Silybum marianum (variegated thistle)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Biology and Ecology
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Causes
- Impact Summary
- Economic Impact
- Environmental Impact
- Social Impact
- Risk and Impact Factors
- Uses List
- Similarities to Other Species/Conditions
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Silybum marianum (L.) Gaertn.
Preferred Common Name
- variegated thistle
Other Scientific Names
- Carduus mariae Crantz.
- Carduus marianus L.
- Carthamus maculatum (Scop.) Lam.
- Cirsium maculatum Scop.
- Mariana lactea Hill
- Mariana mariana (L.) Hill
- Silybum maculatum (Scop.) Moench,
- Silybum mariae (Crantz.) Gray
International Common Names
- English: blessed milk thistle; blessed milkthistle; Holy thistle; Lady's thistle; St. Mary's thistle
- Spanish: Cardo asnal; Cardo blanco; Cardo lechero; Cardo mariano; Cardo santo; Poma
- French: Chardon argente; Chardon marie; Chardon Notre Dame
- Portuguese: cardo-leiteiro
Local Common Names
- Arabic: shawk el-gamel
- English: blessed thistle; bull thistle; cabbage thistle; gundegai thistle; gundy; holy thistle; lady's thistle; Marian thistle; Mary's thistle; Marythistle; milk thistle; spotted thistle; St. Mary's thistle; variegated artichoke; variegated thistle
- French: chardon argente; chardon Marie; lait de Nore Dame; silybe de Marie
- Russian: ostro-pestro; rastoropsa pjatnistaja
- Spanish: cardo de Maria; cardo lechero; cardo mariano; cardo santo
- Germany: Gemeine Mariendistel; Mariendistel
- Italy: Cardo di Maria; Cardo mariano
- Netherlands: Mariadistel
- Portugal: cardo-leitero
- Sweden: mariatistel
- SLYMA (Silybum marianum)
Summary of InvasivenessTop of page
S. marianum is a very large and conspicuous thistle, already reported as a highly invasive weed in the 1800s. It can grow up to 2 m tall and has a very spiky flowerhead and large variegated, prickly leaves. Originally native to the Mediterranean region, parts of Asia and Russia, it has been introduced to Japan, sub-Saharan Africa, the Americas, parts of Europe, Australia and New Zealand. S. marianum has become a seriously invasive weed in Australia, parts of the USA and South America, South Africa and in New Zealand. It invades pastures where, once established, it can eliminate most other plant species. It can also hinder movement of livestock and people. It is a weed of cultivated land as well as being a nuisance along roadsides and in waste areas. In addition to problems and injuries caused by its spiny thistles, S. marianum can, under certain conditions, accumulate nitrogen and become seriously toxic to livestock.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Asterales
- Family: Asteraceae
- Genus: Silybum
- Species: Silybum marianum
Notes on Taxonomy and NomenclatureTop of page
S. marianum can apparently hybridise with S. eburneum. S. marianum may be a former cultivar descended from S. eburneum, a suggestion supported by its large achenes, which are unsuited to wind dispersal but would be encouraged by selection as an oil plant (Goeden, 1976).
DescriptionTop of page
Slightly modified from Webb et al. (1988):
Annual or biennial. Stems branched above, ridged, with sparse mealy hairs at least above, 0.5-2 m tall, not winged. Leaves elliptic to lanceolate, lyrate-pinnatifid to pinnate, sinuate, coarsely dentate, green with conspicuous white markings along veins, (10)-20-60 × (5)-10-30 cm, with sparse short mealy hairs on lamina, and sparse long tangled multicellular hairs on midrib; base amplexicaul, auriculate, with very spinous margins; prickles marginal, yellowish, spreading, 5-12 mm long. Capitula ovoid, erect, 4-6 × 5-7 cm, solitary, terminal and pedunculate, and also sessile in axils of uppermost lvs; peduncles with appressed cobwebby tomentum. Involucral bracts sparsely covered with short mealy hairs; margins with sparse cobwebby hairs. Outer bracts leaflike, obovate with spinous apex and margins. Middle bracts oblong; appendage ovate, subulate, with spinous margins and a long spreading to recurved apical spine. Inner bracts lanceolate; appendage becoming linear-lanceolate, entire. Corolla normally reddish purple, 20-28 mm long; lobes unequal, 4-6 mm long. Occasionally albino forms with white flowers are found (Healy, 1969). Anther filaments joined at margins into a tube which encloses the basal appendages of the anthers. Style exserted 1-2 mm beyond corolla lobes. Achenes brown or black-streaked, obovoid, weakly transversely flattened, smooth, about 6 × 3 mm; outer pappus bristles scabrid, about 15 mm long; inner hairs very fine.
