Cotton Thistle Control Guide

Do's and Don'ts of cotton thistle control

Cotton thistle, whole plant with flower


Do's

  • 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'ts

  • 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.

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    To the extent permitted by law, the Tasmanian Department of Primary Industries, Parks, Water and Environment (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using information or material (in part or in whole) contained on this website.

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