Quick Facts

Quick facts

  • Original from Europe, north Africa and western Asia, Star Thistle (Centaurea calcitrapa) is an annual or biennial herb with spineless divided leaves, with clusters of pale purple, rarely pink or white flowers, surrounded by spines.
  • Invades open unshaded areas such as roadsides, crops, pastures and wasteland in temperate regions.
  • Dispersal is by wind or water, as well as through the spiky flower-heads containing the seed becoming lodged in machinery, vehicle tyres, animal hair, or in agricultural produce and being spread by those ways.
  • The whole mature plant can also break off at the base and blow around in the wind spreading the seed which can then be dispersed further by water.
  • A weed of agriculture, both cropping and pastures, and sometimes native open vegetation, commonly found on roadsides and waste place in southern and eastern Australia
  • The spiky flower-heads cause injuries to grazing animals and make it difficult to access the adjacent pasture species.
  • There are several effective physical control methods and Star Thistle is also susceptible to chemical control.

What Does It Look Like?

What is it?

Star Thistle (Centaurea calcitrapa) is an upright, much branched, bushy annual or biennial herb, usually growing 400 to 800 mm high but sometimes up to 1 m. The taproot is fleshy and 2 to 3 cm in diameter. Two types of leaves are produced, rosette leavers and stem leaves, all spineless. The rosette (basal) leaves are produced first and are short-lived, having a leaf-stalk (petiole), leaves to 250 mm long by 40–50  mm wide, darkish green, pinnatisect (deeply divided almost to midrib), with toothed lobes, pubescent (downy – covered with short, soft, erect hairs) or woolly when young and glandular. Stem and stem-leaves are then produced. The stems are much-branched, sparsely to densely covered with fine hairs. Most stem leaves are also deeply divided,  however, the uppermost leaves of the stem may be undivided. Stem leaves are sessile (without a leaf stalk), scabrous (rough to touch), the lower leaves are lobed to lyrate (deeply lobed, with a large terminal lobe and smaller lateral one) to lanceolate (lance-shaped), 20–80 mm long, 10–30 mm wide, the upper  leaves are smaller, entire (having a smooth margin, not lobed, divided or toothed) linear (very narrow in relation to its length, with the sides mostly parallel) to narrow-lanceolate, 2–4 mm wide. (Parsons & Cuthbertson 2001; Navie 2004; VicFlora 2016).

Flower-heads produce tightly packed groups of pale purple, rarely pink or white flowers on stalkless heads that are 10 to 20 mm long by 6 to 10 mm wide and occur at the ends of the branches or between the upper leaves and stems. Each head is surrounded by numerous greenish to brownish coloured bracts (modified leaves), each ending in a rigid, sharp, white or yellowish spine 15 to 30 mm long and with a further 2 to 6 shorter spines at the base. Flowers mainly Nov.–Apr.

Seeds are whitish or greyish in colour with brownish or darker streaks or blotches, 3 to 4 mm long, 2 mm wide, egg-shaped, smooth and without hairs or scales (Parsons & Cuthbertson 2001; Navie 2004).

For further information and assistance with identification of Star Thistle, contact the herbarium in your state or territory.

Flower colour

Purple, White, Pink

Growth form (weed type/habit)


Where it currently grows? Preferred habitat

Star Thistle grows in open, unshaded situations in temperate, and occasionally subtropical regions, on a wide range of soils as a weed of agriculture especially poorer pastures and cropping land, open native vegetation, roadsides, stockyards, crops, and waste land in areas of 700 to 900 mm annual rainfall (Parsons & Cuthbertson 2001; Navie 2004).

Are there similar species?

Star Thistle (Centaurea calcitrapa) is very similar to Spotted Knapweed (Centaurea maculosa), Black Knapweed (Centaurea nigra), Creeping Knapweed (Rhaponticum repens) [as Acroptilon repens], Soldier Thistle (Picnomon acarna) and White-stemmed Distaff Thistle (Carthamus leucocaulos). It is also relatively similar to St. Barnaby's Thistle (Centaurea solstitialis), Maltese Cockspur (Centaurea melitensis) and Saffron Thistle (Carthamus lanatus) (Navie 2004).

