Onopordum acanthium

Cotton Thistle
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Asterids
Order:	Asterales
Family:	Asteraceae
Subfamily:	Carduoideae
Tribe:	Cynareae
Genus:	Onopordum
Species:	O. acanthium
Binomial name
Onopordum acanthium L.

Onopordum acanthium (Cotton Thistle), is a flowering plant in the family Asteraceae. It is native to Europe and Western Asia from the Iberian Peninsula east to Kazakhstan, and north to central Scandinavia, and widely naturalised elsewhere. It is a vigorous biennial plant with coarse, spiny leaves and conspicuous spiny-winged stems.

Description

Detailed image

Cotton Thistle is a biennial plant, producing a large rosette of spiny leaves the first year. The plants typically germinate in the autumn after the first rains and exist as rosettes throughout the first year, forming a stout, fleshy taproot that may extend down 30 cm or more for a food reserve.

In the second year, the plant grows (0.2–) 0.5–2.5 (–3) m tall and a width of 1.5 m. The leaves are 10–50 cm wide, are alternate and spiny, often covered with white woolly hairs and with the lower surface more densely covered than the upper. The leaves are deeply lobed with long, stiff spines along the margins. Fine hairs give the plant a greyish appearance. The massive main stem may be 10 cm wide at the base, and is branched in the upper part. Each stem shows a vertical row of broad, spiny wings (conspicuous ribbon-like leafy material), typically 2–3 cm wide, extending to the base of the flower head.

The flowers are globe shaped, 2–6 cm in diameter, from dark pink to lavender, and are produced in the summer. The flower buds form first at the tip of the stem and later at the tip of the axillary branches. They appear singly or in groups of two or three on branch tips. The plants are androgynous, with both pistil and stamens, and sit above numerous, long, stiff, spine-tipped bracts, all pointing outwards, the lower ones wider apart and pointing downwards. After flowering, the ovary starts swelling and forms about 8,400 to 40,000 seeds per plant.

Distribution and habitat

The plant prefers habitats with dry summers, such as the Mediterranean, growing best in sandy, sandy clay and calcareous soils which are rich in ammonium salts. It grows in ruderal places, as well as dry pastures and disturbed fields. Its preferred habitats are natural areas, disturbed sites, roadsides, fields, and especially sites with fertile soils, agricultural areas, range/grasslands, riparian zones, scrub/shrublands valleys and plains along with water courses.

Ecology

Cotton Thistle reproduces only by seeds. Most seeds germinate in autumn after the first rains, but some seeds can germinate year round under favourable moisture and temperature conditions. Seeds that germinate in late autumn become biennials. But when they germinate earlier, they can behave as annuals. Buried seed can remain viable in the soil seed bank for at least seven years and possibly for up to twenty years or more. Yearly seed production and seed dormancy are highly variable depending on environmental conditions. The slender and smooth achenes are about 3 mm long and are brown with gray markings. They are tipped with a pappus of slender bristles. Mainly locally dispersed by wind, or more widely by humans, birds, wildlife, livestock or streams, the seeds are sensitive to light and only germinate when close to the surface. Seedlings will emerge from soil depths up to 4.5 cm, with 0.5 cm being optimal.

The leaves of Cotton Thistle provide food for the caterpillars of some Lepidoptera, such as the Thistle Ermine (Myelois circumvoluta).

Taxonomy and naming

Three subspecies are accepted:

• Onopordum acanthium subsp. acanthium. Most of the species' range.
• Onopordum acanthium subsp. gautieri (Rouy) Franco. France, Spain.
• Onopordum acanthium subsp. parnassicum (Boiss. & Heldr.) Nyman. Greece.

The botanical name is derived from the Ancient Greek words onos (donkey), perdo (to consume), and acanthos (thorn), meaning 'thorny donkey food'.

The common name of Cotton Thistle derives from the cotton-like hairs on the leaves. Other names include Scots thistle or Scottish thistle, heraldic thistle and woolly thistle; the name Scots thistle comes from its status as the national emblem of Scotland.

Cultural significance

Scottish thistle as a Heraldic badge.

The thistle has been the national emblem of Scotland since the reign of Alexander III (1249–1286) and was used on silver coins issued by James III in 1470. According to the legend, an invading Norse army was attempting to sneak up at night upon a Scots army encampment. During this operation one barefoot Norseman had the misfortune to step upon a Scots Thistle, causing him to cry out in pain, thus alerting Scots to the presence of the Norse invaders. Some sources suggest the specific occasion was the Battle of Largs, which marked the beginning of the departure of the King Haakon IV (Haakon the Elder) of Norway who, having control of the Northern Isles and Hebrides, had harried the coast of the Kingdom of Scotland for some years. Which species of thistle is referred to in the original legend is disputed. Popular modern usage favours Onopordum acanthium, perhaps because of its more imposing appearance, though it is unlikely to have occurred in Scotland in mediaeval times; the Spear Thistle Cirsium vulgare, an abundant native species in Scotland, is a more likely candidate. Other species, including Dwarf Thistle Cirsium acaule, Musk Thistle Carduus nutans, and Melancholy Thistle Cirsium heterophyllum have also been suggested.

