Onopordum species

What are Onopordum thistles

(Onopordum species)

Cotton thistle, whole plant with flowerGeneric Weed Distribution Map
  • Onopordum thistles are spiny herbs native to Europe, the Mediterranean and Asia. They are significant pasture and cropping weeds.
  • All Onopordum species are declared weeds under the Tasmanian Weed Management Act 1999. The importation, sale and distribution of onopordum thistles are prohibited in Tasmania.
  • There are two species of note for Tasmania:
    Cotton thistle Onopordum acanthium occurs in Tasmania and is discussed below. Cotton thistle is often referred to as Scotch thistle, particularly on herbicide labels.

    Stemless thistle Onopordum acaulon has been recorded as an isolated occurrence in the Southern Midlands and has been eradicated. No other plants have been found at this site.

How to identify Cotton Thistle

  • Cotton thistle is an annual herb growing to about 1.5 metres, and gets its name from the characteristic greyish-white colour of the foliage.
  • Germination occurs mainly during the autumn and winter. Cotton thistle seedlings initially grow into a rosette (a whorl of leaves close to the ground). The leaves are oval, shallowly lobed, and light green, with a light covering of hair giving them a greyish velvety appearance. As the plant matures, the leaf hairs become denser and the foliage becomes greyer in colour (and more 'cotton' like).
  • The rosettes produce a branched flowering stem in early October. The stems bear prominent wings throughout their entire length. Leaves are alternate and spiny. Flowering usually starts in December and continues through to autumn. Mature flower heads are large (30 to 40 mm in diameter) with purple florets.
  • For help in identifying thistles in Tasmania, see Identifying Thistles in Tasmania and search the Dennis Morris Weeds and Endemic Flora database for cotton thistle illustrations. If you are still in doubt about the weed you are dealing with, contact your Regional Weed Management Officer on 1300 368 550 for help.

Cotton thistle in flower
Image top right: Stemless thistle, photo: Andrew Crane, DPIPWE.
Image above: Cotton thistle - flowering plant.


Cotton thistle in Tasmania

  • The main infestations of cotton thistle in Tasmania occur on improved pastures in the lower rainfall areas of the Midlands (see map).
  • Cotton thistle, as with other species of Onopordum, can be very competitive in pasture situations. Large rosettes can prevent the growth of desirable species. Their prickly nature means livestock will not eat them.
  • Cotton thistle often establishes after pasture improvement and in cropping paddocks that have been under improved pasture.
  • Cotton thistle is not generally grazed by stock. But when eaten because better fodder is unavailable it can cause digestive problems and liver damage to stock. The spines can cause injury to the mouths and eyes of animals. Cotton thistle also contributes to vegetable fault in wool.
  • If you locate a cotton thistle, or if you find a plant that you think could be an onopordum thistle different to the cotton thistle described above, immediately contact your Regional Weed Management Officer on 1300 368 550 to report this weed.

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

The legal responsibilities of landholders and other stakeholders in dealing with cotton thistle are laid out in the Statutory Management Plan for Onopordum Thistles (Cotton Thistle and Stemless Thistle).

Use Table 1 (Zone A municipalities) in the Statutory Management Plan for Onopordum Thistles (Cotton Thistle and Stemless Thistle) to find out whether this weed occurs in your municipality.

See also
Cotton Thistle Control Guide
Herbicides for Cotton Thistle Control
Statutory Management Plan for Onopordum Thistles (Cotton Thistle and Stemless Thistle)
Weed Links and Resources

Other useful links
Pest Genie
APVMA


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