Chromolaena odorata (L.) R.King & H.Rob.

China: Chromolaena odorata flowers and fruits from April to December.
Madagascar: C. odorata flowers and fruits from June to September.
West Indies: Chromolaena odorata flowers and fruits from December to February.

Chromolaena odorata is a perennial plant. In cultures where the soil is tilled, it behaves as an annual plant. Reproduction is by seed, disseminated by wind and animals and vegetatively from cut basal shoots.

The seed production of Chromolaena odorata is very important and high densities can be reached (more than 2000 seeds per m2). An important intra-specific competition takes place during the first months of development (Yadav & Tripathi, 1982).
The seeds mature in February-March for the areas composing its range. The duration of production is about two months and corresponds to the period of maturation of all the seeds of the plant. The seed does not have albumen, its longevity is consequently reduced. It is estimated at approximately 26 months (Mouloungou & Sigrist, 1993).
In general, the seeds are adapted to germination in a secondary environment, but a great variability is observed according to the populations studied (Edwards, 1974). Dormancy and shade are necessary for germination. Conditions to promote germination are temperatures above 20°C and humidity. Light quality also plays a role in germination. Green and far infrared light reduce the germination rate, while red or white light has a positive effect (Erasmus & Van Staden, 1986).
Vegetative reproduction may occur in C. odorata during the development period. It is possible that fallen branches develop adventitious roots (CAB-CPC, 2004).
C. odorata has a high regenerative capacity. Numerous stump sprouts can be observed after disturbance of the plant either by fire or by cutting. In particular, fire induces a strong response of the plant in terms of renewal by stump sprouts (Mouloungou & Sigrist, 1993).

The dispersion of the fruits of Chromolaena odorata is mainly anemochore. But it can also be exozoochorous or anthropochorous as the fruits can easily stick to furs, feathers or clothes and vehicles and agricultural equipment. These two types of dispersal ensure an efficient propagation of the seeds of C. odorata over short and long distances (Mouloungou & Sigrist, 1993).
The spread of the plant is relatively rapid. For example, in one field in Ghana, two years were sufficient for the plant to colonize the entire crop from small foci of a few individuals (Mouloungou & Sigrist, 1993).
The movement of agricultural machinery or vehicles in colonized areas where seeds are present on the ground may favour the dissemination of the plant.
In the case of the presence of C. odorata seeds in seed lots intended for planting, sowing will directly promote the spread of the plant.

Chromolaena odorata cannot grow in areas where fires are annual, such as the African savannahs. However, if the fires are irregular, the plant can be favored and form a dense thicket that will limit the sincendies in wet areas (plant not very flammable, contrary to grasses, it keeps a high water content if the dry season is not too long). In a markedly dry season, this bush may burn, but the underground organs will ensure rapid regrowth of the vegetative organs. Fire favors the development of stump sprouts (Mouloungou & Sigrist, 1993).
The large quantity of seeds produced and the variability of dormancy contribute to survival during difficult periods (fire, effective weeding, canopy closure) although the viability of seeds in the soil is not very long.

Chromolaena odorata is found on roadsides, abandoned fields and fallow, in plantations and tree crops.

C. odorata has a wide range of soil tolerance. It can grow on soils ranging from sand dunes to heavy clay soils (Liggit, 1983), however, it seems to prefer well-drained environments. The annual rainfall requirements of C. odorata are between 1000 and 2500 mm. On the other hand, it can grow with a rainfall of less than 1000 mm per year if the dry period is not too long. It has been found that the plant tolerates a marked dry period of 4 to 5 months (Mouloungou & Sigrist, 1993).
The development of C. odorata is strongly dependent on the availability of light, as its competitive strategy is based on a rapid phenotypic plasticity that can only be achieved efficiently with a high energy supply (Internet - (CAB-CPC, 2004)).
The distribution of the plant in altitude is limited to areas below 2000 m, the plant is sensitive to frost (Liggit, 1983).
C. odorata develops in many types of environments such as savannahs or degraded forests. It colonizes cleared or deforested areas, fallows, clearings and forest edges. The anthropized areas such as the edges of roads, tracks, the surroundings of villages are a potential place of development. C. odorata is a heliophilic plant and can develop in any sunny place (Mouloungou & Sigrist, 1993).
C. odorata has a competition strategy based on plasticity. It is therefore able to adapt to the constraints of the environment or to the constraints imposed in a culture. It is also very polymorphic according to the conditions encountered, thus in the West Indies, its height is between 0.30 and 1.50 m whereas in
Indochina, it takes the form of a liana and can rise up to 10 to 15 m (Poilane, 1952). In West Africa, the plant forms dense bushes of 2 to 2.50 m in full sunlight, otherwise it can elongate to find light up to 15 m (Tchoumé, 1980).
It can be considered a major weed in all perennial crops in the humid tropics (fields or forestry).

