What Does It Look Like?
What is it?
Praxelis is an annual or perennial herb 0.2–1 m tall. Its leaves are arranged in opposite pairs along the brittle cylindrical stems, which are covered in short soft hairs. The leaves are tear-shaped or 'ovate' to diamond shaped or 'rhomboid', with a conspicuously toothed margin containing between five and eight teeth. When crushed the leaves emit a pungent odour similar to cat's urine.
The flowers are in conical to bell-shaped heads (a key identifying feature of the species) each with 30–50 lilac or bluish florets and occur in groups at the ends of stems.
The seeds (called achenes) are black, 2–3 mm long, black, with a tuft of 15–40 white pappus-bristles at the apex (Veldkamp 1999; CRC 2003).
For further information and assistance with identification of Praxelis contact the herbarium in your state or territory.
Flower colour
Blue
Growth form (weed type/habit)
Herb
Where it currently grows? Preferred habitat
Praxelis invades a range of habitats in the tropics and subtropics. It is particularly suited to disturbed areas such as roadsides, railway lines and fencelines, and rapidly colonises bare earth following fire. Able to survive on a range of soil types, it invades crops, grasslands and, particularly, over-grazed pastures. It can become the dominant herbaceous plant in open eucalypt woodlands, and grows vigorously along riverbanks. It tolerates partial shade to full sun but does not cope well under heavy shade. Praxelis is well established in areas that have more than 900 mm annual rainfall and is expected to survive in areas with annual rainfall in the range 500–700 mm. In these drier areas Praxelis behaves more like an annual, setting seed and dying off until the next rainy season, when germination takes place. It will probably only exist in cultivated areas or along waterways in areas where annual rainfall is less than 500 mm (CRC 2003).
Are there similar species?
Praxelis is very similar to two related weed species, Ageratum conyzoides and Ageratum houstonianum, which are also found in northern Australia. These species are commonly known as 'Blue Top' and 'Billy Goat Weed', and are often mistaken for each other. Both are found as environmental weeds, especially on roadsides and disturbed areas.
The differences between Praxelis and A. conyzoides are subtle, even to the trained eye. Both have blue flowers, although those on A. conyzoides are often less intense blue and may also be white or pale lilac. Both are covered with hairs, although the hairs on Praxelis are longer and more conspicuous. The main difference is that Praxelis has a conical receptacle for its 'florets', the dense cluster of small flowers that make up the flower head, whereas A. conyzoides has a flat or slightly dome-shaped receptacle. Also, the ring of 'bracts', modified leaves that surround and support the flower head, is deciduous and drops off Praxelis flowers, whereas in A. conyzoides the bracts are persistent. The leaves of Praxelis have a more pungent odour when crushed, and are more triangular and more sharply toothed than those of A. conyzoides, which are more rounded near the tip and have smooth teeth along the edges. Finally, the 'pappus' on Praxelis seeds consists of many more bristles (15-40) than A. conyzoides, which has only about 5 bristles. The flower colour and size of A. houstonianum is usually much closer to that of Praxelis than A. conyzoides, especially in those parts of northern Queensland where the three species occur together (e.g. Atherton Tablelands) (CRC 2003).
