Quick Facts

Quick facts

• Originally from Europe, Asia and Africa, Slender Thistle (Carduus pycnocephalus) is a cool temperate annual erect spiny thistle with purple or pink flowers.
• Slender Thistle germinates and forms a rosette predominantly in autumn, but is also capable of germination in spring with later maturation in late summer.
• It is mostly a weed of pastures but can also compete with annual crops.
• It can be controlled with herbicides, using mechanical means, as well as pasture crop and animal management
• Management to provide a more competitive summer-autumn pasture to reduce germination of seed and establishment of rosettes of the weed is desirable.

What Does It Look Like?

What is it?

Slender Thistle (Carduus pycnocephalus) is an erect annual herb, initially forming a sparse prostrate (growing flat on the ground) rosette but ultimately growing into an erect plant often up to 1 m tall, rarely to 2 m tall. The oblanceolate (lance-shaped with the widest point near the rounded tip and tapering towards the base) rosette leaves are 15 to 25 cm long, deeply lobed and with prominent stiff spines on the margins. They are moderately cobweb-hairy on the top and dense with cobwebby hairs underneath. Erect stems develop from the rosette, usually with long slender erect side-branches. The stems also appear cobwebby and have spiny wings along the stem from the base of the sessile (stalkless) stem leaves. These are usually absent from the stems immediately under the flowerheads (Parsons & Cuthbertson 1992; TPIPWE 2019).

The flower-heads form on the ends of the main stems and side branches in clusters of 1 to 4. The largest heads are up to 1 cm in diameter and about 2 cm long. They are sessile or shortly stalked and have numerous flowers. Each flower-head consists of several rows of spine tipped phyllaries (floral bracts) that are densely cobwebby. Those of the inner row are shorter than the pink-purple narrowly tubular flowers. Each flower is 10-17 mm long and develops a solitary seed with many flowers per flower-head many seeds are produced.

The fruits or 'seeds' (cypselas) are of two sorts, the outer being grey and the inner (the majority) being fawn and finely striate (longitudinally grooved). The seeds are 4-6 mm long, egg-shaped and somewhat flattened, with a terminal but easily detached pappus of unbranched, barbed white hairs to about 15 mm long (Parsons & Cuthbertson 1992; TPIPWE 2019; Navie 2004).

For further information and assistance with identification of Slender Thistle (Carduus pycnocephalus) contact the herbarium in your state or territory.

Flower colour

Purple or pink

Growth form (weed type/habit)

Herb

Where it currently grows? Preferred habitat

Slender Thistle (Carduus pycnocephalus) is a cool temperate annual species. It is found on moderately to highly fertile soils in areas with predominantly winter rainfall, with annual rainfall in excess of 500 mm. It prefers improved pasture, neglected areas and areas of animal or mechanical disturbance within this climatic zone. It also invades dry coastal vegetation, mallee shrubland, lowland grassland and grassy woodland, dry sclerophyll forest and woodland and rock outcrop vegetation following disturbance (Parsons & Cuthbertson 1992).

Are there similar species?

Carduus pycnocephalus is difficult to distinguish from its sister species Carduus tenuiflorus. C. tenuiflorus has broad stem-wings extending up to the base of the clusters of flower-heads, absent from C. pycnocephalus. The flowerheads of C. tenuiflorus occur in clusters of 3-10 (1-4 in C. pycnocephalus), the phyllaries (floral bracts) of each head are scarcely cobwebby or lacking hairs altogether and the flowers are mostly shorter than the phyllaries (longer in C. pycnocephalus). C. tenuiflorus also has shorter seeds with less longitudinal grooves (Navie 2004; Slee 2007, pers. comm.).

Two other species of Carduus, C. nutans and C. thoermeri  occur in Australia as weeds also. Both are more robust plants than C. pycnocephalus and C. tenuiflorus with larger diameter rosettes, coarser stems and leaves with larger flowerheads that frequently nod at a right angle to the stem (Navie 2004; Slee 2007, pers. comm.).

Why Is It A Weed?

What are its impacts?

Slender Thistle (Carduus pycnocephalus) is a weed of disturbed sites, and mainly occur in agricultural areas, where they invade pasture, crops and neglected areas with fertile soils and high rainfall.

Agriculture: In pastoral areas Slender Thistle (Carduus pycnocephalus) forms dense patches of spiny plants and may occupy considerable areas of pasture out-competing desirable pasture species and reducing carrying capacity. The spines can also deter stock. There is some suspicion that that the leaves may develop compounds harmful to livestock in some circumstances (Parsons & Cuthbertson 1992; TPIPWE 2019). Slender thistle seedlings compete with pasture and crops and reduce productivity. Slender thistles are not readily grazed by stock because of their spines, and infestations of slender thistle will discourage animals from grazing pasture grass adjacent to infestations. Slender Thistle impacts on the trade of agricultural products between states. If products (e.g. fodder, wool or pasture and crop seeds) are contaminated with seed of this weed, the product is devalued and in some cases prohibited from sale (Parsons & Cuthbertson 1992; TPIPWE 2019).

Native ecosystems: It is invasive in disturbed areas of grassland, shrubland and woodland along tracks and areas of animal disturbance (Parsons & Cuthbertson 1992; TPIPWE 2019). It also invades dry coastal vegetation, mallee shrubland, lowland grassland and grassy woodland, dry sclerophyll forest and woodland and rock outcrop vegetation following disturbance (Parsons & Cuthbertson 1992).

