Quick Facts

Quick facts

• Originally from the Mediterranean region, south-western Europe, Asia Minor and Russia, Variegated Thistle (Silybum marianum) is an erect annual or biennial spiny herb to 2.5 metres tall
• It grows large green and white variegated spiny-edged leaves (main and secondary veins blotched white), and large flower-heads surrounded by stout spines.
• A highly competitive and invasive agricultural weed on fertile soils in temperate Australia
• Can cause serious loss of production on normally productive farmland.
• Spread by seed that may remain viable in the soil for at least 9 years.
• Most effective control is the maintenance of a healthy perennial pastures, particularly in summer going into autumn when seeds germinates.
• Most effective herbicide control is at seedling and rosette stage.

What Does It Look Like?

What is it?

Variegated Thistle (Silybum marianum) is an erect annual or occasionally a biennial herb, to 2.5 m tall, with shiny green leaves with white veins and blotches, and with a stout taproot with many lateral roots. Two types of leaves are produced; first the basal rosette of leaves; and subsequent stem-leaves. All leaves are shiny green with white veins and blotches (variegated) above, the lower leaf-surfaces are duller and somewhat hairy. The rosette leaves are spiny, spines up to 8 mm long on leaf margins and over leaf veins, oblanceolate in outline (lance shaped, about 4 times as long as broad, broadest in the upper half, tapering to a narrow base), up to 600 mm long and 250 mm wide. They are deeply divided into lobes with undulating margins with long and short spines on the margins. Stems are produced from the centre of the rosette. Stems are not winged or spiny, but are stout, hollow or filled with pith, longitudinally ribbed, slightly cottony-downy and branched extensively in the lower part. The stem-leaves are similar to the rosette leaves but smaller, without stalks and lobed deeply at the base and stem-clasping. There are more leaves on the lower half of the stems than the upper part (Parsons & Cuthbertson 1992; Jeanes 1999).

The flower-heads (capitula) are large and spiny, up to 30–50 mm diameter and located on the end of long almost leafless stems (called scapes), to 300 mm long. Occasionally, small flower-heads are also sessile (stalkless) in the upper leaf stem junctions (axils). Each flower-head consists of numerous pink-purple (rarely white) tubular florets, surrounded by several rows of spiny bracts (modified leaves at the base of the flower). Bracts are up to 50 mm long without hairs and sharp spines with several smaller spines at the base. The outer bracts are lanceolate to elliptic, have a sharp apical stout spine that are spreading or recurved, and the inner bracts with an ovate (egg-shaped with the widest point near the base), with a sharp point. Each floret develops one seed

The fruits or 'seeds' (cypselas) are 5–8 mm long, more or less flattened with a smooth surface, shiny black to brownish with a yellowish ring at the tip and topped with 12–20 mm long barbed bristles (the pappus). The pappus is easily detached from the ripe seed and probably doesn't aid dispersal much. The florets and seeds are borne on a receptacle that is densely white bristly, most easily seen after the seed have fallen (Parsons & Cuthbertson 1992; Keith Turnbull Research Institute 1998; Jeanes 1999; VicFlora 2016).

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

Flower colour

Purple or White

Growth form (weed type/habit)

Herb

Where it currently grows? Preferred habitat

Variegated Thistle (Silybum marianum) is found in well-watered temperate regions of southern Australia on fertile soils of alluvial or volcanic origin, and on some sandy or sandy loam soils. It is common on sheep and cattle camps in pastures and can spread, out-competing desirable species. It is also found on cultivated land, roadsides, stockyards, and disturbed neglected areas and disturbed areas in woodlands and open forests (Parsons & Cuthbertson 1992).

Are there similar species?

Variegated Thistle (Silybum marianum) is the most instantly recognisable species at both rosette and flowering stages of growth of all the thistles that have invaded Australia, because of its robust habit and variegated spiny shiny leaves. It is highly unlikely to be confused with any other species (Slee 2007, pers. comm.).

Why Is It A Weed?

What are its impacts?