Central Africa: Chromolaena odorata grows along roadsides, in fallow land, in ruderal environments, on the edge of villages and in degraded savannahs.
French Guiana: C. odorata is a species native to South America, so it is naturally present in French Guiana. It is found preferentially on the pioneer fronts and on the forest edge.
Madagascar: C. odorata is a good indicator of relatively rich soil. It grows on alluvial and rich humus ferrallitic soils, on sunny or lightly shaded ground. It is found along the roads and cultures, around the dwellings, on fallow land and disturbed areas after forest clearing. It is a weed of crops in wet areas, in upland cultures of rice, corn and fruit, more or less extensive of the East Coast up to 500 m altitude.
Mauritius: The species is abundant on roadsides, invading wasteland and forest clearings in the driest area of Mauritius.
Reunion: Absent.
South Africa: Chromolaena odorata forms thickets in wooded wasteland or at forest edges, in scrubland and along roadsides. 
West Indies: Chromolaena odorata is a ruderal species that grows between 0 and 500 m altitude.

Host of diseases and pests of crops

Chromolaena odorata is a major host of Zonocerus elegans, and Zonocerus variegatus (stink locust).

It is also a minor host of Brachycaudus helichrys, Homona coffearia (coffee tortrix, tea tortrix, Camelia tortrix), Aphis spiraecola (thistle aphid, artichoke aphid), Anoplocnemis curvipes, Apion brunneonigrum, Acalitus adoratus, Melanagromyza eupatoriella, Mycovellosiella perfoliata, Pareuchaetes pseudoinsulata, Septoria ekmaniana, Tetranychus urticae (two-spotted spider mite), Hypolixus truncatulus. It is also a host of Fusarium oxysporum f.sp. elaeidis (vascular fusariosis of oil palm), and Alternaria zinniae.

Risk analysis for Reunion island

Chromolaena odorata having evolved in areas where the competition pressures are strong, has important competitive advantages towards the species of Reunion. It develops more quickly than the native species in the first phenological stages. It thus has a certain competitive advantage and could take the ascendancy on the local species and colonize the open or degraded natural environments. These environments represent important surfaces of the island.
C. odorata could develop without constraints in all vegetation types where it can access the light resource, provided that the rainfall and the temperature are sufficient. In Reunion Island, this corresponds to the agricultural areas and to the sparse or degraded forest formations of low and medium altitude located preferentially on the east coast of the island. This represents a surface of about 30% of the island.

Global harmfulness

Chromolaena odorata is a common and dominant species in the upland fields and fallow vegetation of the forest zone and reported by many as a troublesome weed (Johnson, 1997; Becker and Johnson, 2001; Kent et al., 2001; Anthofer and Kroschel, 2007). C. odorata gradually replaces indigenous species in fallow vegetation (Weise, 1995) and this in turn has consequences for the weed community in subsequent crops as it causes a predominance of broad-leaved species such as Ageratum conyzoides, Tridax procumbens and Phyllanthus amarus (Ikuenobe and Anoliefo, 2003). It is considered one of the most important weeds in the world (Holm & al, 1977).

Local harmfulness

Africa: Introduced in the 1940s as a cover crop in cocoa plantations, Chromolaena odorata has rapidly become invasive in most countries of tropical Africa, in Ghana (J.A. Timbilla & H. Braimah, 2000), Nigeria, Cameroon, Central Africa, Côte d'Ivoire, but also in South Africa (Mouloungou & Sigrist, 1993). Considered a particularly damaging weed to the "plantation economy" of some countries, its harmfulness in this context is currently under discussion, as it contributes to the restructuring of degraded soils and is less difficult to eliminate than species such as Imperata cylindrica when rehabilitating fallow land (Ird, 1996).
Australia: Chromolaena  odorata is an invasive, toxic plant, considered as a quarantine pest and control actions are carried out against it. The direct control program in 1994 cost $460000 and a 4-year biological control program cost $493 684
Benin: C. odorata is rare and not abundant.
Comoros: It grows as a ruderal in secondarized environments.
Ghana: Rare and scarce.
French Guiana: Chromolaena odorata does not pose any particular problem in pastures or in vegetable crops. It occurs occasionally in orchards with spontaneous plant cover but is not considered a constraint.
Hawaii: The plant has been evaluated and is considered a quarantine pest.
Madagascar : Chromolaena odorata is a species recently introduced in Madagascar; it is still rare but locally abundant. This is particularly the case of primary infestation in East Coast, up to 500 m altitude in cropping systems of rainfed rice or corn on slash-and-burn land or poorly maintained fruit crops. It recolonizes fallows and is often considered an invasive plant.
Mali : Rare and scarce.
Mauritius : Due to the very low - frequency abundance in crops, this plant is not considered harmful. It grows as a ruderal in secondarized environments. But it demonstrates very high invasive capacity because of its large production of seeds that are easily dispersed by wind .
Nigeria : Rare and scarce.
Reunion: Absent .
South Africa: Chromolaena odorata smothers and completely suppresses native vegetation and has been known to replace up to 100% of native vegetation in some areas. It is also highly flammable, which encourages fires. Common in the lowlands of KwaZulu-Natal and in the provinces of Swaziland, Mpumalanga and Limpopo.

