Quick Facts

Quick facts

• Originally from the northern hemisphere, Scotch and Illyrian Thistles (Onopordum acanthium and O. illyricum) are very similar looking herbs to 1-2 metres tall, annuals to perennials, with spiny leaves and stems and flower bracts, with purple flowers
• In  New South Wales often grow together and are known to hybridize producing plants of a wide variety of forms.
• All Onopordum spp. are spiny, with or without stems, and purple to pale flowers, found around Australia, but mainly absent from the arid areas.
• They produce seed in large quantities (up to 20 000 per plant in Scotch Thistle) and seed is stored in the soil for many years when buried and will germinate when a paddock is cultivated.
• Infestations degrade pastures and crops, contaminate hay and seed crops and thus start new infestations, and degrade wool, injuring workers and animals that graze spiny thistles.
• Spread by machinery soils and gravel and in agricultural produce .
• Often difficult to control using herbicides because of the woolliness of the leaves.
• Maintaining good cover and healthy dense pastures prevents widespread establishment.
• Control by cultivation in early growth stages, repeating this, and subsequent follow-up using other methods is required
• Bio-control options are also available for large infestations but may require several years to yield noticeable results.
• Success in control and containing infestations required the use of long-term integrated weed management practices (using a combination of control methods over many years).

What Does It Look Like?

What is it?

Scotch Thistle (Onopordum acanthium) is an erect annual or biennial herb commonly to 1.2 m tall, rarely to 2 m tall. Initially the plant forms a flat rosette of stalked leaves. The rosette leaves are grey-green, toothed to lobed with spiny edges and a whitish appearance because of a covering of white woolly hairs, which are dense on the underside. The rosette can be 50 cm in diameter. The erect stems are winged to 15 mm wide, with spines to 5 mm long on lobes, whitish, woolly or densely cobwebbed. The stem leaves grey-green, spiny, without a leaf stalk, sparsely woolly, lower surface paler and more densely woolly oblong-ovate (oblong – length a few times greater than width, with sides almost parallel and ends rounded; ovate – egg-shaped attached at the wider end) to broad-lanceolate  (lance shaped, about 4 times as long as broad, broadest in the lower half and tapering to the tip) or ovate in outline, to 350 mm long and 200 mm wide, lobed or toothed, with 6–8 pairs of broad-triangular lobes or teeth, each tipped by a yellowish spine, upper surface.

The purple to mauve flower-heads are up to 60 mm in diameter, at the ends of the main branch and side-branches. Each flower-head has numerous purple to mauve flowers, 14-25 mm long, and can produce a single seed (achene) which is grey mottled, four-angled, wrinkled and about 4-5 mm long.

The fruits or 'seeds' (cypselas) are 4–5 mm long, 4-ribbed, wrinkled, grey with darker mottling Seeds have pappus , of fine toothed hairs about 7–10 mm long on top, but this usually detaches at the time of the seed is shed and it probably doesn't aid dispersal much (Parsons & Cuthbertson 1992; Hussey et al. 1997; Dellow & Holtkamp 2005).

Stemless Thistle (Onopordum acaulon) is an annual species to 1 m diameter, without a stems. It produces a rosette very similar to of the Scotch Thistle but with quite deeply dissected leaves and it does not develop a stem. It produces its flower-heads, many clustered together, in the centre of the rosette.

The white to purple flower-heads are similar to scotch thistle, produced individually or in clusters at centre of rosette, rounded 40-60 mm in diameter,  stalkless or on short flower-stalks to 30 mm long. The involucral bracts (leaf-like structures surrounding the flower-head)  are spiny, ovate -lanceolate, 20–30 mm long, all erect or outer ones bent back, tapered into a spine. Flowers (florets) are also similar to scotch thistle, are shorter than the bracts, but the flowers may be white to purple.

The fruits or 'seeds' (cypselas) are brown-grey, 4-5 mm long, four-angled (or ribbed) and wrinkled. The pappus can be to 20-30 mm long and is easily detached from the seed (Parsons & Cuthbertson 1992; Hussey et al. 1997; Dellow & Holtkamp 2005).