Variegated Thistle (Silybum marianum) is found on roadsides and in waste areas, in pastures and occasionally in crops. It is a highly competitive weed once it starts to establish and due to the size of its rosette and its ability to compete for light, nutrients and moisture it shades out other species. Establishment is favoured by bare ground or sparse pasture, consequently infestations often occur on stock camps, rabbit warrens and cultivated fire breaks. Variegated thistle has a high early growth rate, which allows it to out-compete many pasture seedlings (Government of South Australia 2021). When the thistle plants die in summer the ground is left bare and open for re-establishment of Variegated Thistle and other weeds in autumn. Variegated Thistle grows best in areas of high fertility and a weed of cultivated land, roadsides, neglected areas, generally replaces existing plants. In addition to its actual impacts, it also receives some attention because of its size and spiny, non-native appearance

Agriculture: A highly competitive weed once it starts to establish in pastures. Due to the size of its rosette and its ability to compete for light, nutrients and moisture it shades out more desirable pasture species. Where established in areas cut for hay it will contaminate the fodder reducing its value and enabling further dispersal of the weed. Where Variegated Thistle can get established in natural plant communities it is also able to displace native species through its competitive ability (Parsons & Cuthbertson 1992). Its prickly nature and its ability to form a dense sward make access and control using hand tools difficult, and can also deter stock and wildlife while harbouring vermin like rabbits. Variegated Thistle is often eaten by stock after being cut or sprayed, as are dead plants in summer. Stock will also eat the leafy green plants. However grazing of fresh green growth in winter can cause serious stock poisoning because of the high nitrate content of the foliage, especially on cloudy days in wet weather or when soil moisture is high. The spines on the leaves can damage the digestive tract of grazing animals (Parsons & Cuthbertson 1992), especially of cattle (Government of South Australia 2021). Also grows in some cops and place such as sheep camps and stock-yards.

Native ecosystems: Occasionally found in native grassy lands or open or degraded areas.  Can be found on the edge of and in roadside vegetation. Normally not found in dense healthy woodlands. During seed development the seed-heads are sought after by Sulphur-crested Cockatoos, Galahs and Crimson Rosellas as a food (Parsons & Cuthbertson 1992).

Urban areas: Found on roadside and disturbed are like machinery yards and abandoned areas, little cared for grassy areas, and on the edge of creek lines.

How does it spread?

The seed of Variegated Thistle (Silybum marianum) may be wind-dispersed but the pappus easily falls off the seed so most probably don't go very far. Contaminated fodder, contaminated crop seed and machinery probably account for most dispersal over greater distances (Parsons & Cuthbertson 1992).

Machinery and  good weed hygiene will prevent further spread.

What is its history in Australia?

Variegated Thistle (Silybum marianum) was a nineteenth century introduction to Australia probably as a medicinal plant. It was first recorded as a problem plant in Tasmania (noted in the Hobart Town Courier in 1832) and first collected in 1867 on one of Tasmania islands (AVH 2021). It was known in South Australia in the 1840s, was declared noxious there in 1851, but no collections were made until 1886. Victoria followed suit in declaring it a noxious weed in 1856 with the first collection made in 1890. It was recognised as a serious problem in NSW by 1857with the first collection made in 1895. The time of introduction to Queensland and Western Australia is not known (Parsons & Cuthbertson 1992)  but it was first collected in WA in 1900 and in Queensland in 1912 (AVH 2021).

How To Manage It?

Best practice management

Variegated Thistle (Silybum marianum) can be controlled by herbicides to kill plants when young, by physical means; grubbing killing if roots are moved, and managed but rarely killed by mechanical means to reduce seed set, and with cultural practices, with grazing animals (caution is and can be toxic to animal in larger quantities). Predominantly a weed of cultivated land on fertile soils, on these areas with existing infestations control will obviously depend on acreage infested and weed density. The best means of control is prevention through good pasture management with of cover of desirable species.

Chemical control: Herbicide applications at seedling and rosette stages are most effective, as the plants become more tolerant of the herbicide as they mature. Spray-grazing, where spraying with a herbicide is followed by a short interval of a couple of weeks of no grazing, then grazing the herbicide-affected plants, is also effective in reducing infestations. In any herbicide program the habitat once occupied by the weed needs to be filled with productive, competitive pasture species for ongoing farming benefit and continued weed control (Parsons & Cuthbertson 1992; Dellow 1996). Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au 

Non-chemical control: Physical control: Small patches of variegated thistle can be chipped with a hoe, provided most of the taproot is also removed.

Mechanical control: Slashing or mowing may reduce seed set, but is ineffective at killing variegated thistles. Mowing plants of Variegated Thistle before flowers open is also effective but the cut plants, now wilted, can cause nitrate poisoning if eaten by livestock, especially if conditions are moist (Parsons & Cuthbertson 1992).

