Parthenium weed: declared pest

Page last updated: Tuesday, 12 November 2019 - 11:32am

Please note: This content may be out of date and is currently under review.

Parthenium weedragweed, Santa Maria feverfew, bitter weed, carrot grass, congress grass, false ragweed, ragweed parthenium, whitetop (Parthenium hysterophorus) is a declared pest in Western Australia (WA). This article describes the nature of the plant with links to requirements land owners/occupiers must adhere to and pest control methods.

Form: herbaceous annual

Status: found in the Pilbara — currently under eradication program

Native to tropical North and South America. Parthenium weed is a Weed of National Significance (WoNS).


An erect aromatic annual herb 30-150 centimetres tall, with a deep tap root, reproducing by seed.

Stems: Erect, much-branched in the upper half, hairy, longitudinally grooved and becoming woody with age.

Leaves: Pale green, fern-like in appearance, alternate, shortly hairy with some of the hairs containing allergy-causing substances. Rosette leaves are deeply lobed, 8-20 centimetres long and four to five centimetres wide. Stem leaves are shorter and less divided.

Flowers: White florets, in compact daisy-like heads 4-10 millimetres diameter, which are grouped together in many-branched clusters arising from the stem nodes and all terminating about the same height. Each head contains about 45 florets.

Seed: Black, two millimetres long, flattened with two white spoon-shaped appendages at the apex.

Root: A deep tap root with many finely branched feeding roots.

Agricultural and economic impact

An aggressive invader of disturbed land and perennial pastures and roadsides. Very prevalent in drier parts of Queensland and New South Wales, but absent from WA. Causes dermatitis and other allergic reactions to humans and cattle.

Declared pest category

The Western Australian Organism List (WAOL) contains information on the area(s) in which this pest is declared and the control and keeping categories to which it has been assigned in Western Australia (WA). Search for parthenium weed in the WAOL using the scientific name Parthenium hysterophorus.

Requirements for land owners/occupiers and other persons

Requirements for land owners/occupiers and other persons if this pest is found can be sourced through the declared plant requirements link.

Search > detect > report

MyPestGuide™ Reporter
via app or online
(Select 'Send report to MyWeedWatcher' from menu)

Pest and Disease Information Service (PaDIS)
+61 (0)8 9368 3080

Control method

Control methods for this declared plant can be found through the parthenium weed control link.

Contact information

Pest and Disease Information Service (PaDIS)
+61 (0)8 9368 3080
Technical support - MyPestGuide®