Stemless thistle (Onopordum acaulon)

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Stemless thistles have very large, spiny, whitish, woolly-looking leaves that grow in a rosette close to the ground. They can form dense stands that compete with pastures.

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How does this weed affect you?

Stemless thistles:

compete with pastures, reducing productivity
can form dense stands that restrict movement of livestock and people
can damage and devalue wool
can injure livestock and people handling the livestock or fleece

Livestock health

Livestock rarely eat stemless thistles. In situations where there was little feed and livestock have eaten stemless thistles, it caused impaction and suspected liver damage.

What does it look like?

Stemless thistles are biennial herbs that have a large rosette of leaves on the ground. Plants can be up to 1 m in diameter.

Leaves are:

covered in woolly hairs that give a whitish or pale blue-green appearance
up to 50 cm long and 12 cm wide
deeply lobed with triangular lobes
spiny with yellow spines up to 1 cm long along the margins.

Flowerheads are:

stalkless or on stalks up to 3 cm long
in the centre of the rosette
white turning yellowish brown as they mature
4- 6 cm in diameter
surrounded by spiny green bracts up to 3 cm long
present from October to December.

Seeds:

are 4-5 mm long
have barbed bristles 2-3 cm long.

Roots:

Stemless thistle has a deep taproot and many lateral roots.

Similar looking plants

Stemless thistles looks similar to other Onopordum species at the rosette stage including Scotch thistles (Onopordum acanthium) and Illyrian thistle (O. illyricum). When flowering, these similar thistles have stems and their flower heads are purple.

Where is it found?

In NSW, most plants have been found in the Murray and Western region. A few infestations have also been found in the North West, Northern Tablelands, Central Tablelands and Riverina regions.

Stemless thistle is native to Europe and northern Africa, bordering the Mediterranean.

What type of environment do they grow in?

Stemless thistles grow well in warm temperate climates. They are mostly found in areas with winter rainfall less than 450 mm per year and are drought tolerant. However, plants in the Northern Tablelands were growing in an area with 800 mm of rain per year. They are commonly found growing in sandy soils.

Stemless thistles in pastures, cultivated sites and on disturbed ground such as roadsides and rocky slopes

How does it spread?

By seed

Stemless thistle infestations can produce up to 15,900 seeds per m2. Thistle seeds can remain viable in the soil for more than 8 years. Seeds are spread by:

wind and water
attaching to animal wool or fur
sheep eating the seed heads and viable seed passes through the gut
attaching to clothing
machinery or vehicles
in hay and other agricultural produce.

References

Parsons, W.T., & Cuthbertson, E. G. (2001). Noxious weeds of Australia. 2nd edition. CSIRO publishing.

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 23 March 2026 from https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Onopordum~acaulon

Swirepik, A. E., & Woodburn, T. L. (2002, November). A new biological control project against stemless thistle (Onopordum acaulon) in Western Australia. In Proceedings of the 13th Australian Weeds Conference (pp. 426-429).

Weed Management Unit NSW DPI. (2008). Scotch, Illyrian and stemless thistle. Primefact 711. New South Wales Department of Primary Industries.

More information

Control

Prevention

To limit the spread of stemless thistles, avoid driving through, or taking machinery through areas infested with plants, especially if they have seed heads. If you cannot avoid these areas clean vehicles and machinery before moving to a non-infested area.

Request a fodder vendor declaration to limit importing contaminated hay or fodder.

Control isolated patches before they set seed.

Pasture management

Where possible, maintain a dense, vigorous and competitive pasture. A vigorous perennial grass pasture provides competition for germinating thistles, reducing seedling establishment. Gaps in the pasture result in an increase in thistle germination and seedling survival.

Fertiliser application (as determined by a soil test) can improve the vigour of annual and perennial introduced grasses, increasing ground cover and reducing future thistle establishment.

Maintain ground cover by managing stocking rates, especially in summer and early autumn.

Lucerne competes well with stemless thistle.

Physical removal

Isolated plants can be dug out. Remove as much of the taproot as possible so that plants do not regrow.

Cultivation

Cultivation is effective on seedlings or young rosettes if they are uprooted. Older rosettes are damaged by cultivation but are able to regrow, especially if the soil is moist.

Chemical control

Spraying

Spray actively growing plants. Herbicides are often recommended before stemless thistles start to flower. Check the labels for the best time and method to spray. When spraying, ensure all of the foliage is covered.

Herbicide options

WARNING - ALWAYS READ THE LABEL
Users of agricultural or veterinary chemical products must always read the label and any permit, before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this information. To view permits or product labels go to the Australian Pesticides and Veterinary Medicines Authority website www.apvma.gov.au

See Using herbicides for more information.

2,4-D LV ester 680g/L (Estercide® Xtra)
Rate: 2.5 to 3.3 L per hectare
Comments: Boom spray application rosette stage to flowering
Withholding period: Do not graze or cut for stock food for 7 days after application.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

Dicamba 750 g/L (Kamba® 750)
Rate: 53 mL per 100 L of water. Add a surfactant.
Comments: Spray prior to flowering. For non crop situations.
Withholding period: Do not harvest, graze or cut for stock food for 7 days after application.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

Dicamba 750 g/L (Kamba® 750)
Rate: 800 mL/ha Use a minimum of 1500 L of water per ha. Add a surfactant.
Comments: Boom spray for non-crop situations. Spray prior to flowering.
Withholding period: Do not harvest, graze or cut for stock food for 7 days after application.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

Fluroxypyr 140 g/L + Aminopyralid 10 g/L (Various products)
Rate: 500 mL in 100 L of water
Comments: Hand gun application
Withholding period: Not required for pastures when used as directed. Do not graze or cut crops for stock food for 7 days after application. See label for export restrictions.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

Biosecurity duty

The content provided here is for information purposes only and is taken from the NSW Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All pest plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.

For technical advice and assistance with identification please contact your local council weeds officer.

Reviewed 2026