Global control

Preventative measures: A Risk assessment of Chromolaena odorata for the Pacific region was prepared by Pacific Island Ecosystems at Risk (PIER) using the Australian risk assessment system (Pheloung, 1995). The result is a score of 34 and a recommendation of: reject the plant for import (Australia) or species likely to be of high risk (Pacific). Using a revised climate model (Kriticos et al. 2005) of the estimated potential distribution of C. odorata it was predicted that Mediterranean, semi-arid and temperate climates are unsuitable for its establishment. Much of tropical Africa, the north-eastern coast of Australia and most Pacific islands are at risk of invasion. The distribution of C. odorata in South Africa extends further south than predicted by the model based on Asian and American distribution records, supporting the claim that the South African variety of C. odorata has different climatic requirements to the varieties elsewhere (EPPO 2005).

Physical: Manual slashing and use of bush-cutter or tractor-drawn implements are commonly used methods of control. Slashing causes regeneration unless followed by other control methods. Manual weeding is labour intensive. The use of tractor drawn equipment is limited to areas that are accessible (Ecoport).

Chemical: Chemical control using herbicides applied at the seedling stage or on regrowth has given encouraging results. Triclopyr has proven to be the most effective. However, problems in herbicide use include the high cost of the chemicals and their application, ecological concerns and, non-compatibility in many cropping and other environmental situations (Ecoport). Removing seed and flower heads and spraying with 2,4-D Amine plus Picloram (Tordon in Australia) kills top growth and (picloram kills the root system is recommended (Rod Randall, pers. comm. 2000).

Biological: The biological control agent Pareuchaetes pseudoinsulata has been introduced into Guam, where it effectively defoliates pure stands. It is less successful in scattered plants and patches. It has also been introduced into Palau, Kosrae, Pohnpei, Yap and Saipan Island (Mariner Islands) where it has been effective in reducing C. odorata. It has also been released on Sumatra, Indonesia, where it is effective in reducing densities of the weed. Releases into other parts of Indonesia appear to have failed. Another species, the stem gall fly Cecidochares connexa (originally collected from C. odorata in Mexico, Brazil and Bolivia Cruttwell 1974) is a suitable biological control agent for C. odorata (Cruttwell McFadyen Chenon and Sipayung 2003). Most gall-forming species of the tephritid generaCecidochares Bezzi are highly host specific, sometimes attacking only a single plant species (Foote et al. 1993, in Cruttwell McFadyen Chenon and Sipayung 2003). Based on the results of host testing of C. connexa was granted Indonesian Government allowance for field release in 1995 and is now established on most of the larger Indonesian islands (Tijitrosemito 2002, Wilson and Widayanto 2002, in Cruttwell McFadyen Chenon and Sipayung 2003). Since then it has been released in Palau, Papau New Guinea and the Philippines (Esguerra 2002, Orapa et al. 2002, in Cruttwell McFadyen Chenon and Sipayung 2003; Dr. Muniappan, pers. comm.). Die-back and death of plants have been recorded at many sites within 3 to 5 years of release, especially in low altitude sites (less than 300m) with a short dry season (Cruttwell McFadyen Chenon and Sipayung 2003). At higher altitude sites (over 600m) or where cloudy conditions, cold temperatures or long dry seasons limit the number and activity of flies control is slower and less adequate (Cruttwell McFadyen Chenon and Sipayung 2003).

List of biological regulators of C. odorata (CAB-CPC 2004):
Pathogens (Anhelia niger in Brazil; Cionothrix praelonga in Colombia, Dominica, Mexico, Trinidad, Venezuela; Mycovellosiella perfoliata in Brazil; Septoria ekmania in Brazil).
Pests (Acalithus adoratus in Bolivia, Brazil, Trinidad, Indonesia (introduced), Malaysia (introduced), Philippines (introduced), Thailand (introduced); Apion brunneonigrum in Argentina, Trinidad, Venezuela; Cecidochares connexa, Actinote anteas in Indonesia; Melanagromyza eupatoriella in Bolivia, Brazil, Trinidad; Mescina parvula in Argentina, Mexico, Trinidad; Pareuchaetes aurata in Argentina, Bolivia, Brazil, Paraguay; Pareuchaetes pseudoinsulata in USA, Bahamas, Mexico, Ecuador, Guyana, Venezuela, Tobago, Trinidad, introduced in India, Indonesia, Malaysia, Thailand, Sri Lanka, Guam, Ghana).

Management options for perennial weeds in irrigated rice: http://portal.wikwio.org/document/show/26


Local control

Herbarium pictures ReCOLNAT: https://explore.recolnat.org/search/botanique/simplequery=Chromolaena%2520odorata