Plant TypeTop of page
DistributionTop of page
S. marianum is native to the Middle East, Arabian peninsula, the Indian subcontinent, North Africa and parts of Europe. It has been introduced to Japan, sub-Saharan Africa, the Americas, parts of Europe, Australia and New Zealand.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 25 Feb 2021
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|South Africa||Present||Introduced||Eastern cape, Gauteng|
|-Jammu and Kashmir||Present||Native|
|Israel||Present||Native||Invasive||Weed in ornamental crops|
|Bosnia and Herzegovina||Present||Native|
|Russia||Present||Present based on regional distribution.|
|-Central Russia||Present||Native||Ciscaucasia, Kurgan|
|Canada||Present||Present based on regional distribution.|
|-District of Columbia||Present||Introduced|
|-New South Wales||Present|
|-South Australia||Present, Widespread||Introduced||Invasive||South-eastern parts; Original citation: Weeds of Australia (2013)|
|-Tasmania||Present||Introduced||Invasive||Original citation: Weeds of Australia (2013)|
|-Western Australia||Present, Widespread||Introduced||Invasive||South-westernareas; Original citation: Weeds of Australia (2013)|
|-Rio Grande do Sul||Present||Introduced|
History of Introduction and SpreadTop of page
S. marianum originated in the Mediterranean region, western Asia and Russia, from where it has been spread to most temperate areas of the world.
Parsons and Cuthbertson (1992) suggested it was introduced to Australia as a medicinal plant. In Tasmania in 1832 the Hobart Town Courier expressed concern at it overrunning fertile soil (Parsons and Cuthbertson, 1992). Elsewhere in Australia it was of enough concern to be included in the South Australian and Victorian noxious weed legislation in 1851 and 1861.
Darwin reported the invasiveness of S. marianum in South America in 1873: ‘very many (probably several hundred) square miles are covered by one mass of these prickly plants, and are impenetrable to man or beast. Over the undulating plains, where these great beds occur, nothing else can now live’ (Parsons and Cuthbertson, 1992).
In California S. marianum spread rapidly through the central valleys in the 1940s (Cal-IPC, 2013). King County (2013) reported that it was first found in King County, Washington, USA, in 1999 in an urban medicinal garden.
IntroductionsTop of page
|Introduced to||Introduced from||Year||Reason||Introduced by||Established in wild through||References||Notes|
|Natural reproduction||Continuous restocking|
|Australia||1857||Yes||Australia’s Virtual Herbarium (2013); Royal Botanic Gardens Sydney (2004); Royal Botanic Gardens Sydney (2013)||In Tasmania|
|California||1854||Yes||Cal-IPC (California Invasive Plant Council) (2013)|
Risk of IntroductionTop of page
S. marianum has already been introduced to many countries, often in the first instance as a medicinal plant, where it has then become naturalised and spread to become a problem. S. marianum may not yet have naturalised in all the countries where it is grown as a crop and in such places there is a high risk of its naturalisation and its spread as a problem weed. PIER give the species a high risk assessment of 17.5 (PIER, 2013).
Trumble and Donald (1938) predicted that thistles would become more dominant in pastures in Australia as superphosphate application and biological nitrogen fixation, associated with increased clover growth, led to increased soil fertility; Austin et al. (1985) commented that ‘their prediction seems to have been borne out.’
HabitatTop of page
S. marianum grows in crop fields, pastures, forestry plantations, coastal areas, roadsides, disturbed areas and waste areas in temperate, sub-tropical and sometimes semi-arid regions (Weeds of Australia, 2013). It grows best in areas of high fertility, such as alluvial flats, sheep camps, stock yards and other places with high soil nitrogen levels (Parsons and Cuthbertson, 1992; Holm et al., 1997).