Spotted Knapweed, Black Knapweed and Creeping Knapweed all have similar spineless stems and leaves to Star Thistle, however the purplish flower heads of these species are enclosed in several rows of spineless bracts (modified leaves), whereas those of Star Thistle end in spines. The margins of the bracts of Black Knapweed and Creeping Knapweed are fringed with black hairs up to 2.5 mm long, whereas those of Creeping Knapweed are papery only. The seeds of all three of these species are topped with a pappus (tuft) of bristles or hairs, although in Creeping Knapweed these can easily become detached. The seeds of Star Thistle have no appendages like bristles or hairs (Navie 2004).

Soldier Thistle differs from Star Thistle in having spiny, winged stems and spiny leaves. Its pinkish or purplish flower heads are similar as they are enclosed in several rows of spine-tipped bracts but the seeds of Soldier Thistle are topped with a pappus of numerous feathery white bristles (Navie 2004).

White-stemmed Distaff Thistle has spineless stems like Star Thistle but its leaves are spiny and the undersides are often without hairs (although the young leaves sometimes have a cobwebby appearance). Like Star Thistle, White-stemmed Distaff Thistle's purplish flower heads are enclosed in several rows of spine-tipped modified leaves but the outer rows are turned downwards (Navie 2004).

Like Star Thistle, St. Barnaby's Thistle and Maltese Cockspur, have spineless stems and leaves which have a covering of hairs on the undersides. Unlike Star Thistle, their flower heads are yellow rather than purple and the seeds may or may not be topped with a pappus of numerous bristly hairs (Navie 2004).

Saffron Thistle also has spineless stems like Star Thistle but generally there are no hairs on the undersides of its spiny leaves (although the young leaves sometimes have a cobwebby appearance). The flower heads are yellow rather than purple and are enclosed in several rows of spiny bracts, the outer rows of which are turned downwards. The greyish-brown coloured seeds of Saffron Thistle are topped with a pappus of numerous bristles or narrow scales unlike the seeds of Star Thistle which are whitish and have no pappus at all (Navie 2004).

Why Is It A Weed?

What are its impacts?

Star Thistle (Centaurea calcitrapa) impacts agriculture and is found in native vegetation. A weed of pastures, roadsides, wastelands, disturbed sites and cultivated areas in southern Australia, but also invades native grasslands, open woodlands, rangelands and conservation areas.

Agriculture: Star Thistle Star thistle forms a dense mat that competes with, and excludes desirable crop and pasture species (DPI NSW 2019). Once heads are formed, their spines that surround the flower-heads may injure grazing animals particularly in the eyes and mouth, and make access to pasture species for grazing difficult in the immediate vicinity. The spines also make clumps of the plant an effective harbour for rabbits. It is believed that Star Thistle may be toxic to animals but as it is rarely grazed, this is not of great importance (Parsons & Cuthbertson 2001).

Native ecosystems: It may invade native grasslands, open woodlands, and conservation areas especially degraded open native vegetation. Occurs in open woodlands, grasslands and conservation area and is regarded as an environmental weed in southern Australia. Does not normally invade healthy dense vegetation. In Victoria it is known to invade lowland grassland, grassy woodland, dry sclerophyll forest and woodland. It is seen as a serious threat to one or more of these vegetation formations and has the potential to displace species in the lower stratum (Carr et al. 1992; VRO 2007).

Urban areas: A weed of roadsides and waste places especially in poorer or degraded soils. The spiny nature of Star Thistle has the potential to injure humans that come in contact with it (VRO 2007).

How does it spread?

Star Thistle seeds lack hairs so they are less equipped for wind dispersal than most other thistles but are spread to some extent by water along channels and as a contaminant in agricultural produce and equipment. The seeds tend to remain in the spiny heads which can break off and become lodged in machinery and vehicle tyres or attached to animals or fabric. The whole plant can also break off at ground level and shed seeds while being blown around as a tumbleweed (Parsons & Cuthbertson 2001).

What is its history in Australia?

Information on the arrival of Star Thistle in Australia is not available but it was recorded in South Australia from as early as 1862 (Parsons & Cuthbertson 2001).

