Stemless thistle: declared pest

Page last updated: Monday, 11 May 2020 - 1:12pm

Please note: This content may be out of date and is currently under review.

Stemless thistle (Onopordum acaulon) 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 or biennial

Status: present in WA

Stemless thistle originated in the western Mediterranean region. It is a weed only in Australia.

Appearance

Leaves: It develops a large rosette that may spread to cover an area up to 60 ccentimetres or more in diameter. The numerous whitish-green leaves are densely covered in woolly hairs. They are deeply divided with numerous spiny waxy lobes.

Flowers: Borne in globular flower heads four to six centimetres in diameter. Each head contains white or purple florets and is surrounded by sharp spines. Several flower heads are formed as a cluster at the centre of the rosette, close to the ground. The flower heads have no stalks.

Seeds: Grey or brown in colour and about four millimetres long. They are four-sided with a ridged seed coat and 25 millimetres long pappus (parachute) of pale barbed hairs. Seeds may remain dormant for several years. Several germinations are common during autumn-spring.

Stemless thistle germinates mainly in the autumn. It persists through winter as a small rosette, which grows rapidly in spring. It flowers from October to November and then dies off. Seedlings arising from winter and spring germinations seldom produce viable seed;  they die off in early summer.

Agricultural and economic impact

Weed of pastures. It is found in pastures, roadsides, disturbed areas, irrigated vegetable crops and cultivated land in south eastern Australia. Infestations are found in both high rainfall and semi-arid areas. In Western Australia infestations have been found mainly in the southern cereal growing areas and near Esperance.

Dense infestations of stemless thistle reduce the carrying capacity of pastures. The plant is unpalatable except when wilted. It spreads over large areas, shading out more useful plants with its large flat leaves. Hungry stock forced to eat stemless thistle suffer from stomach ailments, and liver and kidney damage also occurs at times.

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 stemless thistle in the WAOL using the scientific name Onopordum acaulon.

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)
mypestguide.agric.wa.gov.au

Pest and Disease Information Service (PaDIS)
+61 (0)8 9368 3080
padis@dpird.wa.gov.au

Control method

Control methods for this declared plant can be found through the stemless thistle control link.

Contact information

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