S. marianum can establish in grasslands if summers are dry enough to cause breaks in the soil cover (Holm et al., 1997).
Gabay et al. (1994) observed that S. marianum tends to dominate around the nests of the harvester ant Messor semirufus. This ant takes the seeds back to its nest, removes the elaiosome and dumps the seed in the nest refuse zone.
Habitat ListTop of page
|Terrestrial||Managed||Cultivated / agricultural land||Principal habitat|
|Terrestrial||Managed||Managed grasslands (grazing systems)||Principal habitat|
|Terrestrial||Managed||Disturbed areas||Principal habitat|
|Terrestrial||Managed||Rail / roadsides||Principal habitat|
|Terrestrial||Natural / Semi-natural||Riverbanks||Principal habitat|
|Littoral||Coastal areas||Principal habitat|
Biology and EcologyTop of page
2n = 34 (Flora of North America, 2013).
Groves and Kaye (1989) found that some degree of winter cold was needed before the plants of S. marianum could flower in southern Australia. Thus plants sown in August or earlier (autumn) flowered in the following summer (early November to early February); those sown in September or later did not flower until their second summer. The cotyledons of the seedlings are large (1.2-1.9 x 1.9-2.5 cm) and succulent, which probably gives plants an early advantage over seedlings of other species (Bean, 1985).
Field populations of S. marianum in South Australia have a flowering season that lasts about 2 months and ends in late November or early December, but the length of the flowering season can vary considerably in response to environmental factors, primarily rainfall or water supply (Dodd, 1989). In Tasmania, for example, during favourable seasons flowering can continue throughout the summer or even into winter.
The primary flower head (capitulum) produces an average of 190 seeds; later capitula produce fewer as the season progresses, down to about 100 (Dodd, 1989). Estimates of seed produced per plant ranged from 4 per plant to 10,340, and seed viability was high at an average of 94% (Dodd, 1989). Although seed production is high, most seeds fall to the ground within a few metres of the parent plants. In California, dense stands of S. marianum can produce 1.4 million seeds per acre (3.5 million ha-1). Seed production per plant can vary from 1.2 heads producing 42 seeds to 8.8 heads producing 876 seeds (Cal-IPC, 2013).
Physiology and phenology
Young et al. (1978) established that fresh seeds of S. marianum required a period of after-ripening before they could germinate. After-ripening widened the range of temperatures at which the seeds would germinate readily. Optimum germination occurred with 16 hour cold periods (2o to 15oC) alternating with 8 hour warm periods (10o to 30oC). Emergence of seedlings decreased with increased burial depth, but substantial emergence occurred from as deep as 8 cm. Germination on the surface of the soil or of litter was much lower than if the seeds were slightly covered by soil or litter. Potassium nitrate tended to enhance germination.
S. marianum seeds germinate most readily in open areas such as sheep camps, rabbit warrens and cultivated soil (Phung and Popay, 1981). Bare ground in autumn provides the perfect conditions for germination and early growth (Parsons and Cuthbertson, 1992).
Groves and Kaye (1989) assessed the germination of fresh seed of S. marianum and found that seeds germinated over a wide temperature range (15o/5oC to 40o/30oC), but that germination was higher in 1-year old seed. When placed under water stress induced by polyethylene glycol in solution, seeds showed almost no germination at -0.75 MPa and only limited germination at -0.5 MPa. Radicle extension was much reduced at a water potential of -0.5 MPa, although it still proved possible to some extent even at -1.5 MPa.
Ghavani and Ramin (2007) tested the salt tolerance of seed germination in S. marianum to understand its ability to grow and produce silymarin in the saline coastal areas of Khuzestan in Iran. They tested seeds of an Iranian wild type and a German cultivated line ‘Royston’ at seven salt concentrations and three temperatures and found that germination and numbers of normal seedlings were higher at 15oC than at 25o or 35oC. Seeds of both types germinated satisfactorily up to a conductivity of 6 dS m-1 (deciSiemens per metre) at 15oC but at higher temperatures germination was only satisfactory at up to 3 dS m-1.