Illyrian Thistle (Onopordum illyricum) is an erect annual to 2 m tall. It also forms a rosette of leaves that are more deeply dissected (lobed) to the midrib and not quite as woolly as those of Scotch Thistle, but are generally similar otherwise. Rosette leaves are oblanceolate (lance shaped, about 4 times as long as broad, broadest in the lower half at the base) to elliptic (oval) in outline, to 550 mm long and 150 mm wide, pinnatisect (deeply lobes almost to the midrib), with 8–10 pairs of toothed lobes, lobes tipped by a yellowish spine with spines to 8 mm long, white- or grey-woolly on both surfaces. The stems are winged to 10 mm wide, unbranched below, white-woolly. The stem leaves narrow-lanceolate  in outline, to 300 mm long and 100 mm wide, are more deeply dissected than in Scotch Thistle.

The purple flower-heads are 25-30 mm long, longer than those of Scotch Thistle on a short flower stalk in clusters at the end of stems. Flower-heads are rounded 40-60 mm in diameter. They also have spiny involucral bracts (leaf-like structures surrounding the flower-head) that are lanceolate (lance shaped, about 4 times as long as broad, broadest in the lower half and tapering to the tip), to 30 mm long, purplish, innermost ones erect and smooth lacking hairs, outer bracts reflexed, tapered into a short yellow spine, cobwebbed at the base. Florets longer than involucre, purple.

The fruits or 'seeds' (cypselas) are brown, 4-angled and wrinkled. The pappus is 8–10 mm long, silky and similar to that of Scotch Thistle, also detaching easily (Parsons & Cuthbertson 1992; Hussey et al. 1997; Dellow & Holtkamp 2005).

Hybrids between Scotch and Illyrian Thistles are common especially in southern New South Wales (Parsons & Cuthbertson 1992).

For further information and assistance with identification of Onopordum spp. contact the herbarium in your state or territory.

Flower colour

Purple, White

Growth form (weed type/habit)

Herb

Where it currently grows? Preferred habitat

Scotch Thistle and Illyrian Thistle are weeds of pasture, and irrigated and dryland cropping areas in temperate south-eastern Australia with predominantly winter rainfall of about 500 and 900 mm per year. They prefer soils of moderate to high fertility especially with elevated levels of nitrogen. They do not grow well on waterlogged soils but prefer well-drained soils and tolerate stony soils (Parsons & Cuthbertson 1992; Hussey et al. 1997; Dellow & Holtkamp 2005).

Stemless Thistle is commonly found in dryland and irrigated horticultural areas, and dryland cropping areas, with predominantly winter rainfall up to 450 mm, often on sandy or calcareous soils (Parsons & Cuthbertson 1992). The occurrence on the Northern Tablelands of New South Wales where rainfall is much higher (to 800 mm per year) indicates the potential for the species to spread. The recent new occurrence in Tasmania in an area with about 550 mm rain per year reaffirms this potential. Stemless Thistle is generally found in drier, though still arable areas, than is usual for both Scotch and Illyrian Thistles (Parsons & Cuthbertson 1992; Hussey et al. 1997; Dellow & Holtkamp 2005).

Are there similar species?

Scotch Thistle and Illyrian Thistle are difficult to tell apart. The rosette and stem leaves of Illyrian Thistle are more deeply lobed than in Scotch Thistle and the individual purple flowers are longer than 25 mm in Illyrian Thistle, less than 25 mm in Scotch Thistle. Dellow and Holtkamp (2005) provides a useful comparison table.

Stemless Thistle generally has broader rosette leaves than the other two species and never develops a long stem. Instead it bears the flowerheads on the middle of the rosette, has white or purple individual flowers in the flowerheads, and has a very long (about 2.5 cm) tuft of bristles (the pappus) on top of each newly formed seed, whereas the other two species have a pappus only about 1 cm long.

