Plant of the Month ~~ JULY 2010
- Common Name(s): California Thistle, Bigelow's Thistle, Cobwebby Thistle
- Scientific Name: Cirsium occidentale var. californicum
- Family: Asteraceae, Sunflower family
- Plant Type: biennial or perennial herb
- Size: up to 6 feet high
- Common Habitat: Chaparral, Woodlands, Coastal Sage Scrub
California Thistle is native to the Santa Monica Mountains and is not as invasive as some other thistles. It can grow rather tall and has showy pink, purple or white flowers.
Most leaves of California Thistle are found at the base of the plant. Spiny but otherwise elliptical, deeply creased or cleft, and grayer underneath, they can be up to 14 inches long. Stems are long and branching, and atop each branch there grows a single flower. The larger, more colorful upper part of the flower, a tuft of pink, purple or white hairy structures about 1 inch high, is essentially a cluster of disk flowers. There are no ray flowers. The base of the flower head - the greenish, bristly, spherical structure just below the pink-purple disk flowers and above the stem - is called an involucre. Specialized leaf-like structures called bracts - also called phyllaries in plants like this - are what make up the involucre and give it its bristly appearance. Frequently some thin white fibers are also present here, which look like cobwebs.
The genus name Cirsium is a Greek word for thistle, and the species name occidentale means "from the west". In addition to California Thistle, you may also find Cirsium occidentale var. occidentale, commonly called Red Thistle or Cobwebby Thistle, in the Santa Monica Mountains. (While California Thistle is occasionally called Cobwebby Thistle as well, this common name is more likely to refer to the occidentale variety.) Red Thistle often has a sturdier appearance (bigger stems), redder disk flowers and more cobwebby involucres; also the size of the disk flowers is smaller in relation to the involucre in Red Thistle, and it is more likely to grow nearer to the coast than California Thistle.