Cirsium occidentale
Flower head of Cirsium occidentale

Vulnerable  (NatureServe)[1]
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Eudicots
Clade: Asterids
Order: Asterales
Family: Asteraceae
Genus: Cirsium
Species:
C. occidentale
Binomial name
Cirsium occidentale
Synonyms[2]

Cirsium coulteri

Cirsium occidentale, with the common name cobweb thistle or cobwebby thistle, is a North American species of thistle in the family Asteraceae.[3]

Description

Cirsium occidentale is a biennial plant forming a taproot. It may be short or quite tall, forming low clumps or towering to heights approaching 3 meters (10 feet). The leaves are dull gray-green to bright white due to a coating of hairs, and the most basal ones on large plants may be nearly 0.5 m (1+1⁄2 ft) in length.[4] The petioles are winged and spiny and the leaves are toothed or edged with triangular lobes.[5]

The inflorescence at the top of the whitish stem holds one to several flower heads. Each head is sphere-like, covered in large phyllaries with very long, spreading spines which are laced, often quite heavily, in fibers resembling cobwebs.[5]

The head is packed with disc florets which may be white to blood red to shades of purple. The largest flower heads exceed 8 centimeters (3 inches) in diameter.[4] The heads do not open in synchrony, perhaps allowing greater likelihood of being pollinated.[5]

Varieties

There are several varieties, which differ from each other in range and form:[4]

  • Cirsium occidentale var. californicum — California thistle[6]
  • Cirsium occidentale var. candidissimum — snowy thistle[7][8]
  • Cirsium occidentale var. compactum — compact cobwebby thistle; a short, clumpy California endemic that grows only along the coast of the San Francisco Bay Area and the Central Coast of California[9]
  • Cirsium occidentale var. coulteri — Coulter's thistle[10]
  • Cirsium occidentale var. lucianum — Cuesta Ridge thistle; a California endemic from the Santa Lucia Range[11]
  • Cirsium occidentale var. occidentale — cobwebby thistle[12]
  • Cirsium occidentale var. venustum[13][14]

Distribution and habitat

The plant is widespread and fairly common across most of California: in its mountain ranges, valleys, and the Mojave Desert; and in the western Great Basin region in western Nevada, southern Oregon, and southwestern Idaho.[15][4][16]

Unlike many introduced thistles, this native species is not a troublesome weed.

Ecology

It is a larval host to the California crescent, mylitta crescent, and the painted lady butterfly.[17]

References

  1. ↑ "NatureServe Explorer". NatureServe Explorer. NatureServe. 2022. Retrieved 19 May 2022.
  2. ↑ The Plant List, Carduus occidentalis Nutt.
  3. ↑ Calflora taxon report, University of California, Cirsium occidentale (Nutt.) Jepson, Cobweb Thistle, western thistle
  4. 1 2 3 4 Jepson Flora Project (ed.). "Cirsium occidentale". Jepson eFlora. The Jepson Herbarium, University of California, Berkeley.
  5. 1 2 3 Flora of North America, Western thistle, Cirsium occidentale (Nuttall) Jepson
  6. ↑ Jepson Flora Project (ed.). "Cirsium occidentale var. californicum". Jepson eFlora. The Jepson Herbarium, University of California, Berkeley.
  7. ↑ "Plants Profile for Cirsium occidentale candidissimum (snowy thistle)". plants.usda.gov. Retrieved 2019-02-18.
  8. ↑ Jepson Flora Project (ed.). "Cirsium occidentale var. candidissimum". Jepson eFlora. The Jepson Herbarium, University of California, Berkeley.
  9. ↑ Jepson Flora Project (ed.). "Cirsium occidentale var. compactum". Jepson eFlora. The Jepson Herbarium, University of California, Berkeley.
  10. ↑ Jepson Flora Project (ed.). "Cirsium occidentale var. coulteri". Jepson eFlora. The Jepson Herbarium, University of California, Berkeley.
  11. ↑ Jepson Flora Project (ed.). "Cirsium occidentale var. lucianum". Jepson eFlora. The Jepson Herbarium, University of California, Berkeley.
  12. ↑ Jepson Flora Project (ed.). "Cirsium occidentale var. occidentale". Jepson eFlora. The Jepson Herbarium, University of California, Berkeley.
  13. ↑ "Plants Profile for Cirsium occidentale venustum (cobwebby thistle)". plants.usda.gov. Retrieved 2019-02-18.
  14. ↑ Jepson Flora Project (ed.). "Cirsium occidentale var. venustum". Jepson eFlora. The Jepson Herbarium, University of California, Berkeley.
  15. ↑ C.Michael Hogan ed. 2010. Cirsium occidentale. Encyclopedia of Life
  16. ↑ Biota of North America Program 2014 county distribution map
  17. ↑ The Xerces Society (2016), Gardening for Butterflies: How You Can Attract and Protect Beautiful, Beneficial Insects, Timber Press.
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