The white woolly to cottony hairs on leaves and stems of these three species of Onopordum thistle distinguish them as a group from all other thistles that have invaded this country. The fourth Onopordum species, Taurian Thistle (Onopordum tauricum) differs from these three species in lacking the white woolly hairs, being generally green-leaved instead with many minute glandular hairs.

Also see Navie (2004).

Why Is It A Weed?

What are its impacts?

Onopordum spp. were included in the list of 71 species that were nominated by state and territory governments for assessment as Weeds of National Significance (WONS). Following an assessment process, Onopordum species were not included as one of the 20 WONS. However, they remains a weed of potential national significance. Once established thistles are difficult and expensive to control. Dense thistle populations can reduce property values.

Agriculture: Onopordum spp. are significant pasture and cropping weeds. All three species of Onopordum have a serious impact on agricultural production by occupying space in pastures and crops thus reducing production.  Large rosettes can prevent the growth of desirable species. Their prickly nature means livestock rarely eat them, or if they do cause injury to stock. They also reducing the value of grain and hay by increasing the likelihood of contamination with seeds of these declared noxious weeds. (Parsons & Cuthbertson 1992; Hussey et al. 1997; Dellow & Holtkamp 2005). 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 (DPI NSW 2008)

Native ecosystems: Although a serious weed of agriculture, also invades open native vegetation and degraded and disturbed vegetation.

Urban areas: Found along roadsides and waste areas, abandoned blocks, and in parks and in gardens.

How does it spread?

All three species of Onopordum spp. have a seed with a pappus on top to aid wind dispersal. However in all species this pappus readily detaches from the seed, so may be of limited use in wind dispersal. Observations show that wind dispersal is locally important none-the-less, especially in Stemless Thistle, which has the longest pappus (Parsons & Cuthbertson 1992). Otherwise dispersal is by contaminated grain and hay, machinery and wool.

Scotch Thistle may also spread very locally by root fragments cut up and transported by cultivation equipment provided soil moisture is adequate (Parsons & Cuthbertson 1992; Dellow & Holtkamp 2005).

What is its history in Australia?

Scotch Thistle was probably introduced to Australia as an ornamental. It showed weed potential in Victoria by 1850 and was declared noxious in 1856 and is now well established as a weed of grazing lands and crops in areas of moderate, predominantly winter, rainfall in south-eastern Australia (Parsons & Cuthbertson 1992; Hussey et al. 1997; Dellow & Holtkamp 2005).

Stemless Thistle was also probably introduced as an ornamental species and was recorded growing in Adelaide in 1845. It had reached the South Australian cereal belt by 1890 and Victoria by 1925. Its time of arrival in New South Wales is not documented but it was well established in the south-west of the state by 1963 and now has sporadic occurrences throughout the Western Slopes and Tablelands. It was first recorded in Western Australia in 1955 and is found now in the southern wheatbelt and around Esperance. It was recently recorded in Tasmania at Melton Mowbray in 2002 having most likely been introduced in grain sourced from mainland Australia and fed to livestock during drought between 1999 and 2001 (Parsons & Cuthbertson 1992; Hussey et al. 1997; Dellow &Holtkamp 2005).

Illyrian Thistle is known to have occurred in north-central Victoria in the 1930s but its time of arrival to Australia is unknown. Anecdotal evidence suggests it may have been sold locally as an ornamental 60 years before this. Infestations are small in this area of Victoria, but somewhat larger in New South Wales where it is a serious weed on the Central & Southern Tablelands and Slopes, less so on the Northern Tablelands and Slopes. Infestations in New South Wales often include plants of Illyrian Thistle and Scotch Thistle and hybrids between the two, and are particularly troublesome to control (Parsons & Cuthbertson 1992; Hussey et al. 1997; Dellow & Holtkamp 2005).

How To Manage It?

