Billygoat weed

Ageratum conyzoides
dense infestation (Photo: Sheldon Navie)
habit (Photo: Sheldon Navie)
leaves (Photo: Sheldon Navie)
hairy stems and young flower-heads (Photo: Sheldon Navie)
close-up of flower-heads showing their sparsely hairy bracts (Photo: Sheldon Navie)
seedling (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)
Herb
Opposite
Simple
Purple
Green
Common names 
Also known as: ageratum, billy goat weed, billygoat weed, chick weed, goatweed, floss flower, white weed,
Family 
Asteraceae
Deciduous 
No
Flowering time 
Year round
Native/Exotic 
Exotic
Origin 
This species probably originated in Central and South America, but its exact native enrage is obscure. It is now very widespread in tropical and sub-tropical regions of the world.
Notifiable 
No
Council declaration 
SIL – Special Investigation List
Known distribution 

Widely naturalised in the wetter parts of northern Australia. It is common and widespread throughout the coastal districts of eastern Queensland, from Cape York to the New South Wales border. It is relatively common in the north-western parts of the Northern Territory and in the coastal districts of northern New South Wales, north from the Maclean district. Also occasionally recorded in the Kimberley region in northern Western Australia and in other parts of Queensland. Also naturalised on Lord Howe Island, Norfolk Island and Christmas Island.

Habitat 

A weed of gardens, roadsides, disturbed sites, waste areas, pastures, crops, wetlands and waterways in the tropical, sub-tropical and warmer temperate regions of Australia.

Impact and control methods 
Similar species 

billygoat weed (Ageratum conyzoides) is often confused with another very similar species, Blue billygoat weed (Ageratum houstonianum). These species can be distinguished by the following differences:

blue billygoat weed (Ageratum houstonianum) has numerous sticky hairs on the bracts surrounding its flower-heads (i.e. the involucral bracts are glandular pubescent). Each of the tiny flowers (i.e. florets) which make up the flower-heads have two short and narrow projections (i.e. style branches) that are about 1-2 mm long. The bases of the flower-heads are relatively small (3-6 mm across).

billygoat weed (Ageratum conyzoides) has only a few hairs on the bracts surrounding its flower-heads (i.e. the involucral bracts are glabrous or sparsely pubescent). Each of the tiny flowers (i.e. florets) which make up the flower-heads have two long and narrow projections (i.e. style branches) that are about 5 mm long. The bases of the flower-heads are relatively large (5-8 mm across).
Another environmental weed, praxelis (Praxelis clematidea), and a native plant, vernonia (Cyanthillium cinereum), are also very similar to blue billygoat weed (Ageratum houstonianum). However, praxelis (Praxelis clematidea) leaves have deeply toothed (i.e. coarsely dentate) margins and the bracts surrounding each flower-head (i.e. involucral bracts) fall off when the seeds are shed (i.e. they are deciduous). Vernonia (Cyanthillium cinereum) leaves have finely-toothed (i.e. crenate) or almost entire margins and the seeds are topped with about 20 relatively large bristles (4-5 mm long).