Scotch thistle (Onopordum acanthium)

Scotch thistle is an erect annual or biennial plant. It is a major weed of fertile pastures in winter dominant rainfall areas.

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How does this weed affect you?

Thistles compete with pastures and reduce their carrying capacity. Rosette leaves smother desirable pasture species in spring reducing their early growth. Dense stands of mature thistles create barriers that hinder livestock movement. Parts of the plant can break off causing vegetable fault and thus a loss in value for the wool. Thistles can cause injury to livestock and people handling the livestock or fleece. Thistles are prolific seeders and can spread quickly if not controlled. Once established thistles are difficult and expensive to control. Dense thistle populations can reduce property values.

What does it look like?

Scotch thistle is similar in appearance to other Onopordum species, in particular Illyrian thistle (Onopordum illyricum).

Scotch thistle is an erect annual or biennial herb to 2 m high, commonly 1 to 1.2 m. Generally one main stem with numerous branches, covered with dense, appressed, woolly hairs giving it a whitish-grey appearance. It has broad spiny wings along the stems. 

The flower heads are purple and round with spiny bracts. These bracts, surrounding the flowers, are the main distinguishing feature. They are sharp, needle-like and less than 3 mm broad at the base, yellow in colour, and only the outer bracts are reflexed (bent sharply backwards). 

Where is it found?

Scotch thistle is a native of Europe, western and central Asia. In NSW, Scotch thistle is a major weed of pastures on the Central and Southern Tablelands and the Central and South-West Slopes. It is also present to a much lesser extent on the coast, Northern Tablelands and North–West Slopes.

How does it spread?

The main form of spread is by seed. The seeds have fine hairs which can easily attach to stock and clothing. The seed is often spread in hay, on machinery or vehicles. The seed can also survive the digestive tract of sheep. 

The Scotch thistle seeds are poorly spread by wind except when the seed head breaks off the plant and is blown about by strong winds. It may be spread when root fragments are moved by cultivation equipment. This is rare as plants will only survive if the fragments are fresh and the ground is moist and soft.

What type of environment does it grow in?

Scotch thistle flourishes on soils that are well drained and of moderate to high fertility and is particularly common on high fertile soils associated with introduced perennial pastures in the winter dominant rainfall zone. It prefers moderate to high rainfall. 

Acknowledgements

Authors: J Dellow, former Technical weeds specialist, Orange, Annie Johnson, Project Officer (Weeds), Orange and Royce Holtkamp, State Biological Control Coordinator, Tamworth.

Technical reviewers: Linda Ayres, Mich Michelmore and Andrew Storrie.

More information

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Control

Integrated weed management

An integrated management program is the key to successful thistle management. The use of a range of tools such as the maintenance of a strong perennial pasture, strategic herbicide application, chipping by hand and use of biological control agents where appropriate is the best strategy for long term control. The key to managing thistles is control of seed set. By preventing plants from ripening seed the following year’s population will be greatly reduced. Dense populations of thistles may have to be controlled in stages. Consistency of control is also important. Trials show that preventing seed set for five years reduced a large thistle population, however plant numbers returned to pre-trial levels after only two years when no control was undertaken.

Preventing spread

Control isolated patches before they have a chance to spread. If transporting hay from areas where these weeds occur, monitor the area where the hay was spread out for thistle seedlings and kill them before they set seed. Quarantine sheep in holding paddocks if moving from infested to uninfested areas to allow seed to pass through the digestive tract. A minimum of five days is recommended. Monitor and kill any thistles before they set seed in quarantine paddocks.

Pasture management

The most important part of any thistle control program is maintaining a dense, vigorous and competitive pasture. A vigorous perennial grass pasture provides competition for germinating thistles, reducing seedling establishment. Gaps in the pasture result in an increase in thistle germination and seedling survival so maintaining excellent ground cover at all times is desirable. Maintenance of adequate ground cover especially in summer and early autumn is essential to reduce the potential for the establishment of thistles.

Establishing a new pasture

Establishing introduced perennial pastures is expensive so thorough preparation and research into suitable species and varieties is required. The establishment of adequate plant numbers is the first step to a successful pasture. Seek advice from your agronomist on the best pasture establishment steps for your situation. Some perennial grasses, particularly phalaris compete strongly with Scotch and Illyrian thistles. Lucerne competes well with stemless thistle.

Sown perennial pastures also require regular maintenance fertiliser applications (as dictated by a soil test) to maintain their vigour and competitiveness. Grazing management and ground cover is a critical factor to the maintenance of adequate perennial plant density in the years following pasture establishment.

Maintaining the current pasture

Management of existing native and naturalised pastures should aim to maintain perennial grass content and ground cover. Identify the species present, their growth cycles and their response to grazing, and fertiliser to formulate a management regime that is most suitable to maximising their competitive behaviour. The addition or dominance of too many legumes, combined with fertiliser and a trend towards an annual pasture system can lead to instability in the pasture and potential for thistle invasion.

While thistles respond to increases in soil fertility (and in particular nitrogen), management of all pasture types should aim to maintain pastures that have a good balance of perennial grass to legume content.

Fertilisers

Fertiliser application (as determined by a soil test) will improve the vigour of annual and perennial introduced grasses, increasing ground cover and reducing future thistle establishment.

Grazing management

Thistles are generally avoided by stock. However goats eat thistles at both the rosette and flowering stage as well as post-flowering. Horses, and sometimes cattle graze the flower which can reduce the amount of seed. The amount eaten depends on the grazing pressure and the amount of other feed available.

