Spear Thistle

(Cirsium vulgare)
Spear thistle flower, photo: K. Fenner

What is spear thistle?

  • Spear thistle is a pasture weed.

How to identify spear thistle

  • Spear thistle, (sometimes referred to as 'scotch thistle'), is an annual (living for one year) or biennial (living two years) herb, growing to 60 to 120 cm, and occasionally to 1.5 metres.
  • The leaves are dark green, rough and hairy on the upper surface, and lighter beneath due to a dense covering of whitish hairs. Rosette leaves have spines on the margins and wart-like protuberances on the upper surface. Stem leaves are divided into lobes, with the tip of each lobe ending in a spine. The stems are winged and spined and are covered with hairs. The root is a branched tap root.
  • Spear thistle flowers are reddish to purple. The flower heads are surrounded by a large number of spiny bracts, and occur singly or in groups of two or three heads at the ends of branches.
  • Spear thistle seeds germinate mainly after autumn rains. The plant over-winters as a rosette (a whorl of leaves close to the ground); rosettes grow rapidly through spring and can reach a diameter of 60 cm.
  • The rosette may produce stems, flower and die in their first summer. Alternatively, rosettes may persist through summer and continue growth into a second autumn and winter. Dead plants often remain standing for several years.
  • For help in identifying thistles in Tasmania, see Identifying Thistles in Tasmania and search the Dennis Morris Weeds and Endemic Flora database for spear thistle illustrations. If you are still in doubt about the thistle you are dealing with, contact Biosecurity Tasmania on 03 6165 3777 for help.
Spear thistle rosette, photo: K. Fenner Spear thistle flower heads, photo M. Baker
Image top: Spear thistle flower, © K. Fenner.
Images above, left to right: Spear thistle rosette, © K Fenner; Spear thistle flower heads, © M. Baker.

Spear thistle in Tasmania

  • Spear thistle is not declared in Tasmania.
  • Spear thistle is found in all settled parts of Tasmania, but is more common in the lower rainfall areas. Heavy infestations occur in the Midlands in some seasons, depending on rainfall, grazing pressure and soil fertility.
  • Heavy spear thistle infestations in pasture suppress pasture growth, and can virtually exclude grazing stock, leading to pasture grasses becoming rank and vulnerable to insect pests.
  • Thistle spines in the fleece of sheep can cause difficulties at shearing.
Detailed management and control guidelines for spear thistle can be found in the Spear Thistle Control Guide. Refer also to Herbicides for Spear Thistle Control. For further information see DPIPWE's Weed Links and Resources.


See also:

Herbicides for Spear Thistle Control

Other useful links:

Pest Genie

Spear Thistle Control Guide

Do

  • 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't

  • 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.

Herbicides for Spear Thistle Control

Herbicides for Spear Thistle Control


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