Slender Thistle

(Carduus pycnocephalus and Carduus tenuiflorus)
Slender thistle rosette
Image above: Slender thistle rosette.

What are slender thistles?

  • Slender thistle is a weed of pasture and crops.
  • Slender thistle is a declared weed under the Tasmanian Weed Management Act 1999. The importation, sale and distribution of slender thistle are prohibited in Tasmania.

How to identify slender thistle

  • Slender thistle (Carduus pycnocephalus) is a spiny, erect herb that grows to between 60 cm and 100 cm high. Another species of slender thistle, Carduus tenuiflorus (winged slender thistle), also occurs in Tasmania but is far less common. The two slender thistle species look very similar and are treated together in these information sheets.
  • Mature slender thistles are upright plants that may branch considerably or be un-branched. The stems have white cobwebby hairs while the flower heads form a cluster on the end of the stem.
  • Slender thistles form a rosette, (a whorl of leaves close to the ground), during winter, and only produce a flowering stem during late spring. Flowering and seed production occur in early summer and the life cycle is usually completed by the end of the year.
  • For help in identifying thistles in Tasmania, see Identifying Thistles in Tasmania and search the Dennis Morris Weeds and Endemic Flora database for slender thistle illustrations. If you are still in doubt about the weed you are dealing with, contact Biosecurity Tasmania on 03 6165 3777 for help.
Slender thistle flower infestation, photo: S. Leighton Slender thistle flower Winged Slender Thistle
Image above left: Slender thistle infestation in flower, © S. Leighton.
Image above middle: Slender thistle flower.
Image above right: Winged slender thistle flower.

Slender thistle in Tasmania

  • Slender thistles are widely distributed in Tasmania. They occur in most agricultural areas, where they invade pasture, crops and neglected areas with fertile soils and high rainfall. Slender thistles readily establish on disturbed sites.
  • Slender thistle seedlings compete with pasture and crops and reduce productivity. The traditional approach to pasture improvement of applying superphosphate tends to encourage slender thistles and worsens the infestation.
  • 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.

What is the legal status of slender thistle in your area?

  • The legal responsibilities of landholders and other stakeholders in dealing with slender thistle are laid out in their Statutory Weed Management Plans (SWMP). See Carduus pycnocephalus SWMP and Carduus tenuiflorus SWMP
  • Use Table 1 (Zone A municipalities) and Table 2 (Zone B municipalities) in the Statutory Weed Management Plans to find out whether your area falls in an eradication or containment zone.
Detailed management and control guidelines for slender thistle can be found in the Slender Thistle Control Guide. Refer also to Herbicides for Slender Thistle Control. For further information contact the Department's Weed Management Section.

See also:

Herbicides for Slender Thistle Control
Statutory Management Plans for Slender Thistle:
- Carduus pycnocephalus
- Carduus tenuiflorus (Winged Slender Thistle)
Weed Links and Resources

Other useful links:


Slender Thistle Control Guide


  • Plan your control program, this will save time and money in the long-run;
  • Maintain a healthy unbroken pasture - this is the best way to avoid slender thistle infestations;
  • Carefully time your use of herbicide for best results (see Herbicides for Slender Thistle Control for more information);
  • Regularly inspect treated areas in subsequent years to locate and remove any new thistles;
  • Get in early - if you have slender thistles, eradicate them before they set seed (in summer).


  • Don't use seed or hay contaminated with slender thistle;
  • Don't start your control program without first planning your approach;
  • Avoid bare patches of earth where thistles can take hold;
  • Don't allow thistles to flower and set seed before removal.

Spread of slender thistle

  • Slender thistles are spread by seed. The seeds are well equipped for wind dispersal with the seed acting as a small parachute. Slender thistle seeds can be carried more than 100 meters by wind.
  • Seed can also be dispersed by animals, especially on the wool of sheep. Several birds, notably goldfinches, eat slender thistle seeds and the seed may be deposited with the faeces and germinate.
  • Slender thistle seed can also be spread in contaminated pasture seed, hay, and in water.
  • See the Washdown Guidelines for Weed and Disease Control for detailed information on how to wash equipment and personnel to reduce the chance of spreading slender thistle.

Avoiding the introduction of slender thistle

  • Avoid introducing slender thistle seed into clean areas, or into areas from which the weed is being eradicated.
  • Machinery, equipment and vehicles which have been used on infested areas should be thoroughly cleaned on leaving.
  • Ensure that all machinery, equipment and vehicles coming onto your property are not contaminated.
  • All feed grains or hay should be free of spear thistle seed.
  • Any livestock suspected of carrying seed on their bodies or in their digestive system should be held in a suitable area for approximately two weeks before being put on clean paddocks.
  • Special care should be exercised when buying sheep from other properties as seed is readily carried in wool.

Physical removal

  • Slender thistles are not generally killed by cutting or slashing as cut plants are likely to regrow.
  • However, cutting or slashing when plants are in late bud or early flower (late spring to early summer) can reduce the amount of seed.
  • Cutting and slashing should not be left too late as seed can mature on cut plants lying on the ground. If the plant is in full flower, heads should be collected and burnt to destroy the seed.


  • The best way to control slender thistle is by encouraging competition from useful plants such as pasture. Vigorous pasture minimises slender thistle establishment and growth.
  • Patches of bare ground (for example, from rabbits, overgrazing, insect attack) can lead to an invasion by slender thistles. Patches of bare ground should be revegetated, preferably by insect resistant grasses such as phalaris or cocksfoot.
  • In areas which are heavily infested, growing a cereal crop for one or two seasons can reduce the thistle population, provided a suitable herbicide program is used (see Herbicides for Slender Thistle Control for more information).


  • Goats will graze slender thistles. Over a period of several seasons goats prevent seed production and can produce a reduction in thistle numbers.
  • Slender thistles are not normally grazed by cattle or sheep. However, where thistle germination occurs mainly in the autumn, deferred grazing can be an effective control measure in established perennial ryegrass pastures. Deferred grazing involves the closing off of an infested area to grazing in the autumn before the first thistle seedlings emerge. The grass is then allowed to grow to at least 15 cm. Competition with the grass during the early stages of thistle growth forces the thistles to elongate prematurely and produce soft and palatable plants instead of the usual hard prickly rosette. The area is then heavily grazed with sheep using at least twice the normal stocking rate.

Biological control

  • Biological control is the use of a living species, usually an insect, mite or disease, to control a weed.
  • Several strains of slender thistle rust fungus have been introduced to Tasmania to help control slender thistles. The rust works by infecting the leaves and flowering stems which weakens the plant, making the thistles more susceptible to competition from desirable pasture species.
  • Biological control will not eradicate slender thistles, but can be used in conjunction with other control methods as part of an integrated management program.
  • For more information on biological control programs in Tasmania contact the Tasmanian Institute of Agriculture.

Chemical control

  • A number of herbicides are registered for use on slender thistle in Tasmania (see Herbicides for Slender Thistle Control for more information).
  • Where slender thistles germinate in autumn before the temperature falls, autumn spraying is very effective. Spring treatment can be carried out in September or October when thistles begin to germinate.
  • An infestation may contain plants at various stages, including seedlings, small rosettes, large rosettes, flower-stem growth, and full flower. Be aware that the best type of herbicide to use will depend on the stages of growth in an infestation.

Herbicides for Slender Thistle Control

Herbicides for Slender Thistle Control

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