Silybum is from the Greek silybon, a thistle like plant.
Marianum is after the Virgin Mary, who, according to legend sprinkled the leaves with milk giving them their variegated appearance.
Variegated Thistle describes the white blotched leaves and its thistle like nature.
Other names:
Bull Thistle
Blessed Milk Thistle (USA)
Boerkwasdissel (South Africa)
Cabbage Thistle (Victoria)
Gundagai Thistle (NSW)
Gundy (NSW)
Holly Thistle
Holy Thistle (Europe)
Lady's Thistle (Europe)
Milk Thistle (USA, Europe)
Spotted Milk Thistle
Spotted Thistle
St. Mary's Thistle (UK).
Summary:
Variegated Thistle is a large, prickly annual to biennial plant.
The stems of are neither spiny nor winged. The basal leaves form large rosettes in winter and are variegated with white veins and patches on the upper surface and are shallowly to deeply lobed with prickly margins. Similar smaller and clasping leaves are on the flowering stems. There are usually several stems and on each stem there is a single purple flower head which is extremely spiny and very large (up to 9 cm in diameter including the long spines). The fruits are topped by simple to minutely barbed bristles.
Native to Europe, south-west Asia and northern Africa, it is an occasional weed of pasture, plantations and roadsides. It flowers in spring and early summer.
Description:
Cotyledons:
Two. Oval, 25-40 mm long, large veins. Tip round. Base tapered. Short, broad, merging stalk, 5 mm long. Hairless. The seedling has a very short hypocotyl and no epicotyl.
First Leaves:
Grow singly, oval, 40-80 mm long. Tip round. Broad, merging stalk, 10-25 mm long. A few long multicellular hairs are on both surfaces. White blotches on top that don't follow the veins. Edges spiny and shallow lobed.
Leaves:
Forms a large rosette up to 1600 mm wide.
Petiole - Broad and merging, on lower leaves only.
Blade - Light, shiny grey-green with pale blotches and streaks along the veins on upper surface, duller on the lower surface, 150-800 mm long by 50-400 mm wide, oblong, toothed or deeply lobed with lobes ending in a fine yellow spine, spiny and wavy edges. Tip round. Veins stick out underneath. A few long multicellular hairs may be on both surfaces.
Stem leaves - Alternate, similar to rosette leaves but less lobed. Lower ones up to 500 mm long. Upper ones bent back, triangular, spiny, tip pointed, well spaced, clasp stem with rounded, spiny lobes. Short, mealy hairs.
Stems:
Erect, up to 500-3000 mm tall, fluted with lengthwise ridges, pithy, hollow toward the base, simple or much branched from the base and along their length. Hairless or a few mealy, cobweb hairs. No wings or spines.
Flower head:
Single, urn shaped flower heads, on the ends of branches, sub globular, 30-40 mm long, 30-130 mm diameter(including spines) with many long, spiny, overlapping bracts.
Flowers:
Purple, thistle type.
Bracts - Outer bracts concave around the spiny base, egg shaped to oblong, with a spreading spiny appendage and a white mealy surface.
Intermediate bracts egg shaped to oblong, up to 50 mm long, rigid, pale claws, green pointed bent back blade with spines on the edges and tip and ending in a long very sharp spine.
Innermost bracts narrow, rough on the back with smaller blades.
Florets - Purple, tubular, many, outer ones bent outwards with the upper part erect.
Ovary - Hairy receptacle.
'Petals' - Purple
Stamens - Filaments united in a tube.
Anthers -
Fruit:
Black to brown, shiny, streaked with black or pale mottled, oval, 6-8 mm long by 2.5-4 mm wide, slightly crosswise wrinkled or smooth, slightly flattened achene. Tip has a yellow or pale angled ring. 50-200 achenes per flower head. Pappus of many, simple, rough bristles, of varying length 10-20 mm long, joined at the base, on top of the achene, and fall off easily.
Seeds:
Enclosed in the fruit.
Roots:
Taproot.
Key Characters:
Stems not spiny winged, milky latex absent.
Head distinct, discoid(all florets tubular)
Involucre bracts ending in sharp tips or spines.
