''Cirsium occidentale'', with the common name cobweb thistle or cobwebby thistle, is a

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

n species of

thistle Thistle is the common name of a group of flowering plants characterised by leaves with sharp prickles on the margins, mostly in the family Asteraceae. Prickles can also occur all over the planton the stem and on the flat parts of the leaves. ...

in the family

Asteraceae The family Asteraceae, alternatively Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the order Asterales. Commonly referred to as the aster, daisy, composite, or sunflower family, Compositae w ...

Description

''Cirsium occidentale'' is a

biennial plant A biennial plant is a flowering plant that, generally in a temperate climate, takes two years to complete its biological life cycle. Life cycle In its first year, the biennal plant undergoes primary growth, during which its vegetative structures ...

forming a

taproot A taproot is a large, central, and dominant root from which other roots sprout laterally. Typically a taproot is somewhat straight and very thick, is tapering in shape, and grows directly downward. In some plants, such as the carrot, the taproo ...

. It may be short or quite tall, forming low clumps or towering to heights approaching . The

leaves A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...

are dull gray-green to bright white due to a coating of hairs, and the most basal ones on large plants may be nearly in length. The petioles are winged and spiny and the leaves are toothed or edged with triangular lobes. The

inflorescence An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Morphologically, it is the modified part of the shoot of seed plants where flowers are formed o ...

at the top of the whitish stem holds one to several

flower heads A pseudanthium (Greek for "false flower"; ) is an inflorescence that resembles a flower. The word is sometimes used for other structures that are neither a true flower nor a true inflorescence. Examples of pseudanthia include flower heads, compos ...

. Each head is sphere-like, covered in large

phyllaries In botanical terminology, a phyllary, also known an involucral bract or tegule, is a single bract of the involucre of a composite flower. The involucre is the grouping of bracts together. Phyllaries are reduced leaf-like structures that form one or ...

with very long, spreading spines which are laced, often quite heavily, in fibers resembling

cobwebs A spider web, spiderweb, spider's web, or cobweb (from the archaic word '' coppe'', meaning "spider") is a structure created by a spider out of proteinaceous spider silk extruded from its spinnerets, generally meant to catch its prey. Spid ...

. The head is packed with

disc floret The family Asteraceae, alternatively Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the order Asterales. Commonly referred to as the aster, daisy, composite, or sunflower family, Compositae w ...

s which may be white to blood red to shades of purple. The largest flower heads exceed in diameter. The heads do not open in synchrony, perhaps allowing greater likelihood of being pollinated.Flora of North America, Western thistle, ''Cirsium occidentale'' (Nuttall) Jepson
/ref>

Varieties

There are several varieties, which differ from each other in range and form: *''Cirsium occidentale'' var. ''californicum'' — California thistle *''Cirsium occidentale'' var. ''candidissimum'' — snowy thistle *''Cirsium occidentale'' var. ''compactum'' — compact cobwebby thistle; a short, clumpy California endemic that grows only along the coast of the

San Francisco Bay Area The San Francisco Bay Area, often referred to as simply the Bay Area, is a populous region surrounding the San Francisco, San Pablo, and Suisun Bay estuaries in Northern California. The Bay Area is defined by the Association of Bay Area Go ...

and the

Central Coast of California The Central Coast is an area of California, roughly spanning the coastal region between Point Mugu and Monterey Bay. It lies northwest of Los Angeles County and south of San Mateo and Santa Clara counties, and includes the rugged, undeveloped ...

*''Cirsium occidentale'' var. ''coulteri'' — Coulter's thistle *''Cirsium occidentale'' var. ''lucianum'' — Cuesta Ridge thistle; a California endemic from the Santa Lucia Range *''Cirsium occidentale'' var. ''occidentale'' — cobwebby thistle *''Cirsium occidentale'' var. ''venustum''

Distribution and habitat

The plant is widespread and fairly common across most of

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

: in its mountain ranges, valleys, and the

Mojave Desert The Mojave Desert ( ; mov, Hayikwiir Mat'aar; es, Desierto de Mojave) is a desert in the rain shadow of the Sierra Nevada mountains in the Southwestern United States. It is named for the indigenous Mojave people. It is located primarily in ...

; and in the western

Great Basin The Great Basin is the largest area of contiguous endorheic basin, endorheic watersheds, those with no outlets, in North America. It spans nearly all of Nevada, much of Utah, and portions of California, Idaho, Oregon, Wyoming, and Baja California ...

region in western

Nevada Nevada ( ; ) is a U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...

, southern

Oregon Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...

, and southwestern

Idaho Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and Wyom ...

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

Ecology

It is a larval host to the

California crescent ''Phyciodes orseis'', the Orseis crescentspot or California crescent, is a species of butterfly in the family Nymphalidae. It is found in western North America, particularly northern California and Oregon. Formerly, The range was thought to exte ...

mylitta crescent ''Phyciodes mylitta'', the Mylitta crescent or Mylitta crescentspot, is a butterfly of the family Nymphalidae. It is found in western North America. The wingspan is 26-37 mm. Wings are orange with black markings and white fringe on the edg ...

, and the

painted lady ''Vanessa cardui'' is the most widespread of all butterfly species. It is commonly called the painted lady, or formerly in North America the cosmopolitan. Description File:Vanessa cardui MHNT CUT 2013 3 14 Pontfaverger-Moronvilliers Dos. ...

butterfly.The Xerces Society (2016), ''Gardening for Butterflies: How You Can Attract and Protect Beautiful, Beneficial Insects'', Timber Press.

Gallery

Asteraceae cirsium occidentale California Thistle.jpg, California thistle Cirsium occidentale var. compactum.jpg, ''C. occidentale'' var. ''compactum'' blooming Cirsium occidentale plant.jpg, ''C. occidentale'' Cirsium occidentale candidissimum (5378322408).jpg, ''C. occidentale'' var. ''candidissimum'' Cirsium occidentale candidissimum (5378323652).jpg, ''C. occidentale'' var. ''candidissimum'' flower Cirsium occidentale v. occidentale.jpg, ''C. occidentale'' var. ''occidentale'' flower with seed head forming

References

External links

Jepson Manual treatment — ''Cirsium occidentale''''Cirsium occidentale'' — Calphotos Photo gallery, University of California
* {{Taxonbar, from=Q19848754 occidentale Flora of the Western United States Natural history of the California chaparral and woodlands Natural history of the California Coast Ranges Natural history of the Mojave Desert Natural history of the Peninsular Ranges Natural history of the San Francisco Bay Area Natural history of the Santa Monica Mountains Natural history of the Transverse Ranges Plants described in 1901 Taxa named by Thomas Nuttall Taxa named by Willis Linn Jepson