Onopordum acanthium

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It 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 (12 in) or more for a food reserve.

In the second year, the plant can grow to as much as 3 m (9.8 ft) tall with a width of up to 1.5 m (4 ft 11 in). The leaves are 10–50 cm (3.9–19.7 in) wide, alternate, spiny, and 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. Fine hairs give the plant a greyish appearance. The massive main stem may be 10 cm (3.9 in) wide at the base and branched in the upper part. Each stem has a vertical row of broad, spiny wings (conspicuous ribbon-like leafy material), typically 2–3 cm (0.79–1.18 in) wide, extending to the base of the flower head.

The flowers are globe shaped, 2–6 cm (0.79–2.36 in) 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.[4][5][6]

Cotton thistle is native to Europe and Asia. The plant prefers habitats with dry summers, such as the Mediterranean region, growing best in 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.[1][4] Temperature and moisture, rather than soil nutrient concentrations, determine the ecological performance of Onopordum species.[7]

In Europe, the plant tends to colonise disturbed pastures. In its native range, cotton thistle is considered a weak competitor that needs regeneration gaps to develop and maintain stands; populations tend to retreat when disturbance ceases.[8] The plant has been widely introduced at mid-latitudes across much of North America.[9]

Scotch thistle can spread rapidly. For example, it was first found in Utah in 1963. By 1981, it covered approximately 6070 hectares in 17 counties. Eight years later, it had spread to cover more than 22,540 hectares in 22 counties.[10]

Onopordum acanthium 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.[4] While some seeds will germinate in the dark, studies indicate that most germination occurs with alternating light/dark cycles, with 8 hours being the optimal day length.[11]

The leaves of cotton thistle provide food for the caterpillars of some Lepidoptera, such as the thistle ermine (Myelois circumvoluta).

Three subspecies are accepted:[4]

• 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 genus botanical name is derived from the Ancient Greek words όνος (ónos=donkey), πέρδω (pérdo=to fart), and άκανθος (ácanthos=thorn), meaning 'donkey fart thorny food'.[12][13][14]

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.[15] The name Scots thistle comes from the flower used as the national emblem of Scotland. Despite its name, it has nothing to do with the actual thistle that is used as the emblem of Scotland, which is the Cirsium vulgare.

It is grown as an ornamental plant for its bold foliage and large flowers.[16] It has been used to treat cancers and ulcers and to diminish discharges of mucous membranes. A recent study of a crude extract from the leaves of the plant demonstrated activity against glioblastoma, a type of very malignant brain tumor.[17] 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 burning and cooking.[18][19]

Thistles produce abundant nectar for pollinators and provide seeds and floss for birds such as the goldfinch. Large-flowered tall thistles in particular provide considerable nectar.[20][21][22][23][24]

A mature Scotch thistle (1.8 m) at Galong, Australia

An example of Onopordum acanthium in southwest Idaho, demonstrating the plant's impressive size

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.[25] 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 plant is imposing enough, but an entire colony can ruin a pasture or destroy a park or campsite, sometimes forming tall, dense, impenetrable stands. This issue is often amplified by the prolific thorns found on this species. Besides creating an impenetrable barrier to humans and animals, the plant nearly eliminates forage use by livestock and some mammal species such as deer and elk.[26]

In the United States, 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.[27] In Australia it commonly hybridises with the related invasive Onopordum illyricum.[26]

• Donald R. Strong (1997). "Fear no weevil?". Science. 277 (5329): 1058–1059. doi:10.1126/science.277.5329.1058. S2CID 83205230.
• Gianfranco Tucci; Marco Cosimo Simeone; Carlo Gregori; Fabio Maggini (1994). "Intergenic spacers of rRNA genes in three species of the Cynareae (Asteraceae)". Plant Systematics and Evolution. 190 (3–4): 187–193. doi:10.1007/BF00986192. S2CID 26309594.

Wikimedia Commons has media related to:
Onopordum acanthium (category)

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