Ageratina adenophora is a perennial herbaceous shrub that may grow to 1 or 2 metres (3.3 or 6.6 ft) high. It has opposite trowel-shaped serrated leaves that are 6–10 cm (2.4–3.9 in) long by 3–6 cm (1.2–2.4 in) in width. The small compound flowers occur in late spring and summer, and are found in clusters at the end of branches. Each flowerhead is up to 0.5 cm in the diameter and creamy white in colour. They are followed by a small brown seed with a white feathery ‘parachute.It is native to Mexico, but it is known in many other parts of the world as an introduced species and often a noxious weed. It has caused great economic loss in agriculture in southwestern China, and is threatening the native biodiversity there. It was first inadvertently introduced to Yunnan around 1940, and its rapid spread is due in part to its allelopathic competition with other plant species. It also a weed in Australia, where it was introduced to Sydney in 1904. It has spread along the coastline of New South Wales and southern Queensland. It is rated a Class 4 Noxious Weed under the NSW Noxious Weeds Act of 1993.Ageratina adenophora has also spread in Hawaii and the mainland USA, where it is recognised as a weed in ten states of the South and Southwest.Elsewhere it is an invasive species in many tropical and subtropical countries, including northeastern India, China, Sri Lanka, Nigeria, Southeast Asia, Australia, New Zealand, the Pacific Islands, the Canary Islands, and South Africa.The plant can spread vegetatively, that is the stems can sprout roots and grow upon contact with earth. The seed is also carried by the wind or water and colonises disturbed areas, such as fields and areas near human habitation, readily. Seed may also be transported on animals and in soil.Ageratina adenophora is regarded as an environmental weed in many parts of the world. It is on the Federal Noxious Weeds List in the USA and the State Noxious Weeds Lists in Queensland and New South Wales, Australia. A. adenophora is probably one of the worst weeds in China where it is rapidly invading the foothills of the Himalayas – “the Chromolaena of the highlands (Arne. Witt. pers. com). It is also very invasive in South Africa and Zimbabwe.
In Queensland and New South Wales, this species colonises forest margins, stream banks and disturbed areas, preferring shaded wetter areas but also growing in open sunny sites. It also thrives in damp areas such as wetland margins, drainage lines, gullies and in clearings in wetter forests. It grows in large dense clumps and will eventually out-compete all other plants in an area, choking out native vegetation and forming a monoculture.
A. adenophora is also an aggressive weed in pastures in eastern Australia. It prefers wetter pastures (e.g. kikuyu grass pastures on wetter slopes), is usually not eaten by cattle, and can reduce the carrying capacity and productivity of invaded areas.
It is also poisonous to livestock, being particularly toxic to horses. In fact, this species is the cause of an acute pulmonary disease in horses which is known as “Tallebudgera horse disease” in Queensland and “Numinbah horse sickness” in New South Wales. This condition can be fatal if enough of the weed is consumed over a long period.