Best practice management

Control of Onopordum spp. is difficult and therefore time consuming and expensive. The best defence against invasion of Onopordum spp. on agricultural lands is maintaining healthy perennial pasture with good ground cover at all times to prevent seedling establishment. for dense infestations an integrated management program using 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 reduce or prevent plants setting viable seed, and in doing so the following year’s population will be greatly reduced (DPI NSW 2019)

Chemical control: Herbicide control of these three Onopordum species can be very effective and is an essential part of the overall management of these thistles and are used on large or dense infestations applying . However, plants can be tricky  to kill  because of the difficulty in wetting the woolly-hairy leaves of the plants. To achieve an effective kill the following should be considered; Timing of application is also important. If a single application of herbicide is undertaken then a springtime application is best (Dellow 1996), as herbicides are most effective on seedlings and young rosettes, Once plants begins 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. When there is a low density of thistles spot spraying  is preferred (DPI NSW 2019). Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information available at http://www.apvma.gov.au 

Non-chemical control: Physical control:  When there is a low density of thistles hand chipping is effective. 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.

Mechanical control: 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. Plants can be mown or slashed before flowering to prevent seed set but this is temporary as plants will regrow and develop new growth from the base. Immature seed heads that are cut and left lying on the ground can contain viable seed and spread seed.

Competition and management: Pasture management is the most important part of any thistle control program and maintaining a dense, vigorous and competitive, and 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 (DPI NSW 2019).

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. 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.

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 (DPI NSW 2019).  With existing infestations of Illyrian Thistle goats are known to be effective in grazing the rosettes, and also the flower-heads once the plant has elongated. This can be a vital step as it reduces seed production and thus limits recruitment of new thistle plants and also reduces the build up of seed in the soil. The soil seedbank is relatively long-lived (Holst & Allan 1996).

Biological control: Onopordum spp. has also been recognised as a target for biological control through a cross-jurisdictional government process. This allows activities to be undertaken to develop effective biological controls. Biological control of all three Onopordum thistles discussed here is under study with three insects released into infestations – a seed head weevil, a stem boring weevil and a crown moth the larvae of which attacks leafstalks, the crown of the rosette and also the roots (Dellow & Holtkamp 2005). The following summary is taken from DPI NSW (2019)

Seed-head weevil. The seed-head weevil, Larinus latus, was released in spring 1992. 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. 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. 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. 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 shriveling, 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.

For further information on the control of Onopordum spp. see Keith Turnbull Research Institute (1998), DPI NSW (2008); DPI NSW 2019; DPIW (2002), and the Department of Agriculture and Food, Western Australia (undated);

Does it have a biological control agent?

YES

When does it grow? (lifecycle/growth calendar)

Scotch Thistle seeds germinate at any time of year but do so mainly in late summer-autumn and also late winter-spring, and start to form rosettes. A population may therefore consist of plants of many ages (sizes). Plants from late summer-autumn germination will flower and seed the following spring-summer and then die (i.e. they are annuals). Plants from winter or spring germination will also behave as annuals if they have enough chilling to allow stem elongation and flowerhead formation in the late spring. If chilling is insufficient then the rosette will persist over summer and the following winter then elongate, flower and seed then die during the second spring-summer (i.e. they are biennials) (Groves & Kaye 1989; Parsons & Cuthbertson 1992; Dellow & Holtkamp 2005).

Most Stemless Thistle seeds germinate in autumn after rain and during winter plants develop into small rosettes. The rosettes grow rapidly in spring with flowerheads forming by October-November and seed shedding by December followed by death of the plant. Sometimes late germinating seeds do not have a large enough rosette to flower by the summer and these rosettes over-winter with flowering, seeding and death occurring in the second spring-summer (Groves & Kaye 1989; Parsons & Cuthbertson 1992; Dellow & Holtkamp 2005).

Illyrian Thistle seeds germinate at any time of year but predominantly in autumn. It forms a rosette during winter with some plants elongating and flowering in the following spring, then seeding and dying or continuing to grow as a rosette through the summer, autumn and winter not flowering until the second spring-summer, then seeding and dying. There seems to be a chilling requirement before flowering can occur (Groves & Kaye 1989; Parsons & Cuthbertson 1992; Dellow & Holtkamp 2005).