Gabay et al. (1994) observed that S. marianum tends to dominate around the nests of the harvester ant Messor semirufus. This ant takes the seeds back to its nest, removes the elaiosome and dumps the seed in the nest refuse zone. The authors compared the germination of S. marianum seeds (achenes) when 1) seeds with elaiosomes attached were collected from parent plants; 2) seeds without elaiosomes were collected in the refuse zone of ants’ nests; and 3) seeds had their elaiosomes removed with forceps. Germination rates were not affected at all by elaiosome removal, whether by ants or humans. The same authors tested the effects of seeds, leaf pieces and leaf extracts of S. marinum on the germination of Erucaria rostrata and Beta vulgaris and found that none of the treatments significantly affected germination of the test species and all seedlings appeared normal. The authors concluded that the dominance of S. marianum on the sites of ants’ nests was not due to allelopathy but to other advantageous traits such as rapid growth and gain of biomass. Gabay et al. (1994) also found that S. marianum seems to have a competitive advantage over most of its companion species close to ants’ nests, largely because of its faster growth and biomass accumulation on soils rich in nitrogen and organic matter. Height, flower heads per plant, receptacle diameter and mean number of achenes per head were all significantly higher on nutrient-enriched sites such as the refuse zones of ants’ nests and in waste places than on nearby ‘control’ sites.
In Australia, S. marianum seeds germinate mainly after autumn rains but there is also some emergence in late winter and spring (Parsons and Cuthbertson, 1992). Autumn seedlings develop into small rosettes by early winter. Rapid growth in winter and early spring produces very large rosettes and ‘cabbage-like growth’, from which flowering stems develop in spring. Flowering begins in late October and continues into early summer. Plants die during summer but the dead stems can remain standing for several months. In parts of Victoria and Tasmania flowering may continue into autumn or even winter if moisture and temperature are suitable. A dry summer followed by a wet autumn appears ideal for seed germination and seedling establishment.
As with other thistle species, infestations of S. marianum vary from year to year, almost certainly because of pasture conditions in the previous late summer and autumn (Parsons and Cuthbertson, 1992).
The plants of S. marianum themselves are annuals or biennials so never live very long, but populations tend to be self-perpetuating since the seedbed formed by dead and decaying leaves of the parent plants provides good conditions for seed germination and early seedling growth. Seeds can remain viable in the soil for at least 9 years (Parsons and Cuthbertson, 1992).
Austin et al. (1985) compared the growth of six species of thistles at 12 different nutrient concentrations (1/64 to 16 times the normal concentration of Hoagland No. 2 nutrient solution) either in monocultures or in mixtures of all six species. S. marianum did not reach its maximum physiological performance, even at the highest nutrient concentration, and performed better in mixtures than it did in monoculture. At the highest nutrient concentration, S. marianum was the most competitive of the six thistle species, a result consistent with those of Doing et al. (1969). In further work, this time on competition between S. marianum and Cirsium vulgare at five different densities and six nutrient concentrations, Austin et al. (1985) found that the shoot yield of S. marianum was similar in both monoculture and mixture, but with higher yields in mixture at relative nutrient concentrations greater than 1.
Gabay et al. (1994) listed plants associated with S. marianum on ant nests in Israel, part of its native range. The predominant species were Avena sterilis, Hordeum spontaneum, Hirschfeldia incana, Avena barbata, Malva parviflora and Lolium rigidum. Elsewhere, S. marianum is associated with a wide range of other species that live in waste places and in pastures.
S. marianum germinates and grows best in full daylight without any shade from other vegetation, and prefers a nutrient-rich substrate, especially high in nitrate. S. marianum grows best on areas of high fertility such as alluvial flats, sheep camps, stock yards and other places with high soil nitrogen levels (Parsons and Cuthbertson, 1992). Trumble and Donald (1938) predicted that thistles would become more dominant in pastures in Australia as superphosphate application and biological nitrogen fixation, associated with increased clover growth, led to increases in soil fertility; Austin et al. (1985) commented that ‘their prediction seems to have been borne out.’
ClimateTop of page
|Cf - Warm temperate climate, wet all year||Preferred||Warm average temp. > 10°C, Cold average temp. > 0°C, wet all year|
|Cs - Warm temperate climate with dry summer||Preferred||Warm average temp. > 10°C, Cold average temp. > 0°C, dry summers|
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
Notes on Natural EnemiesTop of page
Goeden (1976) compared the insects associated with S. marianum in southern California, USA, where 48 species were of minor importance as natural enemies, with those in Italy and Greece, where over 115 species were identified on S. marianum, many occupying a variety of food niches, and about 35% of them reproducing on the species.
