Onopordum thistles (Cotton and Stemless)

(Onopordum species)

What are Onopordum thistles?

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 Biosecurity Act 2019 and associated Regulations. 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 Biosecurity Tasmania on 03 6165 3777 for help.

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.
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 Biosecurity Tasmania on 03 6165 3777 to report this weed.

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

Use grubbing with a hoe or mattock to remove single thistles or small patches;
If thistles are flowering, burn all grubbed material to ensure the severed flower heads do not mature and produce seed;
Aim for zero flowering of cotton thistle every year;
Check all areas which have been treated and grub or spot spray any survivors - Cotton thistle is a tall conspicuous plant and any surviving plants are easily seen.

Don't

Don't start your control program without first planning your approach;
Don't rely on grazing: cotton thistle is avoided by stock because of its dense spines and heavy grazing will aid its survival.

Spread of cotton thistle

Cotton thistle is spread by seed. Seed spread is mainly by livestock, particularly sheep, either entangled in the wool or by passage through the digestive system. Seed may be carried by water, wind and may also be spread by birds.
Spread of seed may also occur via contaminated agricultural produce.
Root fragments of certain Onopordum species are able to develop into new plants so spread by cultivation and earthmoving equipment can also be important.
Seeds can remain dormant in the ground for a long period. Seed buried below a depth of 75 to 100 mm does not germinate until brought nearer the surface.

Avoid the introduction of cotton thistle

Avoid introducing seed into clean areas, or into areas from which the weed is being eradicated.
Implements which have been used on infested areas should be thoroughly cleaned on leaving.
All feed grains or hay should be free of cotton 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 infested properties as seed is readily carried in wool.
See the Washdown Guidelines for Weed and Disease Control for detailed information on how to wash-down equipment and personnel to reduce the chance of spreading cotton thistle.

Physical removal

Use grubbing with a hoe or mattock to remove single thistles or small patches.
Take care to sever the root below ground level to ensure that all growing points are removed.
If flowering, all grubbed material should be collected and burnt to ensure the severed flower heads do not mature and produce seed.
Slashing and mowing of cotton thistle are unreliable methods of control. If the soil is moist or it rains following slashing, many thistles will recover and produce seed.
Slashing or mowing can be used to reduce an infestation which has passed the flower bud stage without being treated. Slashing should not be delayed for more than a few days after the first flowers open or the several heads will continue to mature and produce seed.
Treat any regrowth with herbicide (see Herbicides for Cotton Thistle Control for detailed information).

Cultivation

Cultivation such as ploughing can be used to control cotton thistle on arable land. Because of the expense involved, cultivation should be used only in an area intended for cropping, or where pasture needs to be re-sown.
After ploughing of an infested area, there is likely to be a mass germination of new seedlings: these can be controlled by herbicide application to the crop or pasture.

Grazing

Although most stock avoid cotton thistle, goats will graze flowering plants in summer and autumn which reduces seed production.

Biological control

Biological control is the use of a living species, usually an insect, mite or disease, to control a weed. It will not eradicate cotton thistle, but can be used in conjunction with other control methods;

Biological control agents for cotton thistle that have been released on the mainland include the seed weevil and the stem-boring weevil. At present there are no release sites in Tasmania.
For more information on biological control programs in Tasmania contact the Tasmanian Institute of Agricultural Research.

Chemical control

A number of herbicides are registered for use on cotton thistle in Tasmania. See Herbicides for Cotton Thistle Control for more information.
Both boom and spot spraying can be used to control cotton thistle, the choice depending on the crop, the age and composition of the pasture, the terrain, and the extent and density of the infestation.
Boom spraying is usually the cheapest method for dense or extensive infestations on land where boom spraying is practicable. Depending on seedling germination, treatment can be carried out in autumn or spring.
Spot spraying is recommended for infestations of scattered plants, for small patches, and for any follow-up work after boom spraying where individual plants have survived or been missed.

Herbicides for Cotton Thistle Control

Important Disclaimer

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