Where Is It Found?

Which states and territories is it found?

ACT, NSW, SA, TAS, VIC, WA

What areas within states and territories is it found?

Scotch Thistle is currently found in New South Wales, Victoria, Tasmania and South Australia. In New South Wales it is common on the Central and Southern Tablelands and Slopes; in Victoria it is mainly found in parts of Gippsland and the high country from Omeo south to Bruthen and around the Bairnsdale-Sale area, and central Victoria; in Tasmania mainly in the Midlands and Fingal Valley whilst in South Australia it occurs sporadically in the south, around Port Lincoln, on Yorke Peninsula and in the southern Lofty Ranges (see Parsons & Cuthbertson 1992; Hussey et al. 1997; Dellow & Holtkamp 2005).

Stemless Thistle is currently found in New South Wales, Victoria, South Australia and Western Australia. In New South Wales it mainly occurs in the drier south-west of the state but is also found on the inland plains the Northern Tablelands; in Victoria it is restricted to areas west of Echuca and north of Warracknabeal as well as Lake Hindmarsh to the Murray River; in South Australia it is abundant in the Northern Lofty, Murray and Eastern regions but is also found in the Flinders Ranges, Yorke Peninsula and Southern Lofty regions. In Western Australia it occurs around Esperance, Lake Grace and Bruce Rock areas of the southern wheatbelt and coastal plain (see Parsons & Cuthbertson 1992; Hussey et al. 1997; Dellow & Holtkamp 2005). In Tasmania there was a recent record (2002) of this species from farmland near Melton Mowbray (Hanson & Crane 2003).

Illyrian Thistle is currently found in New South Wales, Victoria and South Australia. In New South Wales it is mainly found on the Central and Southern Tablelands and adjacent western slopes, but also has significant populations on the Northern Tablelands and North-west slopes, with outliers in the Bega Valley on the South Coast. In Victoria the species has not spread much since its initial recorded occurrences in the Maryborough – Smeaton – Daylesford area, but is also found east of Craigieburn. In South Australia there are sporadic records including near Angaston, Sedan and Wirrabarra (see Parsons & Cuthbertson 1992; Hussey et al. 1997; Dellow & Holtkamp 2005).

Where does it originate?

Scotch Thistle occurs naturally from central, south-eastern and south-western Europe (e.g. Germany, France, Bulgaria, Greece) across into Siberia, the Caucasus (e.g. Armenia and Georgia), western Asia (Afghanistan, Iran, Iraq and Turkey) and the Indian Subcontinent (Pakistan).

Stemless Thistle is native to northern Africa (Algeria, Morocco and Tunisia) and south-western Europe (France and Spain).

Illyrian Thistle is found from south-eastern and south-western Europe (e.g. Albania, Greece, Crete, France, Portugal and Spain) (GRIN 2007).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

ACT, NSW, TAS, VIC, WA

Onopordum acaulon, Onopordum acaulon, Onopordum illyricum and Onopordum tauricum (and presumably any other Onopordum species) are declared under the collective name Onopordum spp. in New South Wales.

Similarly in Tasmania the species are declared under Onopordum spp.. Only O. acanthium and O. acaulon have been recorded in Tasmania.

In the ACT O. acanthium & O. illyricum are declared individually.

In Victoria O. acanthium, O. acaulon and O. illyricum are declared individually.

In Western Australia only O. acaulon is declared.

No Onopordum species is declared in South Australia, the Northern Territory or Queensland.

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Onopordum spp.

Other scientific names (synonyms)?

Does it have other known common name(s)?

Heraldic Thistle, Cotton Thistle, Woolly Thistle, Cardo-bastardo (Onopordum acanthium), Stemless Thistle, Horse Thistle, Stemless Onopordon (Onopordum acaulon), Illyrian Thistle (Onopordum illyricum)