Means of Movement and DispersalTop of page
Natural dispersal (non-biotic)
S. marianum is only spread by seed. Although each seed has a large pappus and the parent plants are very tall, the large and heavy seed does not move very far from the parent plants. Seeds may also spread in water.
Vector transmission (biotic)
Guthrie-Smith (1953) deduced that seeds must have been taken to his New Zealand sheep station by feral pigs (Sus scrofa).
In Israel, in its native environment, the seeds of S. marianum are dispersed by ants (Danin and Yom-Tov, 1990; Gabay et al., 1994). The seeds have an oily body (elaiosome) that is attractive to ants. Harvesting ants (Messor semirufus) carry the seeds back to their nests, remove the elaiosome and then discard the seed itself in the nest refuge zone, where conditions of high soil fertility, good aeration and moisture encourage it to germinate and grow free of competition from other species.
Seeds are probably carried in hay, since the plants are in seeds when hay is cut and baled, or transported in mud or dirt carried by vehicles and farm machinery (Parsons and Cuthbertson, 1992). In the UK, S. marianum can be transported in wool shoddy (recycled woollen fabric imported for processing) (Biological Records Centre, 2013).
S. marianum has been intentionally introduced to North and probably South America, Australia and New Zealand. The plants have long been valued as a source of medicines and as a food source.
Pathway CausesTop of page
Impact SummaryTop of page
|Environment (generally)||Positive and negative|
Economic ImpactTop of page
Besides sometimes occupying large areas of pasture space and replacing valuable nutritious pasture grasses and legumes, S. marianum can also restrict access of livestock to areas of pasture and can increase the difficulties of handling livestock. It can provide shelter and refuge for pests such as rabbits, rats and stoats (Parsons and Cuthbertson, 1992; Cal-IPC, 2013). Insects that are economically damaging to various crops shelter in and sometimes feed on its leaves and flowers (Cal-IPC, 2013). Even in its native environment, in Israel, S. marinum has become a problem weed among crops of cultivated ornamental species (Buxbaum et al., 1999). It is regarded as an environmental weed of some importance in New South Wales and Victoria, and subject to special regulations in parts of Victoria, South Australia and Western Australia.
In King County, Washington, USA, S. marianum is a Class A Noxious Weed, harmful to environmental and economic resources. It is regarded as a serious threat to livestock because its spines can injure animals (King County, 2013).
Connor (1977) reported on poisoning in cattle that have eaten S. marianum vegetation in New Zealand, Australia, California and Argentina. Sheep can also be affected. Both young and mature cattle can be affected, particularly hungry or travelling animals. The plant does not become less harmful on drying, so hay infested with the thistle may still be toxic. The toxicity is due to considerable amounts of nitrate in the plant. In the rumen the nitrate is microbially reduced to nitrite, which then reacts with haemoglobin in the blood, leading to the condition known as methaemoglobinaemia, which causes affected animals to stagger, vomit and have laboured breathing, resulting in death if not treated. Nitrate poisoning is associated with a number of crop species and some weeds, and tends to be most serious when heavy rainfall follows a prolonged drought, when heavy dressings of fertiliser are applied to crops or after plants have been treated with herbicide.
Environmental ImpactTop of page
S. marianum can have dramatic impacts on habitats in the countries to which it has been introduced. In Australasia and the Americas it can sometimes occupy large areas of valuable pasture land, replacing useful forage species and hindering access of livestock to those forage plants. Elsewhere, though, it tends to be a weed of roadsides and waste areas, which are both places where wide assemblages of alien plant species tend to congregate.
Sultana and Asaduzzaman (2012) found some effects of both hot and cold water extracts of S. marianum in reducing the germination and growth of Lolium rigidum and Brassica napus. Khan et al. (2011) also found allelopathic effects when cold water extracts of S. marianum at two strengths were tested on germination characteristics of a range of legumes (Phaseolus vulgaris, Vigna radiata, Cicer arietinum, Glycine max). They found that the extracts significantly reduced the germination percentage, mean germination time, germination index and seed vigour index of all the tested legume species, but that P. vulgaris and V. radiata were less affected than the other species. The authors suggested that preventing further spread of S. marianum was important to prevent future contamination of leguminous crops in Pakistan.
Social ImpactTop of page
S. marianum is an important food source in many countries, whether native or introduced. However, dense stands are very uncomfortable for people to move through, even on horseback.
