Crofton weed

Eupatorium adenophorum syn Ageratina adenophora
Shrub
Opposite
Simple
White
Green

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.

Common names 
Also known as: sticky snakeroot, cat weed, hemp agrimony, Mexican devil, sticky agrimony, sticky eupatorium, white thoroughwort,
Family 
Asteraceae
Deciduous 
No
Native/Exotic 
Exotic
Origin 
Native to Mexico and possibly Central America.
Notifiable 
No
State declaration 
Nil
Council declaration 
Class E - Early detection and eradication
Known distribution 

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.

Habitat 

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.

Habit 

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.

Impact and control methods 

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.

Stem and leaves 

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).

Flowers and fruits 

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.

Reproduction and dispersal 

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.

Similar species 

Ageratina adenophora is quite similar to Chromolaena odorata (Chromolaena) and Gymnocoronis spilanthoides (Senegal tea plant). These species can be distinguished by the following differences:

Ageratina adenophora is an upright (erect) plant 1-2 m tall with relatively broad, diamond-shaped (rhomboid) or almost triangular, leaves and young stems that are densely covered in sticky (glandular) hairs. Its 'seeds' (achenes) are tiny (1-2 mm long), have hairless edges, and are topped with a ring (pappus) of whitish hairs (3-4 mm long).
Chromolaena odorata is a large upright (erect) shrubby plant 1.5-5 m tall with relatively broad, egg-shaped with broad end at base (ovate) or triangular, leaves and young stems that are sparsely covered in fine hairs. Its 'seeds' (achenes) are relatively large (4-5 mm long), and are topped with a ring (pappus) of whitish or brownish hairs (about 5 mm long).
Gymnocoronis spilanthoides is a semi-aquatic plant less than 1 m tall with somewhat hollow stems and relatively narrow, ovate or lance-shaped (lanceolate), hairless leaves. Its 'seeds' (achenes) are relatively large (about 5 mm long), and are not topped with a ring (pappus) of hairs.
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.