Star thistle (Centaurea calcitrapa)

Star thistle is found throughout the cropping region of New South Wales. It can be a troublesome weed of crops and pastures.

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How does this weed affect you?

Star thistle forms a dense mat that competes with, and excludes desirable crop and pasture species. Each plant can produce about 1000 seeds, allowing the plant to quickly spread in size and density.

In native grassland habitats, star thistle has the potential to reduce the amount of native ground cover and grass species. 

Where is it found?

Native to southern and western Europe, western Asia and northern Africa. Star thistle is a weed in western USA, Argentina, Chile, Uruguay and New Zealand. It has also spread beyond its native range in Europe.

In Australia it is widespread and patchy throughout most of the wheat belt—extending from southern Queensland, through New South Wales, Victoria and into South Australia. Some small scattered infestations also occur near Perth.

In NSW it occurs throughout most of the state, except for the far western districts. It is mostly a weed of pastures in the Tableland districts. Elsewhere it is more commonly found along roadsides, channel banks and neglected areas.  

How does it spread?

Star thistle reproduces by seed. Most mature seed will fall close to the parent plant as it lacks the mechanism for wind dispersal that many other thistle species have. Mature spiny flower heads and seed may be spread further distances by water along channels and by becoming entangled in the wool and fur of animals. It may also be spread by the movement of contaminated hay and mud attached to vehicles and machinery.

Lifecycle

Seeds may germinate at any time of the year provided there is enough soil moisture. Most germination takes place in autumn following seasonal rains. Rosettes form during winter and into spring. Flowering begins late spring and into summer. Seeds ripen throughout summer and plants die back after seed set. Sometimes plants behave as biennials, only flowering in the second spring.

What does it look like?

An annual, sometimes biennial, bushy thistle growing to 1 m high.

Stem

• branched
• whitish to pale green in colour
• hairy when young, becoming sparsely hairy with age
• no spines

Leaves

• darkish green in colour
• have short hairs
• many deep narrow lobes
• no spines
• up to 25 cm long and 5 cm wide
• arranged in a rosette at the base, becoming smaller and narrower, growing directly from the base of stems

Flowers

• pinkish-purple in colour
• surrounded by many whitish-yellow spines 10–30 mm long
• flower head 10–20 mm long and 6–10 mm wide
• on short stalks
• occur  individually at the end of branches or in the fork of upper leaves

Seed

• whitish in colour with dark streaks
• 3–4 mm long and 2 mm wide
• oval shape
• smooth 

What type of environment does it grow in?

Star thistle prefers a temperate climate with an annual rainfall of 700–900 mm. It is found on a variety of soil types and mostly persists in open, disturbed areas with little competition. Most likely found around stock yards, channel banks, sheds and roadsides. 

Acknowledgements

Written by Rachele Osmond

Reviewed by Michael Michelmore

References

Australia’s Virtual Herbarium (2007) Council Heads of Australian Herbaria (CHAH) Available at: http://avh.chah.org.au. Accessed August 2014.

Department of the Environment (2011) Weeds in Australia: Centaurea calcitrapa. Australian Government. www.environment.gov.au/biodiversity/invasive/weeds/identification/index.html. Accessed August 2014. 

Department of Environment and Primary Industries (2014) Star thislte, State Government of Vctoria. Available at http://www.depi.vic.gov.au/agriculture-and-food/pests-diseases-and-weeds/weeds/a-z-of-weeds/star-thistle. Accessed August 2014.

Lamp C and Collet F (2004) Field guide to weeds in Australia, Inkata Press, Melbourne.

Hosking JR, Sainty GR, Jacobs SWL & Dellow JJ (in prep) The Australian WeedBOOK.

Parsons, WT and Cuthbertson, EG (2001) Noxious weeds of Australia, CSIRO Publishing, Collingwood.

More information

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Control

In cropping situations, cultivation can stimulate the germination of seeds. Using cultivation combined with herbicides can give positive results as long as thistles are treated at the seedling or rosette stage of growth. This is usually during winter and early spring.

Using crop and pasture rotations can help manage star thistle. In the pasture phase, it is important to maintain a competitive pasture. Treat any new emergence of thistles with either physical measures or herbicide.

Physical control

Individual plants and small infestations can be removed with a hoe and dug from the ground. Remove the entire plant and at least 5–10 cm of the tap root. Treat plants at the rosette stage and before flowering.

Herbicide treatment

Infestations can be treated using a foliar application of a registered herbicide.  Best results are obtained when treated at the seedling or rosette stage of growth.

Always monitor control efforts and treat regrowth as required.

Herbicide options

See Using herbicides for more information.

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2,4-D 300 g/L + Picloram 75 g/L (Tordon® 75-D)
Rate: 300–500 mL in 100 L of water
Comments: Spot spray. Seedling to rosette stage. Use higher rate on older rosettes.
Withholding period: Do not graze or cut crops (except sugar cane 8 weeks) or pastures for stock food for 7 days after application.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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2,4-D 300 g/L + Picloram 75 g/L (Tordon® 75-D)
Rate: 3.5–7.5 L/ha
Comments: Boom spray application. Use higher rate on older rosettes.
Withholding period: Do not graze or cut crops (except sugar cane 8 weeks) or pastures for stock food for 7 days after application.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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2,4-D LV ester 680g/L (Estercide® Xtra)
Rate: 1.15 to 1.7 L per hectare
Comments: Boom spray application, seedling to rosette stage
Withholding period: Do not graze or cut for stock food for 7 days after application.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Dicamba 750 g/L (Kamba® 750)
Rate: 67 mL per 100 L of water
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: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Dicamba 750 g/L (Kamba® 750)
Rate: 1.1 L/ha. Use at least 1500L 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: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Fluroxypyr 140 g/L + Aminopyralid 10 g/L (Hot Shot™ )
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: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the 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.

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Reviewed 2021