Receptacle beset with soft fine hairs, densely setose, originating from the splitting up of the chaffy scales.
Florets all bisexual.
Filaments united in a tube.
Pappus of simple, scabrous bristles united in a short ring at the base and falling off in one piece.
Achenes glabrous attached by their bases to the receptacle with a nearly central, horizontal basal attachment scar.
From J.M. Black and N.S. Lander.
Biology:
Life cycle:
Biennial or annual. Seed germinates mainly in autumn but smaller germinations may occur at any time of the year that moisture is present. A large rosette of leaves is produced over winter that is initially flat and becomes bushy with time. Flowering stems emerge in spring and flowers mainly in from October to December, but may continue to flower into autumn and even through winter under some conditions. Top growth normally dies off over summer and most plants die leaving the dead flowering stems standing for some months.
Physiology:
Reproduction:
By seed.
Flowering times:
Spring and summer in western NSW.
June to February in SA.
October in Perth.
Late spring to early summer in WA.
Seed Biology and Germination:
Seed may remain viable in the soil for 9 years.
Around 50% of the seed produced is viable.
Vegetative Propagules:
Hybrids:
Allelopathy:
Population Dynamics and Dispersal:
The seed has a pappus but it is small relative to the seed and easily detached. Wind plays only a some role in seed dispersal and water is probably a minor agent for spread. Most seed is spread by animals and machinery moving flower heads containing seed or as a contaminant in agricultural produce such as grain and hay. Also spread in mud attached to animals and machinery.
Establishment is greatest on disturbed or bared areas such as hoof marks, camps, warrens, firebreaks and fallows.
Densities of infestations may vary considerably between seasons. Dry summers followed by wet autumns often lead to denser infestations.
Origin and History:
Southern Europe. Mediterranean. North Africa. South western Asia. Soviet Union.
Probably introduced to Australia as a medicinal plant.
Naturalised in Tasmania by 1832, SA and VIC in the 1840's and NSW in the 1850's
Distribution:
ACT, NSW, QLD, SA, TAS, VIC, WA.
Occurs in the south, north and Midlands of Tasmania and is less common in the north-east and north-west.
4.8 million ha are infested in Victoria.
3000 ha in WA. Courtesy Australia's Virtual Herbarium
Habitats:
More abundant in wetter areas.
Climate:
Temperate. Mediterranean. Sub humid warm temperate areas.
Soil:
More abundant on granite derived soils, loams, black earths, volcanic, alluvial and fertile soils.
Plant Associations:
Significance:
Beneficial:
Medicinal herb used in the 19th century for lung, chest and liver complaints. Young leaves and stems were eaten as a vegetable to cleanse the blood, ease catarrh, tune the liver and improve milk flow in nursing mothers. Teas made from roots or seeds were used for jaundice. Flower heads and leaves were used in poultices to relieve pain. Some of these effects have been substantiated but not the practice of having a root necklace for good luck, good health and protection against snakes.
A drug, marianine, is extracted from the plant and oil is extracted from the seeds in India.
Bases of flower heads used as a vegetable.
Early source of pollen for honey bees.
Roots eaten as a Salsify substitute. Seeds roasted as a coffee substitute.
Detrimental:
Serious weed of pastures, crops, irrigated crops, cultivation, roadsides, streams, irrigation channel banks, stock yards, forestry and disturbed areas.
Strongly competitive causing severe yield reductions in pastures and crops.
Forms dense inpenetrable thickets to the exclusion of most other species.
Contaminates of wool.
Relatively unpalatable unless sprayed or wilted by cutting or as it naturally wilts in summer. It is considered to be a low value fodder.
Harbours vermin, especially rabbits.
Toxicity:
It can accumulate nitrate levels high enough to cause toxicity to stock especially after spraying with hormone herbicides like 2,4-D or after wilting. Spraying increase both the palatability and nitrate content. Stock should be excluded from sprayed fields for at least 3 weeks.
It affects cattle, occasionally sheep and rarely horses.
Avoid grazing with cattle, sheep, horses, pigs and young or breeding stock or allowing hungry stock to have access to heavy infestations.
