteh, S. M.; Ndoleh, P.; Dimoh, G. J. S.; Luseni, S. J.; CABI, 2013, English language
Kanteh, S. M.; Ndoleh, P.; Dimoh, G. J. S.; Luseni, S. J.; CABI, 2013, English language
 
External factsheets
BioNET-EAFRINET Invasive Plant Factsheets, BioNET-EAFRINET, 2011, English language
BioNET-EAFRINET Invasive Plant Factsheets, BioNET-EAFRINET, 2011, English language
IITA IPM Field Guides for Extension Agents, International Institute of Tropical Agriculture (IITA), 2000, English language
IITA IPM Field Guides for Extension Agents, International Institute of Tropical Agriculture (IITA), 2000, French language
IITA IPM Field Guides for Extension Agents, International Institute of Tropical Agriculture (IITA), 2000, English language
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