Risk and Impact FactorsTop of page
- Proved invasive outside its native range
- Has a broad native range
- Tolerant of shade
- Has high reproductive potential
- Reproduces asexually
- Negatively impacts agriculture
- Negatively impacts human health
- Competition - monopolizing resources
- Competition - shading
- Pollen swamping
- Produces spines, thorns or burrs
UsesTop of page
S. marinum is sometimes cultivated as an ornamental, a minor vegetable or as a medicinal herb (Flora of North America, 2013). The flowers provide a useful source of pollen for bees in early summer (Parsons and Cuthbertson, 1992).
S. marianum is an important food source in many countries, whether native or introduced. It is grown as a commercial crop in several countries, including Iran and Pakistan. It is being considered as a crop in Saskatchewan (Govt. of Saskatchewan, 2013). S. marianum has also been considered, at least in Sardinia, as a possible biomass crop for the production of bioenergy (Murgia et al., 2008). Until the 1930s it was widely cultivated as an oil-seed plant in Russia (Goeden, 1976).
S. marianum has long been used for its medicinal qualities (Chevalier, 1996, quoted in PFAF, 2013), traditionally for depression and liver problems. In the 19th century it was considered an effective treatment for lung, chest and liver problems. The young stems and leaves of S. marinum can be eaten raw or boiled as a vegetable, and are claimed to have health-giving properties in cleansing the blood, tuning the liver, easing catarrh and improving milk flow in nursing mothers (Parsons and Cuthbertson, 1992). All parts of the plant have been utilised – the roots and seeds are made into infusions to relieve jaundice and the leaves and heads are used as poultices to ease pain. The seeds can be used as a coffee substitute. Plant roots have also been regarded as valuable talisman, and a piece of root worn around the neck supposedly brings good luck as well as protecting against snakes and disease. It can be used internally in the treatment of liver and gall bladder diseases, jaundice, cirrhosis, hepatitis and poisoning (Bown, 2003, cited in PFAF, 2013). Zohary (1962) reported on it use as a vegetable among the bedouin of Palestine.
A flavonoid component of the seed (silybin) is reported to be clinically useful in the treatment of poisoning by mushrooms of the genus Amanita (Foster and Duke, 2008, cited in PFAF, 2013). Seeds contain the bioactive principle silymarin, a collection of alkaloids used for curing a range of spleen, liver and gallbladder problems. A number of papers have been written on improving yields of seeds of S. marianum and their content of silymarin (Omidbaigi and Nobakht, 2001; Zheljazkov et al., 2006; Geneva et al., 2007; Ibrahim et al., 2007). There is a difference in silymarin content between white and purple flowered varieties of S. marianum.
Uses ListTop of page
- Sociocultural value
Human food and beverage
- Leaves (for beverage)
- Source of medicine/pharmaceutical
Similarities to Other Species/ConditionsTop of page
S. marianum can be confused with other thistles. S. marianum itself has ‘wingless’ stems, very distinctive variegated leaves and large flower heads (up to 13 cm across, including the spines) subtended by a circle of long spines. The globe artichoke Cynara cardunculus also has tall wingless stems and large, spiny flower heads, but its leaves are grey-green and are not spiny. The cotton thistle Onopordum acanthium has winged stems, bluish-grey leaves and smaller flower heads with smaller spines. O. illyricum has winged stems, whitish coloured leaves and moderately large flower heads with relatively small downward-pointing spines. Carduus nutans, the musk thistle, has winged stems, greenish coloured leaves, moderately large flower heads that droop with age, and which have small spines on the bracts below the flower head. Spear thistle Cirsium vulgare has winged stems, greenish leaves and moderately sized flower heads with small spines. Creeping thistle Cirsium arvense has wingless stems, greenish coloured leaves and small, slender flower heads.
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Grubbing or digging out plants before they flower helps control the weed, taking care not to disturb the soil more than is necessary (King County, 2013). 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. Dodd (1989) found that mown plants produced viable seed at any stage of flowering and a small but significant numbers of viable seeds developed even in advanced buds. Livestock may be poisoned by eating cut foliage if its nitrogen level is high (Parsons and Cuthbertson, 1992).