Uses

Cotton Thistle is grown as an ornamental plant for its bold foliage and large flowers. It has been used to treat cancers and ulcers and to diminish discharges of mucous membranes. The receptacle was eaten in earlier times like an artichoke. The cottony hairs on the stem have been occasionally collected to stuff pillows. Oil from the seeds has been used for cooking.

Cotton Thistle as an invasive species

In the late 19th century, it was introduced to temperate regions of North America, South America, and Australia as an ornamental plant, and is now considered a major agricultural and wildland noxious weed. It has been recorded from nearly 50 countries. It is difficult to eradicate because of its drought resistance. It can spread rapidly and eventually dense stands prohibit foraging by livestock. Infestations of Cotton Thistle often start in disturbed areas such as roadways, campsites, burned areas, and ditch banks. The weed adapts best to areas along rivers and streams, but can be a serious problem in pastures, grain fields and range areas. A single Cotton Thistle is imposing enough, but an entire colony can ruin a pasture or destroy a park or campsite, sometimes forming tall, dense, impenetrable stands. Besides creating an impenetrable barrier to humans and animals, Cotton Thistle nearly eliminates forage use by livestock and some mammal species such as deer and elk.

Known infestations include most of the Pacific Northwest along with Utah, Colorado, New Mexico, Wyoming, Nebraska, and South Dakota. On western rangeland, infestations directly result in significant economic losses for ranchers. It is also widespread in Australia and New Zealand. In Australia it commonly hybridises with the related invasive Illyrian Thistle Onopordum illyricum.

Control

Mechanical

Small infestations may be physically removed or cut a few centimetres below the soil surface ensuring that no leaves remain attached to prevent regrowth. Mowing during early flowering will not kill the plant but will reduce seed production. Repeated treatments may be required because populations typically exhibit a wide range of developmental stages among individual plants. Slashing should be done prior to flowering since seed may mature in the seed head after cutting. Plants should not be mowed following seed set, as this increases chances for seed dispersal.

Chemical

Because of their shorter life cycle, Cotton Thistle plants can be effectively treated with herbicides. All herbicide treatments should be applied at the rosette stage of the plant. Generally, herbicide applications would be in early spring or autumn. One of the primary difficulties in chemical control of Cotton Thistles is their ability to germinate nearly year round. From autumn to spring a range of plant sizes can be found which may result in variable success from chemical control. Herbicides are very effective on seedlings and young rosettes, but control becomes more variable with increasing plant age. Onopordum spp. seeds may persist for several years in the soil. Buried seed may persist for up to twenty years, and reinfestation is likely without yearly management. Therefore several years of re-treatment may be necessary. Dicamba and 2,4-D will injure or kill other broadleaf plants including legumes. Clopyralid is more selective for controlling plants in the Asteraceae family, but will also injure or kill legumes.

Biological

There are no biological control agents that have been specifically released for Cotton Thistle control in the United States. A thistle head weevil (Rhinocyllus conicus) that feeds on Italian Thistle (Carduus pycnocephalus) has also been shown to feed on Cotton thistle. However, this insect was the object of imprudent biological control introduction, and it became an invasive species that has threatened endangered native thistles in North America (Strong 1997). Establishment of this thistle head weevil as a biological control agent for Cotton thistle has been unsuccessful in the Pacific Northwest. A thistle crown weevil (Trichosirocalus horridus) that feeds on Musk, Bull, Plumeless, Italian, and Creeping Thistles will also feed on Cotton Thistle. In Australia, this insect has been shown to kill Cotton Thistle rosettes. The related Trichosirocalus briese is also being tested as a control agent.

In Australia, a total of seven insects have been released to control Cotton thistle. Two of the seven released insects are weevils, including Larinus latus which feeds on the seeds, and Lixus cardui, which bores in the stems.

Integrated management

A combination of methods (IPM) is often more effective than any single method. An integrated pest management plan deals with prevention as well as control. Eradication of weed species is often not a practical goal, but in many cases reducing infestation to manageable levels is an achievable objective. Seed bank longevity is a major factor in managing Cotton Thistles. Re-establishing competitive perennial grasses and monitoring infested areas on a yearly basis is critical. Herbicides can successfully be used for reducing thistle populations and giving grasses a competitive advantage. However, they cannot be used as a stand alone solution. These techniques must be linked with good grazing practices in rangeland areas. Otherwise, the thistles will recolonise and rapidly replenish the seed bank to pre-control levels.

References

• Grieve, M. 1971. A Modern Herbal: The Medicinal, Culinary, Cosmetic and economic Properties, Cultivation and Folk-Lore of Herbs, Grasses, Fungi, Shrubs & Trees with Their Modern Scientific Uses. Dover Publications, Inc., New York.
• Mucina, L. 1989. Syntaxonomy of the Onopordum acanthium communities in temperate and continental Europe. Vegetatio 81:107-115.
• Strong, D. 1997. ECOLOGY: Enhanced: Fear No Weevil? Science 22 August 1997: 1058-1059.
• Tucci, G., M.C. Simeone, C. Gregori, and F. Maggini. 1994. Intergenic spacers of rRNA genes in three species of the Cynareae (Asteraceae). Plant Systematics and Evolution 190: 187-193.

External Links

• Species Profile- Scotch Thistle (Onopordum acanthium), National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for Scotch Thistle.