Slender Thistle Control Guide

Do's and don'ts of slender thistle control

Slender thistle rosette

Do's

  • 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'ts

  • 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.


Cultivation

  • 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).


Grazing

  • 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.


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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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