Most toxic on high N soils, where other nutrients are limiting growth, low light levels, atmospheric drought and plentiful soil moisture.
Spines may injure stock and subsequent infections may cause production losses.
Symptoms:
Muscle trembling, weakness, staggering, collapse, breathlessness, blue lips, gums and inside eyelids and chocolate coloured blood, collapsed animals may show violent twitching when touched but are unable to rise. Death normally occurs with a few hours of symptoms showing and is often preceded by convulsions.
Treatment:
Nitrate poisoning.
Legislation:
Noxious weed of SA, VIC, TAS and WA.
Management and Control:
Perennial grasses such as Phalaris spp. and Cocksfoot (but not Perennial Ryegrass) and Lucerne have been used to reduce Variegated Thistle infestations.
It readily invades annual pastures and Perennial Ryegrass.
Slashing and mowing can reduce seed set but must be done early before the buds open otherwise fertile seed forms in the cut heads. It usually needs repeating to control regrowth.
Goats have been used to reduce infestations but it takes a few years.
Hormone herbicides provide good control of young rosettes and seedlings but usually requires repeat treatments to control late emerging seeds. Of the hormone herbicides, Lontrel usually gives the best control.
Glyphosate is effective where total vegetation control is desirable.
Thresholds:
Eradication strategies:
Prevent seed set for several years.
Mowing is not very effective because the plant regrows.
Manual removal is very difficult because of the extensive root system and prickly nature.
Blanket wipers applying 3 L/ha glyphosate(450g/L) or 2 L/ha paraquat(250g/L) at flowering can provide partly selective control. Overall spraying with 200 g/ha Lontrel®750 or 4 g in 10 L water provides reasonably selective control in bushland. Spray-grazing with 20 g/ha Lontrel®750 plus 500 mL/ha MCPA(500g/L) provides partial control in Clover pasture. Replant perennial species to increase shading.
For isolated plants, spray with a mixture of 100 mL Grazon® plus 4 g Lontrel®750 in 10 L water. Remove and burn any flowering stems. Spraying a five metre buffer around each plant will help control late seedlings. This treatment will damage most young broad-leaved species.
Cultivation provides variable control. Grazing with sheep to reduce pasture then grazing with goats at flowering provides good control in 3 years.
It is occasionally toxic to stock, especially where there are high levels of soil nitrogen or after spraying.
Herbicide resistance:
Biological Control:
Aphids occasionally damage plants.
A Nodding Thistle weevil has been released which may cause some damage has been released and a fungus is under investigation.
Related plants:
None in this genus.
Plants of similar appearance:
Slender thistle, Sow Thistle, Spear Thistle.
References:
Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P115. Photos.
Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). P934. Diagrams.
Burbidge, N.T. and Gray, M. (1970). Flora of the Australian Capital Territory. (Australian National University Press, Canberra). P393-394. Diagrams.
Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P724-725. Photo.
Everist, S.L. (1974). Poisonous Plants of Australia. (Angus and Robertson, Sydney). P185, Diagrams. Plate 4.
Gardner, G.A. and Bennetts, H.W. (1956). The toxic plants of Western Australia. (West Australian Newspapers Ltd, Perth). P196-201. Diagrams.
Hussey, B.M.J., Keighery, G.J., Cousens, R.D., Dodd, J. and Lloyd, S.G. (1997). Western Weeds. A guide to the weeds of Western Australia. (Plant Protection Society of Western Australia, Perth, Western Australia). P104. Photo.
Hyde-Wyatt, B.H. and Morris, D.I. (1975). Tasmanian weed handbook. (Tasmanian Department of Agriculture, Hobart, Tasmania). P30-31. Diagrams.
Hyde-Wyatt, B.H. and Morris, D.I. (1980) The Noxious and Secondary Weeds of Tasmania. (Tasmanian Department of Agriculture, Hobart, Tasmania). P46-47. Diagrams.
Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).
Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #1134.1.
Marchant, N.G., Wheeler, J.R., Rye, B.L., Bennett, E.M., Lander, N.S. and Macfarlane, T.D. (1987). Flora of the Perth Region. (Western Australian Herbarium, Department of Agriculture, Western Australia). P706.
