Quick Facts

Quick facts

• Originally from western Europe and northern Africa Slender Thistle (Carduus tenuiflorus) is a cool temperate annual thistle with a rosette of leaves before producing an erect stem 1-2 metres tall with spiny leaves and groups of 3-10 purple flowers.
• Spread by seed, with limited dispersal by wind, most long distance dispersal is via contaminated soil or produce, or on machinery and vehicles.
• Germination and rosette formation predominantly in autumn, but also capable of germination in spring also with later maturation in late summer.
• It is mostly a weed of pastures but can also compete with annual crops, especially weedy in Tasmania.
• Management to provide a more competitive summer-autumn pasture to reduce germination of seed and establishment of rosettes of the weed is desirable for effective control.

What Does It Look Like?

What is it?

Slender Thistle (Carduus tenuiflorus) 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 2 m tall. The oblanceolate (Lance=shapped and widest and rounded at the tip (apex) and tapering towards the base) rosette leaves are 150 to 250 mm 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 to 10 mm wide along the stem from the base of the sessile (stalkless) stem leaves, continuing up the plant to beneath the flower-heads (Parsons & Cuthbertson 1992; DPIW 2002).

The flower-heads form on the ends of the main stems and side branches in clusters of 3 to 10. The largest heads are up to 10 mm in diameter and about 20 mm 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 hairless or only sparsely cobwebby. The phyllaries of the inner row are longer than the pink-purple narrowly tubular flowers. Each flower is 10-17 mm and develops a solitary seed with many flowers per flower-head many seeds are produced per flower-head.

The fruits or 'seeds' (cypselas) are of two sorts, the outer being grey and the inner (the majority) being grey-brown and finely striate (longitudinally grooved). The seeds are 3-4 mm long, egg-shaped and somewhat flattened, with a terminal but easily detached tuft of unbranched white hairs (pappus) to about 15 mm long (Parsons & Cuthbertson 1992; DPIW 2002; Navie 2004).

Recognition: This species can normally be recognised by the combination of the following characters; easliy confused with other thistles; erect annual herb 1-2 metres tall; cowwebby stems and underside of leaves; stem wings to 10 mm wide; purples flowers in clusters of 3 to 10, largest flower heads to 10 mm in diameter and about 20 mm long; The seeds are 3-4 mm long, egg-shaped with a terminal but easily detached tuft of unbranched white hairs (pappus) to about 15 mm long.

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

Flower colour

Purple, Pink

Growth form (weed type/habit)

Herb

Where it currently grows? Preferred habitat

Slender Thistle (Carduus tenuiflorus) 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. Both the Slender Thistles (C. pycnocephalus and C. tenuiflorus) are often found growing together (Parsons & Cuthbertson 1992).

Are there similar species?

Carduus tenuiflorus is difficult to distinguish from its sister species C. pycnocephalus. 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 while C. pycnocephalus has only 1-4 per cluster. The phyllaries (floral bracts) of each head are scarcely cobwebby or lacking hairs altogether compared with the densely cobwebby phyllaries of C. pycnocephalus 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  also occur in Australia as weeds. 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 tenuiflorus) 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.

How does it spread?

Slender Thistle (Carduus tenuiflorus) is dispersed by seed. The wind does spread seed to some extent but how important this is, is debated. Dispersal by wind may reach distances of up to 100m because of the pappus (a tuft of silky hairs). However, the pappus on top of the seed is very easily detached and seed may mostly fall near the parent plant. Distribution patterns of individual plants tend to be patchy and clumped reinforcing this idea of localised 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; Slee 2007, pers. comm.).

What is its history in Australia?

Details of the introduction of Slender Thistle (Carduus tenuiflorus) are unknown. It was first recorded in South Australia in 1881, and present in Victoria during the 1880s. It 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. The best control of Slender Thistle (Carduus tenuiflorus) is obtained by integrated management

Does it have a biological control agent?

YES

When does it grow? (lifecycle/growth calendar)

Slender Thistle (Carduus tenuiflorus) seeds germinate in autumn if moisture is available and can continue to do so until winter. Plants form rosettes which cease growth in the 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. Elongation of the stem does not seem to depend on vernalization (exposure to cold temperatures) (Groves & Kay 1989; DPIW 2002).

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 and 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 plant continues through into summer and may even survive into autumn before seed shed and death of the plant (Groves & Kay 1989; DPIW 2002).

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 tenuiflorus) 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), the Australian Capital Territory, Victoria (all regions except Mallee, Wimmera and North-central), Tasmania (especially the Midlands) and South Australia (it is common in all the higher rainfall grazing areas, including the Lower South East, Adelaide Hills and Lower Eyre Peninsula, but more widely scattered in the Mid North and on Kangaroo Island ). In Queensland it is sporadic in the south-east. In Western Australia occurs in the southern agricultural districts especially closer to the coast (Parsons & Cuthbertson 1992; AVH 2020).

Where does it originate?

Slender Thistle (Carduus tenuiflorus) is native to western Europe and northern Africa (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 tenuiflorus

Other scientific names (synonyms)?

Does it have other known common name(s)?

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