Urban areas: May invade roadsides and grassy areas. 

How does it spread?

Slender Thistle (Carduus pycnocephalus) is spread only be seed. The seed is dispersed by wind to some extent but how important this is, is debated. Dispersal by wind may reach distances of up to 100 m because of the pappus (a tuft of barbed hairs). However, the pappus on top of the seed is very easily detached and seed mostly fall near the parent plant. Distribution patterns of individual plants tend to be patchy and clumped, reinforcing this idea of localized dispersal. Seed is known to be dispersed over longer distances by contaminated fodder, contaminated pasture and crop seed, agricultural machinery and vehicles (Parsons & Cuthbertson 1992).

What is its history in Australia?

Details of the introduction of Slender Thistle (Carduus pycnocephalus) into Australia are unknown. It was present in Victoria during the 1880s and now occurs throughout much of eastern and southern Australia (Parsons & Cuthbertson 1992).

How To Manage It?

Best practice management

There is considerable information available on integrated control of the Slender Thistles (Carduus pycnocephalus and C. tenuiflorus). See particularly Faithfull (1998) and DPIPWE (2019). If you have slender thistles, eradicate them before they set seed, as with all annual plants reproducing via seed.

Chemical control: Control via herbsides are used either via hand held applications or via boom spraying. Please see DPIPWE (2019b) for Slender Thistles (Carduus pycnocephalus or  the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au for control of thistles.

Non-chemical control: Physical control: Hand remove isolated or small populations plants through spring and early summer.

Cultivation of the soil will encourage germination that can be followed up with herbicide application.

Pasture management: The best control is obtained by integrated pasture management to produce a vigorous and dense sward of pasture grasses during the main germination period in autumn to prevent successful rosette establishment. The traditional approach to pasture improvement of applying superphosphate tends to encourage slender thistles and worsens the infestation. In heavily infested arable areas growing a cereal crop for one or two consecutive seasons can reduce the thistle population provided a suitable spray program is used (TPIPWE 2019).

Grubbing: where the number of plants involved is small, removal by grubbing is feasible. The thistle should be grubbed out (This method requires weeds to be dug out using a mattock or chip hoe) to a depth of at least 5 cm. 

Slashing: The Slashing or cutting will not generally kill Slender Thistles. However, cutting or slashing when the plants are in the late bud or early flower stage, in late spring to early summer, can prevent or reduce the production or spread of seed (TPIPWE 2019).

Grazing management, Goats will graze Slender Thistles at the flowering stage, eating flowers, seed heads and stems. Over a period of several seasons this prevention of seed production can produce a significant reduction in thistle numbers (TDPI PWE 2019).

Biological control: Slender Thistle has been recognised as a target for biological control through a cross-jurisdictional government process. This allows activities to be undertaken to develop effective biological controls. Use of rust fungi for biological control is currently being evaluated with the release of new rust strains to replace the ineffective strains that have been present in the country since the early twentieth century (TPIPWE 2019).

Does it have a biological control agent?

YES

When does it grow? (lifecycle/growth calendar)

Seeds of Slender Thistle (Carduus pycnocephalus) germinate in autumn if moisture is available and can continue to do so until winter. Plants form rosettes which cease growth in winter but re-commence growth as the weather warms in spring with stems elongating and then flowering before the end of spring. Seed is shed from late spring to mid-summer. Plants die after seedshed. Note that elongation does not seem to depend on vernalization (exposure to cold temperatures) (Groves & Kay 1989; DPIPWE 2020).

In Tasmania further germination may also occur during spring, this being more common in the northern and north-western parts of the state. The autumn germinating plants form an over-wintering rosette and produce a flowering stem during late spring resulting in flowering and seed production in early summer with the life cycle usually completed in the midlands and southern part of the State before the end of the year. In the north-west the spring germinated plants continue through into summer and may even survive into autumn before seed shed and death of the plant (Groves & Kay 1989; DPIPWE 2020).

Where Is It Found?

Which states and territories is it found?

ACT, NSW, QLD, SA, TAS, VIC, WA.

What areas within states and territories is it found?

Slender Thistle (Carduus pycnocephalus) is found in all Australian states and territories except the Northern Territory.

It is considered an important weed in New South Wales (coast, tableland and slopes subdivisions particularly), Victoria (all regions except Mallee, Wimmera and North-central) and Tasmania (especially the Midlands). In Queensland it is confined to the south-east, and in Western Australia to the southern agricultural districts especially closer to the coast (Parsons & Cuthbertson 1992; AVH 2007). In South Australia it occurs in the higher rainfall areas around Adelaide.

Where does it originate?

Slender Thistle (Carduus pycnocephalus) is native to southern Europe, northern Africa, Asia Minor and south-western Asia (Navie 2004; GRIN 2007).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

TAS, VIC.

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Carduus pycnocephalus

Other scientific names (synonyms)?

Does it have other known common name(s)?

Italian Thistle, Slender Winged Thistle, Shore Thistle, Winged Thistle. Note: Carduus pycnocephalus and C. tenuiflorus often are referred to under the same common name Slender Thistle.