Click on images to enlarge
comparison of crofton weed (Ageratina adenophora), left, and mistflower (Ageratina riparia), right (Photo: Sheldon Navie)
seedling (Photo: Sheldon Navie)
the bright green, oppositely arranged, leaves (Photo: Sheldon Navie)
infestation (Photo: Forest and Kim Starr, USGS)
close-up showing the greenish flower-head bracts and the sticky hairs on the young stems (Photo: Sheldon Navie)
close-up of the flower-heads, each with numerous tiny tubular flowers (Photo: Sheldon Navie)
the small white flower-heads are borne in dense clusters (Photo: Sheldon Navie)
the branched flower clusters at the tips of the stems (Photo: Sheldon Navie)
habit in flower (Photo: Sheldon Navie)
close-up of the relatively broad and almost triangular leaves (Photo: Sheldon Navie)
Ageratina adenophora (Spreng.) R.M. King & H. Rob.
Eupatorium adenophorum Spreng.
Native to Mexico and possibly Central America.
Locations within which Ageratina adenophora is naturalised include Australia, southern Europe, Africa, Asia, New Zealand, south-western USA and many oceanic islands in the Pacific Ocean.
Ageratina adenophora is invasive in parts of Kenya including around Lake Naivasha (A.B.R. Witt pers. obs.). The editors are not aware of records of the presence of A. adenophora in Tanzania and Uganda though this does not necessarily mean that it is absent from these countries.
This species is a weed of roadsides, railways, pastures, fence-lines, disturbed sites, waste areas and riparian zones (banks of watercourses) in subtropical and warmer temperate regions. It is also commonly found in urban open spaces, open woodlands, forest margins and rainforest clearings.
A long-lived (perennial) herbaceous plant or small soft-stemmed shrub usually growing 1-2 m tall, but occasionally reaching 3 m in height. It produces numerous upright (erect) stems from a woody rootstock. The branched stems are densely covered in sticky (glandular) hairs when young and may be green, reddish or purplish in colour. They become slightly woody and turn brownish-green or brown in colour when mature. Its roots are yellowish in colour and give off a distinct carrot-like smell when broken or damaged.
The leaves are oppositely arranged along the stems and are borne on stalks (petioles) 1-6 cm long. The broad leaf blades (4-15 cm long and 3-9 cm wide) are trowel-shaped, diamond-shaped (rhomboid), or triangular with bluntly or sharply toothed (crenate or serrate) margins. These leaves have sharply pointed tips (acute apices) and are mostly hairless (glabrous), but their stalks are often covered in sticky hairs (they are glandular pubescent).
The small white flower-heads (capitula) consist of several tiny flowers (tubular florets) surrounded by two rows of greenish bracts (an involucre) 3-5 mm long. These flower-heads (5-8 mm across) are borne in large numbers and arranged in clusters at the tips of the branches (in terminal corymbose inflorescences). The tiny tubular florets (3-5 mm long) are white and contain both male and female flower parts (they are bisexual). The 'seeds' (achenes) are slender, reddish-brown or blackish-brown in colour, and slightly curved. These 'seeds' (1-2 mm long and 0.3-0.5 mm wide) have four or five slight ribs which run lengthwise (longitudinally) and their bodies are hairless (glabrous). However, they are topped with a ring (pappus) of numerous whitish hairs (3-4 mm long), which are readily shed.
Ageratina adenophora reproduces by seeds which are easily dispersed by wind and float on water. They may also be spread in by animals and vehicles and can contaminate agricultural produce.
Ageratum houstonianum (blue billygoat weed), Ageratum conyzoides subsp. conyzoides (billygoat weed) and Praxelis clematidea (praxelis) are slightly similar to A. adenophora when in the vegetative stages of growth, but these species usually have hairy leaves. When in flower, they can be easily distinguished by their bluish, purplish or pinkish-coloured flower-heads.
Ageratina adenophora has been moved around the world as an ornamental but it is now more known for its negative impacts.
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.
The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.
The best form of invasive species management is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Controlling the weed before it seeds will reduce future problems. Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.
Ageratina adenophora can be manually controlled by cutting with a machete (panga), followed by ripping out the plant or ploughing, then sowing desirable pasture species. Small infestations can be dug out and exposing the roots. However, like so many flowering plants, it can set down a significant seed bank in the soil so that other mechanical control can become continuous. The fact that this plant often grows in swamps and difficult to access areas like riverbanks makes control difficult but herbicides have been used to control this plant. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.
A tephritid stem galler, Procecidochares utilis has been introduced to control A. adenophora in Hawaii, parts of Australia, South Africa, New Zealand and India. It is not very effective in South Africa (Arne. Witt. pers. com). In Hawai'i the fungus Entyloma compositarum (Basidiomycetes: Ustilaginales) is considered effective. A combination of fungal infestation of plants during wet periods and the impact of the stem galler has resulted in other plants growing in A. adenophora infestations and in some cases replacing them.
Not listed as a noxious weed by any state or government authorities in Kenya, Tanzania or Uganda.
Global Compendium of Weeds. www.hear.org/gcw. Hawaiian Ecosystems at Risk Project. Accessed March 2011.
Henderson, L. (2001). Alien weeds and invasive plants. A complete guide to declared weeds and invaders in South Africa. Plant Protection Research Institute Handbook No. 12, 300pp. PPR, ARC South Africa.
Pacific Island Ecosystems at Risk (PIER). Agave sisalana Perrine, Asparagaceae: plant threats to Pacific ecosystems. www.hear.org/pier/species/ agave_sisalana.htm. Institute of Pacific Islands Forestry, Hawaii, USA. Accessed March 2011.
Scher, J. (2005). Federal Noxious Weed Disseminules of the U.S.: an interactive identification tool for seeds and fruit of plants on the United States federal noxious weed list. CD-ROM. Animal and Plant Health Inspection Service (APHIS), United States Department of Agriculture (USDA).
Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.
This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).
BioNET-EAFRINET Regional Coordinator: firstname.lastname@example.org