Ageratum houstonianum (Blue Billygoat Weed)

Scientific name

Ageratum houstonianumMill.


Ageratum mexicanumSims.

Common names

Blue billygoat weed, ageratum, bluemink, flossflower, goatweed.


Asteraceae (Compositae)


Native to Mexico, Central America and the Caribbean.

Naturalised distribution (global)

Locations within which Ageratum houstonianum is naturalised include Australia, south-eastern USA, southern Europe, Africa, China, Japan, New Zealand and some Pacific islands.

Introduced, naturalised or invasive in East Africa

Ageratum houstonianum is invasive in parts  Tanzania (Henderson 2002). A. houstonianum is naturalised in Kenya at 1500-2780m altitude. Specifically in the highlands: Mau, Aberdares, parts of Laikipia and Nyeri. The editors are not aware of records of the presence of this species in Uganda, though this does not necessarily mean that it is absent from this country.


Ageratum houstonianum weed is a weed of gardens, roadsides, disturbed sites, waste areas, pastures, crops, wetlands and riparian zones (banks of watercourses).


A short-lived (annual or biennial) herbaceous plant growing 0.3-1 m tall, with showy flower-heads.

The stems are round, mostly green in colour, and softly hairy (pubescent). The leaves are often opposite, and may be alternate in upper parts of the stem. Leaves are borne on stalks (petioles) 0.5-3 cm long and vary from being almost triangular in shape (obovate) to egg-shaped in outline with broad end at base (ovate). These leaves (2-7 cm long and 1.5-6 cm wide) have bluntly toothed (crenate) margins and either blunt or pointed tips (obtuse to acute apices).  Both surfaces of the leaves and the leaf stalks have a scattered covering of hairs (they are pubescent).

The flower-heads (capitula) are arranged in dense clusters at the tips of the branches (in terminal corymbs) and do not have any obvious 'petals' (ray florets). Each flower-head (5-8 mm across) has numerous tiny tubular flowers (tubular florets) that are surrounded by two or three rows of greenish-coloured bracts (an involucre). The florets (2-3 mm long) range from pale lavender to blue, pink or purplish in colour and each has two elongated projections (style branches). The bracts at the base of the flower-head (3-5 mm long) are elongated in shape (linear-lanceolate) and covered in sticky hairs (glandular pubescent). Flowering occurs throughout most of the year.

The 'seeds' (achenes) are about 2 mm long, brown to black in colour, and topped with five awn-tipped scales (pappus). These scales (2-3 mm long) are whitish in colour and resemble short bristles or hairs.

Reproduction and dispersal

This species reproduces by seed. The tiny, light, seeds are often dispersed by wind or water. Also, they readily become attached to animals, clothing and vehicles and may also be spread in contaminated agricultural produce.

Similar species

Ageratum houstonianum is often confused with another very similar species, Ageratum conyzoides (billygoat weed). These species can be distinguished as follows:

  • Ageratum houstonianum has numerous sticky hairs on the bracts surrounding its flower-heads (the involucral bracts are glandular pubescent). Each of the tiny flowers (florets) which make up the flower-heads have two short and narrow projections (style branches) that are about 1-2 mm long. The bases of the flower-heads are relatively small (3-6 mm across).
  • Ageratum conyzoides has only a few hairs on the bracts surrounding its flower-heads (the involucral bracts are glabrous or sparsely pubescent). Each of the tiny flowers (floret) which make up the flower-heads have two long and narrow projections (style branches) that are about 5 mm long. The bases of the flower-heads are relatively large (5-8 mm across).

Economic and other uses

Ageratum houstonianum weed has a long history of use as an ornamental plant has been spread around the world for this reason It is also used in some cultures as a medicinal plant.

Environmental and other impacts

Ageratum houstonianum is prone to becoming a rampant environmental weed when grown outside its natural range. It is considered to a highly invasive species in Australia and South Africa and neighbouring countries (Swaziland, Mozambique and Zimbabwe) where it poses a threat to biodiversity. It is one of several closely-related species that are invading savannas, grasslands, forest margins, riverbanks and wetlands. It often forms dense stands that exclude other species and there is evidence to suggest that it has allelopathic properties that inhibit the growth and germination of other plants. It has been listed as an noxious weed in South Africa.

This species is regarded as a naturalised garden escape in Kenyan highlands. It is a very common weed of farmland in Kenyan highlands, and is also a weed of other cropping systems in Kenya.


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. As Ageratum houstonianum is so widespread this is unlikely to be possible in many instances.

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.

As a common agricultural weed, it is managed in small and large cropping situation by tillage and can contribute to soil fertility as a waste mulch.

Hand weeding of the species is not advisable due to allergenic effects Herbicides can be sprayed in crops but care must be taken to direct the spray at the weeds only. This chemical should not be used when maize is intercropped with legumes and oil-seeds. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert. A 20% solution of common salt can be safely used in non-cropping areas. 

The editors know of no biological control agents available for the control of A. houstonianum.


Not listed as a noxious weed by the state or governments in Kenya, Tanzania and Uganda.


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: