# Horticulture/Onopordum acanthium

Cotton Thistle :
| Binomial: | Onopordum acanthium |
| Type: | Biennial |
| Seed Dispersal: | Wind |
Onopordum acanthium (Cotton Thistle) is a flowering plant in the Family Asteraceae. Other common names include, Scottish thistle, and Scottish cotton thistle. Native to Europe, North Africa and Asia, it is a vigorous, biennial with coarse, spiny leaves and conspicuous spiny-winged stems.
The botanical name is derived from the Greek words onos (donkey), perdo (to consume), and acanthos (thorn), meaning 'thorny plant eaten by donkeys'. The common name Cotton thistle derives from the cotton-like hairs on the leaves; the name Scottish thistle comes from a legend that the plant's thorny thickets helped protect Scotland from the Vikings, though this legend more likely applies to the common Scottish native Spear Thistle Cirsium vulgare, rather than the Cotton Thistle which does not occur naturally in Scotland. Oral folklore holds that as Vikings attempted to sneak up at night to attack and raid Scottish villages, they were stuck by the thistles' thorns and cried out in pain, alerting the townsfolk to the attack and allowing them to fight back and drive back the invaders. Following this the Thistle was adopted as the floral emblem of Scotland.

## Description

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-) 0.5–3 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, with the lower surface more densely covered than the upper. The leaves are deeply lobed with long, stiff spines along the margins. The leaves have winged appearance that continues down the stems of the plant. Fine hairs give the plant a greyish appearance (Global). The massive, main stem may be 10 cm wide at the base, and is branched in the upper part. Each stem shows a vertical rows 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, 3-5 cm in diameter, from dark pink to lavender, and are produced in the summer from July to September. The flower buds form first at the tip of the stem and later at the tip of the axillary branches. The flower heads exist singly or in groups of 2 or 3 on branch tips. They 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. It prefers habitats with dry summers, such as the Mediterranean, growing in sandy, sandy clay and calcareous soils, 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 (Encycloweedia), agricultural areas, range/grasslands, riparian zones, scrub/shrublands, water courses.
Cotton thistle reproduces only by seeds. Most seeds germinate in fall after the first rains, but some seeds can germinate year round under favorable moisture and temperature conditions. Seeds that germinate in late fall 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 7 years and possibly 20 years or more. Yearly seed production and seed dormancy are highly variable depending on environmental conditions. These slender and smooth achenes are about 3 mm long and are brown with gray markings. They are tipped with a pappus of slender bristles. They are 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.

## Ecology

In the late 19th century, it was introduced to North America and temperate Australia as an ornamental plant, and is now considered a major agricultural and wildland noxious weed problem in areas it has been introduced. It is difficult to eradicate because of its drought resistance. It can spread rapidly and in the end 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 Colorado, Wyoming, Nebraska, and South Dakota. On western rangeland, infestations directly result in significant economic losses for ranchers.

## Uses

Cotton thistle is sold as an ornamental plant. It has reportedly 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 Cotton thistle seeds has been used in Europe for burning and cooking. Cotton thistle is the national emblem of Scotland, dating from the time of James III of Scotland. The thistle is also part of the logo of Encyclopædia Britannica.

## Control

* Mowing: Mowing by early flowering will not kill the plant but will reduce seed production, but may require repeated treatment because populations typically exhibit a wide range of developmental stages among individual plants.
* Coppicing: 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 dispersal.
* Pulling: Small infestations should be physically removed or cut a few centimeters below the soil surface so that no leaves remain attached, or it will grow back.
* Contact herbicides (organic): Herbicides can successfully be used for reducing thistle populations and giving grasses a competitive advantage. However, they cannot be used as a standalone solution.
* Contact herbicides (synthetic): 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 chemical control. These herbicides are all 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 retreatment 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. 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.
* Biocontrols (animal): 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. 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.
* Biocontrols (plants): Eradication of weed species is often not a practical goal but in most cases reducing infestation to manageable levels should be the objective. Seedbank longevity is a major factor in managing Cotton thistles. Reestablishing competitive perennial grasses and monitoring infested areas on a yearly basis is critical.
* Grazing: The above techniques must be linked with good grazing practices in rangeland areas. Otherwise, the thistles will recolonize and rapidly replenish the seed bank to pre-control levels.

## References

* 1 2 3 Encycloweedia
* ↑ Global Invasive Species Database
* ↑ Strong, D. 1997. ECOLOGY: Enhanced: Fear No Weevil? Science 22 August 1997: 1058-1059.
* 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.
* 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.