Chromolaena odorata (L.) R.M.King & H.Rob., Phytologia 20(3): 204 (1970)

Synonyms
Eupatorium affine Hook & Arn.
Eupatorium brachiatum Wikstrom
Eupatorium clematitis DC.
Eupatorium conyzoides M. Vahl
Eupatorium conyzoides var. angustiflorum Cuatrec.
Eupatorium conyzoides forma angustiflorum Cuatrec.
Eupatorium conyzoides var. floribunda (Kunth) Hieron.
Eupatorium conyzoides forma glabratum Hassl.
Eupatorium conyzoides var. glabrescens Steetz
Eupatorium conyzoides var. heterolepis Griseb.
Eupatorium conyzoides var. incanum Baker
Eupatorium conyzoides var. paucidentatum Baker
Eupatorium conyzoides var. pauciflorum Baker
Eupatorium conyzoides var. phyllocephalum Sch.Bip. ex Baker
Eupatorium conyzoides var. scaberulum Hassl.
Eupatorium conyzoides var. tambillense Hieron.
Eupatorium dichotomum Sch.Bip.
Eupatorium divergens Less.
Eupatorium floribundum Kunth
Eupatorium graciliflorum DC.
Eupatorium incisum Rich.
Eupatorium klattii Millsp.
Eupatorium odoratum L.
Eupatorium odoratum var. brachiatum (Sw. ex Wikstr.) DC.
Eupatorium odoratum var. cubense DC.
Eupatorium odoratum var. mallotophyllum B.L.Rob.
Eupatorium odoratum forma scandens Kuntze
Eupatorium sabeanum Buckley
Eupatorium stigmatosum Meyen & Walp.
Osmia clematitis (DC.) Sch.Bip.
Osmia conyzoides (Vahl) Sch.-Bip.
Osmia conyzoides Small [Illegitimate]
Osmia divergens (Less.) Schultz-Bip.
Osmia floribunda (Kunth) Schultz-Bip.
Osmia graciliflora (DC.) Sch.-Bip.
Osmia odorata (L.) Schultz-Bip.

Description
Herbaceous perennial that forms dense tangled bushes 1.5-2.0 m in height. It occasionally reaches a maximum height of 6m (as a climber on other plants). Its stems branch freely, with lateral branches developing in pairs from the axillary buds. The older stems are brown and woody near the base; tips and young shoots are green and succulent. The root system is fibrous and does not penetrate beyond 20-30cm in most soils. The flowerheads are borne in terminal corymbs of 20 to 60 heads on all stems and branches. The flowers are white or pale bluish-lilac, and form masses covering the whole surface of the bush. Leaves opposite, flaccid-membranous, velvety-pubescent, deltoid-ovate, acute, 3-nerved, very coarsely toothed, each margin with 1-5 teeth, or entire in youngest leaves; base obtuse or subtruncate but shortly decurrent; petiole slender, 1-1.5cm long; blade mostly 5-12cm long, 3-6cm wide, capitula in sub-corymbose axillary and terminal clusters; peduncles 1-3cm long, bracteate; bracts slender, 10-12mm long; involucre of about 4-5 series of bracts, pale with green nerves, acute, the lowest ones about 2mm long, upper ones 8-9mm long, all acute, distally ciliate, flat, appressed except the extreme divergent tip; florets all alike (disc-florets), pale purple to dull off-white, the styles extending about 4mm beyond the apex of the involucre, spreading radiately; receptacle very narrow; florets about 20-30 or a few more, 10-12mm long; ovarian portion 4mm long; corolla slender trumpet form; pappus of dull white hairs 5mm long; achenes glabrous or nearly so. The seeds of Siam weed are small (3-5mm long, ~1mm wide, and weigh about 2.5mg seed-1. [data from Global Invasive Species Database]

Ecology
A notorius invasive plant that can quickly take over open, sunny places. It grows on a wide range of soils and vegetation types, e.g. forests (annual rainfall 1500mm), grassland and arid bushveld (annual rainfall less than 500mm). In arid areas, it is restricted to riverbanks and it will only become invasive in the frost-free areas of medium to arid woodland which are not water-stressed in the growing season. For good growth of Siam weed seedlings, the relative humidity should be in the range of 60 to 70%; at values higher than 80% the growth performance is poor. Experiments show that Siam weed seedlings grew well at 30C and even better on mulched soils at 25C. In heavy shade, Siam weed will not seed. It has a negative relationship with tree canopy cover and appears to be most abundant on the edge of forested areas. In north-eastern India, Siam weed is regarded as a nutrient-demanding early successional species. It takes advantage of the flush of soil that becomes available after a disturbance, such as fire or land clearing for agriculture, and exhibits relatively high foliar N, P and K contents. [data from Global Invasive Species Database]

Distribution
Native to South and Central America and parts of the southern USA, but now spread all over the tropics.

Uses
Medicinal, the young leaves are crushed and used to treat skin wounds in Indonesia. Also an ornamental plant that is sometimes encouraged for use in shifting slash-and-burn agriculture to compete with Imperata cylindrica (alang alang or cogon grass), which is harder to control.

Local names
Chamorro-Guam: Kesengesil, Masigsig.
Chuuk: Otuot.
English: Bitter bush, Christmass bush, Chromolaena, Common floss flower, Jack in the bush, Siam weed, Triffid weed.
French: Herbe du Laos.
German: Siam-Kraut.
Indonesia: Rumput belalang, Rumput golkar, Rumput putih.
Kosrae: Mahsrihsrihk.
Palau: Ngesngesil.
Philippines: Agonoi, Hagonoy, Huluhagonoi.
Pohnpei: Wisolmatenrehwei.