Spear Thistle Control Guide

Do's and Don'ts of spear thistle control

Spear thistle flower, photo: K. Fenner

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 spear thistle infestations;
  • Carefully time your use of herbicide for best results (see Herbicides for Spear 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 spear thistles, eradicate them before they set seed (in summer).

Don'ts

  • Don't bring in spear thistle seed in contaminated seed or hay;
  • 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 spear thistle

  • Spear thistle reproduces by seed. The seed has a pappus or parachute of long hairs (the 'thistle down'), and can be spread long distances by wind. However, the pappus readily separates from the seed, and most seed falls within a few feet of the parent plant. Up to 200 flower heads and 8000 seeds can occur on an individual plant.
  • Most seed is distributed by farm machinery and vehicles, on the fleeces and coats of livestock, by run-off after heavy rains, and by feeding out contaminated hay.
  • Seeds germinate mainly after the autumn rains and during winter an extensive root system develops. Rosettes grow rapidly through spring. In a dry summer the rosette dies off and a new rosette forms at the crown in the next autumn and winter. Normally, rosettes persist through summer and continue growth into the second autumn and winter. In both cases, a flowering stem is produced in late spring of the second year and plants flower during summer and late autumn before dying.
  • Dead plants often remain standing for one or two years. Seed germination in early autumn produce plants which over winter as rosettes before making rapid growth in early spring. They may flower and die in summer and autumn, thus behaving as annuals.
  • 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 spear thistle.


Avoiding the introduction of spear thistle

  • Avoid introducing spear 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

  • Hand hoeing is effective for individual plants and small patches provided the growing point and the top 20 to 40 mm of the tap root are removed.
  • Cutting or slashing spear thistle in late bud or early flower stage can reduce seed production.
  • However, because spear thistles mature over an extended period, slashing is unlikely to stop all seed production. In years when there is adequate soil moisture, thistles are also likely to regrow after slashing.

Cultivation

  • The best method for control of spear thistle is to encourage competition from useful plants.
  • Vigorous pasture restricts seedling establishment and growth, and management practices should aim to maintain a vigorous and unbroken sward during the period when thistles are germinating.
  • In general, cell or rotational grazing should be used in preference to set stocking.
  • Areas of bare ground (for example after insect attack) can result in an invasion of spear thistles. Areas left bare by insect attacks should be re-sown with insect resistant pasture grasses such as phalaris or cocksfoot.
  • In heavily infested arable areas, growing a cereal crop for one or two seasons can reduce spear thistle infestations, provided a suitable herbicide spray program is used (see Herbicides for Spear Thistle Control for more information). Any thistles not killed by the herbicide will be suppressed by a vigorous crop.

Grazing

  • Spear thistles are not normally grazed by cattle or sheep. However, cell or rotational grazing can give some control of seedlings.
  • Goats will graze spear thistles at the flowering stage, eating flowers, seed heads and stems, and several seasons of goat grazing can significantly reduce thistle numbers.

Chemical control

  • A number of herbicides are registered for use on spear thistle in Tasmania. See Herbicides for Spear Thistle Control for more information)
  • Spear thistles at the seedling stage are susceptible to growth regulator type herbicides applied either as spot or boom sprays.
  • Thistles must be growing actively for herbicides to be effective. Plants subject to stress due to waterlogging, drought or low temperatures are less susceptible to herbicides.
  • In years when there is an early autumn break and large numbers of thistles germinate before the temperature falls, autumn spraying can be effective. Spraying at this time also requires lower herbicide rates, is less damaging to clover, and allows pasture to grow with reduced competition from the thistles.
  • In winter, spear thistles become dormant and are less susceptible to herbicides. Susceptibility increases again with spring growth and spraying can usually be resumed effectively in September or October.
  • Spraying should be completed before the flowering stem develops. Although spear thistles remain susceptible up to flowering, treatment at this late stage requires more active herbicides and higher application rates, with greater risk of pasture legume damage.


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