Cirsium occidentale, with the common name cobweb thistle or cobwebby thistle, is a North American species of thistle in the family Asteraceae.
Description
Cirsium occidentale is a biennial plant or perennial plant forming a taproot. It may be short or quite tall, forming low clumps or towering to heights approaching . 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 in length. The petioles are winged and spiny and the leaves are toothed or edged with triangular lobes.
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.
The head is packed with disc florets 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
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 and the Central Coast of California 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: in its mountain ranges, valleys, and the Mojave Desert; and in the western Great Basin region in western Nevada, southern Oregon, and southwestern Idaho.C.Michael Hogan ed. 2010. Cirsium occidentale. Encyclopedia of LifeBiota of North America Program 2014 county distribution map
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.The Xerces Society (2016), Gardening for Butterflies: How You Can Attract and Protect Beautiful, Beneficial Insects, Timber Press.
Gallery
References
External links Jepson Manual treatment — Cirsium occidentale Cirsium occidentale — Calphotos Photo gallery, University of California