Competition and management: The best defence against invasion by Variegated Thistle (Silybum marianum) on agricultural lands is to maintain healthy perennial pasture with good ground cover at all times, especially in summer going into autumn to prevent seedling establishment. On the highly fertile sites where this species is often found, summer-growing pasture species such as Phalaris and lucerne are effective in out-competing germinating and establishing thistle seedlings. Winter annual species such as ryegrass and clover generally are not effective (Parsons & Cuthbertson 1992).

Cultivation of the ground when young seedlings are present will kill those seedlings, but establishing a competitive pasture is then vital for continued control of the species.

Grazing by goats has been shown to be effective against Variegated Thistle as they selectively graze leaves and heads that are setting seed and thus can reduce overall seedset and prevent buildup of the soil seed store. Sheep grazing is ineffective (Holst & Allan 1996).

Quick grazing with high stocking rates of sheep in autumn just after thistles have germinated will greatly reduce seedling numbers. Grazing must be managed carefully however as overgrazing may weaken the pasture, allowing the thistles to colonise bare areas and successfully compete with the weakened grass.

Biological control: Variegated Thistle has also been recognised as a target for biological control through a cross-jurisdictional government process. This allowed activities to be undertaken to develop effective biological controls. The receptacle weevil Rhinocyllus conicus has been released in Victoria but its extent and effectiveness is unknown.   It is likely that this will be at its most effective in large infestations so populations of the weevil can build up (see Bruzzese et al. 1998).

Does it have a biological control agent?

YES

When does it grow? (lifecycle/growth calendar)

Seed of Variegated Thistle (Silybum marianum) mainly germinates after autumn rain, especially if the summer has been dry, but some germination will also occur in late winter and spring. Growth of the rosette stage is rapid during winter and early spring with large densely-leaved plants forming which block out light from pasture species. Stems elongate in spring with flowering starting about late October and continuing to early summer with plants dying after seed shed. If there is summer rain and temperatures are suitable flowering may continue over summer and into autumn, especially in southern Victoria and Tasmania. Plants germinating and forming a rosette after about August will over-winter before elongating and flowering (Parsons & Cuthbertson 1992; Groves & Kaye 1989).

Seed may remain viable in the soil for at least 9 years (Parsons & Cuthbertson 1992).

Where Is It Found?

Which states and territories is it found?

ACT, NSW, QLD, SA, TAS, VIC, WA

What areas within states and territories is it found?

Variegated Thistle (Silybum marianum) is found in all states, except the Northern Territory.

In the ACT occurs in  the northern areas in and around Canberra area.

In the ACT and New South Wales it is common on the coast and tablelands, western slopes and in the Riverina, with disjunct occurrences recorded in the inland around Cobar.

In Queensland it is found mostly in the Darling Downs area in the south-east of the state but occurs very sporadically in suitable habitat through the higher rainfall areas as far north as the Atherton Tableland and inland as far as Springsure and the Carnarvon Range, north of Injune.

In South Australia it occurs in the wetter agricultural areas such as the Lofty Ranges, Fleurieu Peninsula, Kangaroo Island, southern parts of the Flinders Ranges and the south-eastern region, but sporadic occurrences have been recorded in the eastern region and on the Eyre Peninsula (eFlora 2021).

In Tasmania small infestations occur in most areas but Variegated Thistle is particularly abundant in the Midlands.

In Victoria it is widespread throughout, in all but the driest areas, being particularly common in pastoral areas on the basalt plains.

In Western Australia the major occurrence is on agricultural and forestry land in the lower south-west, but it also occurs further north near Northampton (Parsons & Cuthbertson 1992; Hussey 1997; AVH 2021).

Where does it originate?

Variegated Thistle (Silybum marianum) originated in the Mediterranean region, south-western Europe, Asia Minor and Russia (i.e. Eurasia) (Navie 2004).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

SA, VIC, WA

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

Silybum marianum

Other scientific names (synonyms)?

• Carduus marianus L.
• Mariana mariana (L.) Hill

Does it have other known common name(s)?

Cabbage Thistle, Spotted Thistle, Gundagai Thistle, Gundy, Blessed Milk thistle, Holy Thistle, Milk Thistle, St Mary's Thistle, Lady's Thistle