Slender Thistle

Slender thistle rosetteGeneric Weed Distribution Map
  • 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, C. 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 thistle you are dealing with, contact your Regional Weed Management Officer on 1300 368 550 for help.
Slender thistle flower infestation, photo: S. LeightonSlender thistle flowerWinged Slender Thistle
Image top right: Slender thistle rosette.
Images above, left and middle: Slender thistle flower infestation, photo: S. Leighton; Slender thistle flower.
Image above right: Winged slender thistle

Slender thistle in Tasmania

  • Slender thistles are widely distributed in Tasmania (see map). 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:

Slender Thistle Control Guide
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:

Pest Genie

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