Although mown plants still produce some seeds, mowing before the start of flowering would reduce production of viable seeds, provided that none of the buds were fully developed and near opening (Dodd, 1989). The same author also suggested that cutting the plants into small pieces may reduce production of viable seeds still further, provided the cut rosettes did not resprout and resume flowering, as can happen in wetter summers. Following the removal of plants, the site should then be monitored and any new plants rigorously removed.
Care must be taken not to spread seed on agricultural machinery such as tractors, mowers or other vehicles, or in agricultural produce such as hay (Jefferson County, 2013).
Parsons and Cuthbertson (1992) recommended a vigorous, dense pasture as the best protection against S. marianum. Appropriate perennial pasture species such as perennial ryegrass (Lolium perenne), cocksfoot (Dactylis glomerata) or phalaris (Phalaris aquatica) can both utilise high levels of nitrogen in the soil and form swards dense enough to limit the germination of seeds of S. marianum and outcompete thistle seedlings. Using the correct species and cultivars for local conditions is important, as is managing pastures so that good cover is available in autumn, the time when thistle seeds tend to germinate. Well-established and well-managed lucerne (Medicago sativa) also competes well with S. marianum.
Classical biological control efforts have been limited, but some strains of the nodding thistle receptacle weevil Rhinocyllus conicus, which severely damage heads of S. marianum, have been released in the USA and in Australia (Parsons and Cuthbertson, 1992). However, this weevil can also attack native American thistles. Furthermore, Dodd (1989) pointed out that the timing of oviposition in R. conicus may not coincide with development of the flower heads in S. marianum, at least in Australia. Strains of a plant pathogen in the genus Alternaria have been studied in the USA as a possible mycoherbicide (Parsons and Cuthbertson, 1992).
The introduction of synthetic herbicides like 2,4-D in the 1950s had a dramatic effect in reducing S. marianum populations in New Zealand (Arthur Healy, pers. comm). S. marianum is susceptible to 2,4-D or MCPA when it is young, but its resistance to herbicides increases with age. Although most seedlings emerge in autumn in Australia and New Zealand, some seeds will germinate in late winter or early spring, so treating infestations in early spring may kill two cohorts of seedlings (Parsons and Cuthbertson, 1992). Dicamba is sometimes added to 2,4-D or MCPA to make them slightly more effective, and to give a certain amount of residual activity.
Non-selective herbicides like glyphosate, triclopyr or picloram are often used as herbicides for spot-treatment of individual plants or of dense patches.
Zheljazkov et al. (2006) described herbicides which can be used in cultivated crops of S. marianum in Bulgaria. These include metribuzin, pendimethalin, pendimethalin plus metribuzin, and trifluralin plus linuron. They found that effective weed control with chemicals or by hand weeding increased the amount of silymarin in the seeds but decreased the amount of seed oil (linoleic and oleic acids).
In winter cereals methabenzthiazuron or a bromoxynil/MCPA mixture is used for selective control, or 2,4-DB for crops undersown with a legume. Bentazone is used for control in pea and bean crops and simazine can be used as a residual herbicide in fruit and ornamental crops (Parsons and Cuthbertson, 1992).
In Pakistan, Marwat and Khan (2007) examined the effects of seed rates of wheat (Triticum aestivum) in suppressing S. marianum and found that in one year a high seed rate (up to 160 kg ha-1) reduced the height, leaf area, biomass and seed yield of S. marianum, but that in the following year, when rainfall was higher and temperatures lower, seed rates did not affect S. marianum growth. The authors suggested that lowering the seed production of S. marianum and other weeds is the best long term management solution.
Control by utilization
The leaves and especially the flower heads of S. marianum are very prickly and are usually avoided by livestock, except when livestock are very hungry or are being driven; this can be dangerous because, under some circumstances, the vegetation accumulates nitrates which can be poisonous. Goats happily eat the flower and seed heads; goats should be moved so that they can eat new heads as they emerge.
Bean (1985) stated that, with proper management, affected areas can be restored to more desirable vegetation. Recommended management practices are to reduce grazing or remove the source of disturbance and introduce a native replacement plant. A dense, uniform sward of grass can be a very effective barrier to S. marianum seedlings.
ReferencesTop of page
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ContributorsTop of page
16/11/13 Original text by:
Ian Popay, consultant, New Zealand, with the support of Landcare Research.
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