Click on images to enlarge
close-up of flower-heads showing their long floral projections (Photo: Sheldon Navie)
comparison of billygoat weed (Ageratum conyzoides), with sparsely hairy bracts and short floral projections on the left, and blue billygoat weed (Ageratum houstonianum), with very hairy bracts and long floral projections on the right (Photo: Sheldon Navie)
close-up showing the numerous tiny flowers in each flower-head (Photo: Sheldon Navie)
branched clusters of flower-heads at the tips of the stems (Photo: Sheldon Navie)
paired lower leaves with slightly heart-shaped bases (Photo: Sheldon Navie)
close-up of hairy stems, alternately arranged upper leaves, and young flower-heads with hairy bracts (Photo: Sheldon Navie)
young plant and seedlings (Photo: Sheldon Navie)
infestation along a waterway (Photo: Sheldon Navie)
plants with white flower-heads are occasionally seen (Photo: Sheldon Navie)
habit (Photo: Sheldon Navie)
large infestation in a pasture (Photo: Sheldon Navie)
Blue billygoat weed, ageratum, bluemink, flossflower, goatweed.
Native to Mexico, Central America and the Caribbean.
Locations within which Ageratum houstonianum is naturalised include Australia, south-eastern USA, southern Europe, Africa, China, Japan, New Zealand and some Pacific islands.
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.
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.
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.
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.
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: firstname.lastname@example.org