How To Manage It?

Best practice management

Star Thistle (Centaurea calcitrapa) can be managed by chemical and physical means. 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, usually during winter and early spring.

Chemical control: Star Thistle is susceptible to various chemical treatments in the seedling, rosette and pre-flowering stages (Parsons & Cuthbertson 2001). See DPI NSW (2019) & Parsons & Cuthbertson (2001) for further herbicide information. Please see the Australian Pesticides and Veterinary Authority for chemical information http://www.apvma.gov.au 

Non-chemical control: Physical control: Isolated plants should be grubbed out, ensuring that the bulk of the taproot is removed. If near the seeding stage, these plants should be heaped and burnt to destroy seeds.

Competition and management:  For larger infestations, cultivation is effective particularly in the early growth stages. Once the plants reach the rosette stage, cultivation must be at least 10 cm deep to sever the taproot at a depth from which regrowth does not occur. Cultivation can be combined with growing a fodder crop or establishing a pasture which should be adequately top dressed and only lightly grazed in order to provide maximum competition to seedlings (Parsons & Cuthbertson 2001). In the pasture phase, it is important to maintain a competitive pasture. Treat any new emergence of thistles with either physical measures or herbicide. 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, usually during winter and early spring.

Mechanical control:  Slashing is effective if carried out just before flowering. Mowing is equally effective but, if left until early flowering, there may be enough food material in the cut stems to allow the seeds to mature. Slashing on the other hand, has the advantage of destroying many of the seed heads. If either slashing or mowing is carried out too early, the plants may regrow from buds at the base and produce new flowering stems.

Does it have a biological control agent?


When does it grow? (lifecycle/growth calendar)

Seeds will germinate at any time of the year if moisture is available, however most germination occurs after autumn. Rosettes are formed during winter and spring, and in most cases flowering stems are produced in late spring. In some cases, however, plants do not produce flowering stems until the second spring, thus becoming biennials. In either case, flowering commences about November and continues until February or even later, after which the plants die. The seed is relatively short lived, surviving in the soil for only two to three years (Parsons & Cuthbertson 2001).

Where Is It Found?

Which states and territories is it found?


What areas within states and territories is it found?

Star Thistle (Centaurea calcitrapa) is widely distributed across south-eastern Australia with scattered occurrences in southern and south-western Australia (AVH 2021).

It is most common in New South Wales and the Australian Capital Territory, occurring from the coast to the tablelands and slopes as far west as Wanganella (Murray 2008).

In Queensland it occurs mainly sub-coastal areas of south-eastern Queensland (Western Australian Herbarium 1998; Parsons & Cuthbertson 2001; Navie 2004).

In South Australia it is common in the higher rainfall areas in and around Adelaide and in the  south-east of the State, and also extends to semi-arid areas,  to the Riverlands and north to Port Augusta, scattered on Eyre Peninsula and on roadsides across the Nullabor (eFlora 2021).

Although Star Thistle was known to occur in Tasmania in the late 19th century and mid 20th century in the Oatlands and Sheffield areas, and the only other report from Tasmania in recent times was from Launceston in 1981  (AVH 2021; TDPIW 2008). DPIPWE Tasmania (2019) state that Star thistle has not naturalised in Tasmania. 

In Victoria, it occurs in scattered localities throughout the state with the largest populations in the Mallee and Wimmera areas to the west (VRO 2007).

In Western Australia, it is recorded from around Perth and Merredin and on the southern coast (Western Australian Herbarium 1998; AVH 2021).

Where does it originate?

Star Thistle originated in southern and western Europe, north Africa, and western Asia (POWO 2019).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?


Where is it a declared weed?


Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?


Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?


Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Centaurea calcitrapa

Other scientific names (synonyms)?


Does it have other known common name(s)?

Purple Star Thistle, Purple Star-thistle, Purple Starthistle, Red Star Thistle, Red Star-thistle, Red Starthistle, Uncommon Star-thistle, Two-star Thistle, Augusta Thistle, Chinese Thistle, Purple Cockspur, Saucy Bob, Caltrop, Caltrops, Maize Thorn.

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