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== Synonyms ==
*''Eupatorium odoratum'' L. (1759), *''E. conyzoides'' Vahl (1794), *''Osmia odorata'' (L.) Schultz-Bip. (1866).
== Vernacular names ==
== Properties ==
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.
== Description ==
*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 .*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.
== Growth and development ==
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'' ).
== Diseases and pests ==
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.
== Prospects ==
== Literature ==
* 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.