Slender Thistle

Carduus pycnocephalus L.

Family: - Asteraceae.


Carduus is derived from the Greek kandos meaning thistle.

Pycnocephalus is from the Greek words for thick or dense, pyknos and cephale meaning head.

Slender thistle

Other Names:

Corsican Thistle

Italian Thistle

Plymouth Thistle

Rabbit Thistle

Sheep Thistle

Shore Thistle

Slender-flowered Thistle

Slender Winged Thistle


Slender Thistle is a spiny, erect, purple flowered annual to biennial thistle, usually about 1 m tall, with clusters 1-3(4) flower heads that are 10-15 mm wide. The stems have discontinuous wings that are spiny and no wings on the 100 mm of stem below the flowers. The seeds have about 20 ribs and the bristles topping the fruits are only very minutely barbed. The leaves have spiny lobes a cobweb-like hairs.

It originates from Europe and is a common weeds of pasture, roadsides and disturbed bushland, flowering in spring and early summer.

Overall it has a finer habit than Winged Slender Thistle.



Two. Oval. 15-25 mm long with a short, broad merging petiole. Tip round. Base tapered. May be pale central streak. Hairless. The seedling has a very short hypocotyl and no epicotyl. Larger than Winged Slender Thistle.

First leaves:

Oval. About 45 mm with a broad merging petiole that tends to elongate as the leaf ages. Hairy. Tip round. Edges spiny. Distinctive pale veins. The leaves grow singly at first and carry a few, long, multicellular hairs on the upper and lower surfaces.

Leaf is longer and broader and the tip not as pointed as Winged Slender Thistle at the same age.


Stipules -

Petiole - Green.

Blade - Elliptical to egg shaped. 100-250 mm long. 2-5 pairs of many spined, deep lobes that usually cut more than half way to the midrib. Edges spiny and wavy. Upper surface shiny and often mottled lighter along the veins especially at the base of larger spines on the lobes. Few hairs on upper surface, woolly or cobweb hairs underneath. Tip sharply spined.

Leaves lighter green and shinier than Winged Slender Thistle.

Stem leaves - Similar to rosette leaves but smaller to 130 mm long. Spiny edges extend down the stem. Woolly hairs on the upper and lower surfaces.

Stem leaves more deeply lobed and with narrower wings than Winged Slender Thistle.


Erect, green. Elongates in spring to 600-2000 mm tall and is variable in diameter. Unbranched or branching near the top and sometimes from the base. Solid, pithy and five sided and striped or ribbed with 3 rows of spiny wings (to 10 mm high) along most of its length but not right up to the flower head. Woolly or cobweb hairs and may become relatively hairless with age. Tend to be grey near the top.

Stems webbier than Winged Slender Thistle.

Flower head:

Purple or pink, slender, cylindrical, up to 15-25 mm long by 7-20 mm diameter. In clusters of 1-4 flowers on short stalks on the ends of the stems or in leaf axils. Surrounded by several rows of spined, bracts, with pale margins or sometimes coloured with yellow, tiny hairs, rough to touch on midrib and upper edges. Bracts are faintly 3 nerved near their base, green and papery. Inner bracts shorter than adjacent florets. No wing like bracts on the 100 mm of stem just below the flower head.


Florets - Ligulate, pink or purple, 10-14 mm long, all tubular and bisexual.

Ovary -

Receptacle - Hairy.

Stamens -

Anthers - Arrow shaped.



Inner and outer types of seed depending on placement in the head.

Inner seed is about 85% of seed produced and has about 20 longitudinal ribs and is yellow fawn. Outer seeds are grey.

Both are 4.5-5 mm long, flattened, and may have minute transverse wrinkling, prominent small tubercle(wart) on top and rough, barbed pappus bristles (15-20 mm long) joined in a short ring at their bases. Pappus bristles fall off in a single piece.


Branched, slender taproot.

Key Characters:


Life cycle:

Annual or biennial. Germinates mainly in autumn with some germinations occurring in winter and spring. They form a large rosette of 300-400 mm diameter over winter then stems emerge in spring to bear flowers from September until summer drought kills the plant. Flowering may continue up to April.


They are more competitive than clover and ryegrass over a wide range of nutrient conditions. Applying phosphate fertilisers usually encourages thistle infestations.


By seed.

Flowering times:

October to December in WA.

Seed Biology and Germination:

The germination pattern is strongly influenced by the autumn rains and the density of infestations may vary considerably between years.

Some viable seed passes through the gut of birds but not sheep.

Seedlings establish most readily on bare or disturbed sites.

Some seed may remain dormant in the soil for more than 10 years. In Victoria seed only germinates within 6 weeks after the first autumn rains. In other areas, winter and spring germinations may occur.

Vegetative Propagules:

May re shoot from the crown.



Population Dynamics and Dispersal:

Seed is carried by sheep in their wool. Wind occasionally causes long distance dispersal under cyclonic conditions. In these situations seed appears to be spread many kilometres and new infestations appear on the leeward side of hills and timber belts. In most years it is only responsible for localised spread as the pappus separates from the seed relatively easily. At low humidity and a wind speed of 19 kph, seed are blown on average 23 m with some travelling over 100 m. Most seed falls within a few metres of the parent plant normally.

Seed is also spread in contaminated produce such as hay and pasture seed. Dispersal by water and birds can also be important.

There is a large variation in the density of infestations from year to year, which is poorly understood.

It rarely invades strong perennial pastures.

