Cirsium occidentale, with the common name cobweb thistle or cobwebby thistle, is a North American species of thistle in the family Asteraceae.[1]
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 3m (10feet). 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 0.5m (01.6feet) in length.[1] 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 8cm (03inches) in diameter. The heads do not open in synchrony, perhaps allowing greater likelihood of being pollinated.[2]
There are several varieties, which differ from each other in range and form:
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.[5] [6]
Unlike many introduced thistles, this native species is not a troublesome weed.
It is a larval host to the California crescent, mylitta crescent, and the painted lady butterfly.[7]