Physical removal

Isolated plants should be removed using a hoe or mattock (chipping or grubbing). Remove as much of the taproot as possible so that regrowth does not occur. Cultivation is effective on seedlings or young rosettes if they are uprooted. Older rosettes are damaged by cultivation but are able to regrow, especially if the soil is moist. Slashing or mowing is not usually effective as plants develop new growth from the base. Immature seed heads that are cut and left lying on the ground can contain viable seed.

Biological control

Biological control of thistles is a long term tactic and should only be seen as part of an effective integrated weed program. Up to ten insect species were trialled for released for the biological control of Onopordum thistles. The successfully established species shown below were released several years ago and are now present in most areas affected by Onopordum thistles. There should be no need for redistribution of these species. These biological control agents have a direct impact on the seed set of thistles. This is important for the long term control of thistles.

Seed-head weevil

The seed-head weevil, Larinus latus, was released in spring 1992. The adult weevil is up to 25 mm long, 10 mm wide and is black with a yellow waxy coating. Adults hibernate over winter until mid to late spring when they become active and mate. Each lay up to 30 eggs on the thistle flower head. Larvae hatch and tunnel into the flower head where they feed on the tissue supporting developing seeds. After feeding for about six weeks the larvae pupate.

A single larva is capable of destroying all the seed in a flower head of 3 cm diameter or less and more than one larva may develop in larger heads. This weevil has only one generation per year so it has taken a number of years for populations to grow large enough to destroy most of the seed in a flower head. At some NSW sites this insect has reduced seed production by more than 80 per cent.

Stem-boring weevil

The stem-boring weevil, Lixus cardui, was first released in November 1993 and is now widely established. Adults grow up to 15 mm long and are brown in colour. They emerge from stems in early spring and chew holes in the leaves of thistle rosettes. Females lay eggs into the flowering stems then larvae bore into the plant. Adults continue to feed on the leaves. Combined heavy adult and larval feeding can reduce plant growth and subsequent seed production.

The stem-boring weevil is not capable of killing Onopordum thistles. Its activity weakens the plant, makes it less competitive and reduces seed production. This action allows insects such as the seed head weevil to have a greater impact on the plant.

Crown moth

The crown moth, Eublemma amoena, was first released in 1998. Adult moths are mottled, light brown and white and up to 15 mm in length. There are three adult generations per year, commencing in spring when adults emerge from pupal cells in thistle rosettes. Females lay eggs on leaves and larvae feed in the leaf petioles, causing leaves to shrivel and die. Larvae can also bore into the crown and root of the plant. This may lead to the death of smaller plants. Larvae of subsequent generations feed in the leaves of stems, causing similar leaf shrivelling, reducing plant vigour.

Rosette weevil

The rosette weevil, Trichosirocalus briesei, was first released in 1997. It is now established in many areas and is starting to have an impact on thistle populations. Adults are 3–5 mm long and a mottled brown colour. There is one generation per year. Adults emerge from a summer dormancy period following autumn rains and commence feeding on rosette leaves. Females lay eggs near the base of the rosette leaves. After hatching, larvae destroy the growing point of the rosette either killing the plant or severely reducing its vigour.

Herbicides

Herbicide control can be very effective and is an essential part of the overall management of these thistles. When there is a low density of thistles spot spraying or chipping is preferred.

Onopordum thistles can be difficult to kill, and a number of critical factors should be taken into consideration when using herbicides. These include:

• Correct rate and timing of application – herbicides are most effective on seedlings and young rosettes however once plants begin stem elongation herbicides may be less effective.
• The hairy leaves of the thistles can reduce herbicide effectiveness through reduced absorption. Therefore it is important to follow the herbicide label and use the recommended rate, application volume and adjuvant.
• Control will be reduced if plants are stressed.

Always read the herbicide label prior to applying herbicides.

For populations of thistles where there is a mixture of sizes, spray in the spring before the flowering stem develops. If there is a flush of seedlings in the autumn an early spray can prevent the rosettes from colonising the ground and competing with the pasture.

Take care in selecting the herbicide product, as many registered and effective herbicides for control will damage pasture legumes leaving room for other weeds to establish. Regular herbicide use affects the long term composition of the pasture.

Resistance

In New Zealand the total reliance on 2,4-D ester (MOA Group I) for the control of nodding and slender thistles for many years has led to the development of herbicide resistance, however this has not yet been recorded in Australia. Dense thistle populations with large seed banks are a higher risk to develop resistance compared with sparse weed populations.

Herbicides should only be used as part of an overall management strategy.

Spray grazing

Spray grazing is the use of a sub-lethal rate of herbicide to ‘sweeten’ the weeds making them palatable to livestock. Only use this method when there are low levels of the weeds present and there is plenty of alternative feed available. Weeds must be in the early rosette stage. For spray grazing to be effective sheep stocking rates must be increased to 8-10 times the normal rate. Grazing should be continued until the weeds have had the growing points eaten but not to the point where the pasture species are at risk. Spray grazing is only effective if there is a competitive pasture to replace the weed.

Observe the stock health warnings on the herbicide label when using the spray grazing technique. There is a risk of stock poisoning if a large proportion of the pasture consists of other weeds such as Paterson’s curse (Echium plantagineum) or variegated thistle (Silybum marianum).

Herbicide options

See Using herbicides for more information.

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Clopyralid 300 g/L (Lontrel®)
Rate: 250 mL in 100 L of water
Comments: Handgun application
Withholding period: 1-12 weeks (see label).
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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Dicamba 750 g/L (Kamba® 750)
Rate: 53 mL in 100 L of water. Add a surfactant.
Comments: Handgun application: Spray prior to flowering. For non crop situations.
Withholding period: Do not harvest, graze or cut for stock food for 7 days after application.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All pest plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.

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Reviewed 2021