Infestations tend to be worse where sheep compared to cattle graze and this leads to one of its common names being Sheep Thistle. It reaches maximum density at moderate to heavy grazing pressures.

Origin and History:

Europe and North Africa.

First recorded in Victoria in the 1880's





Sub humid warm temperate regions with a winter dominant rainfall of more than 500 mm.


Prefers loams, gravels and sandy soils. Occurs on a wide range of soil types.

Plant Associations:



Produces pollen for bees in late spring.


Weed of pasture, crops, roadsides and disturbed areas.

Strongly competitive in pastures.

Spiny nature of dense stands restricts access to pasture

Contaminates wool.


May contain toxic levels on nitrate, but generally is not a problem.


Nitrate toxicity.


Remove animals from densely infested areas.


Noxious weed of VIC and TAS

Management and Control:

Establish perennial pastures. Phalaris and cocksfoot have given good suppression of thistles. Mowing or slashing can reduce seed set, but the plants often re grow and set more seed, especially if there are late rains. Goats can effectively control infestations over a few years but often require a combination with sheep to reduce pasture levels before introducing the goats. Deferred grazing is promoted in Tasmania as an economical control technique.

Diquat or MCPA applied early in the season generally give good control. 2,4-D amine followed by heavy grazing in winter also provides high levels of control. Clopyralid or wick applied glyphosate are useful for late season control.


Eradication strategies:

Prevent seed set for several years. Mowing or preferably slashing before the bud stage is effective if it is repeated to control regrowth. Seeds can develop from nutrient reserves in the stem if substantial amounts of stem remain attached to the buds. Manual removal is also effective but often unpleasant due to the spiny nature of the plant.

Blanket wipers or wick applicators using 1 part glyphosate(450g/L) to 2 parts water or spot spraying with 50 mL glyphosate(450g/L) in 10 L water can provide partially selective control. Overall spraying with 200 g/ha Lontrel®750 (or 4 g in 10 L for spot sprays) provides reasonably selective control in bushland situations. Spray grazing when young with 500 mL/ha 2,4-D amine(500g/L) provides cheap control in pasture and partial control in bushland.

As these thistles are annuals, control of the seed bank is the key to success. Control neighbouring infestations to reduce spread by birds. Wind rarely takes seed more than 100 metres from the parent plant.

Replant to ground covering species and avoid disturbance to reduce the bare areas present at the break of the season.

Cultivation is effective.

Grazing with sheep to reduce pasture then grazing with goats at flowering provides good control in 3 years.

Herbicide resistance:

Biological Control:

Bio control has had varying success overseas and research is continuing in Australia.

Related plants:

Nodding Thistle (C. nutans, C. thoermeri)

Plumeless Thistle (C. acanthoides)

Winged Slender Thistle (C. tenuifolius) has prickly wing like bracts on the stem that go all the way up to the flower head.

Plants of similar appearance:

Winged Slender Thistle is very similar to Slender Thistle. The following helps with field identification.

Slender Thistle often has white patches on the leaves, Winged Slender Thistle usually doesn't or they are inconspicuous.

Slender Thistle usually has 2-5 pairs of leaf lobes, Winged Slender Thistle has 6-10.

Slender Thistle has green stems and petioles, Winged Slender Thistle often has red or purple colouration or flecks.

Slender Thistle has flower heads in groups of 2-4 on short stalks, Winged Slender Thistle has sessile heads in groups of 3-10.

Slender Thistle has inner flower head bracts that are shorter than the adjacent florets whereas Winged Slender Thistle has bracts that are longer than the adjacent florets.

Slender Thistle has wing like bracts on the stem but these disappear below the flower head whereas Winged Slender Thistle has wing like bracts on the stem that go all the way up to the flower head.

Californian Thistle has a branched rooting system and different flowers.

Nodding Thistle has a leaf that is much longer in proportion to its width, has more lobes and is much more deeply divided, often right down to the midrib and different flowers.

Perennial Thistle has hairs but no spines on its woody stems.

Spear thistle has 'warts' on the leaves, no pale blotches on the leaves, spines that extend downwards from the leaves and different flowers and a pappus with feathery hairs.

Variegated Thistle has white blotches that don't follow the veins and different flowers.


Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P88. Photo.

Burbidge, N.T. and Gray, M. (1970). Flora of the Australian Capital Territory. (Australian National University Press, Canberra). P392. Diagram.

Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P724.

Everist, S.L. (1974). Poisonous Plants of Australia. (Angus and Robertson, Sydney). P161.

Gilbey, D. (1989). Identification of weeds in cereal and legume crops. Bulletin 4107. (Western Australian Department of Agriculture , Perth). P48. Photos.

Hyde-Wyatt, B.H. and Morris, D.I. (1975). Tasmanian weed handbook. (Tasmanian Department of Agriculture, Hobart, Tasmania). P26-27. Diagrams.

Hyde-Wyatt, B.H. and Morris, D.I. (1980) The Noxious and Secondary Weeds of Tasmania. (Tasmanian Department of Agriculture, Hobart, Tasmania). P42-43. Diagrams.

Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).

Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #252.2.

Marchant, N.G., Wheeler, J.R., Rye, B.L., Bennett, E.M., Lander, N.S. and Macfarlane, T.D. (1987). Flora of the Perth Region. (Western Australian Herbarium, Department of Agriculture, Western Australia). P665.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P189-193. Photos.

Wilding, J.L. et al. (1987). Crop weeds. (Inkata Press, Melbourne). P44-45. Diagrams. Photos.


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