Russian thistle, Salsola tragus, also known as tumbleweed, is in the goosefoot family (Chenopodiaceae). Its scientific name is Salsola tragus, but it also has been known as Salsola iberica, Salsola kali, and Salsola australis. It is a summer annual native to southeastern Russia and western Siberia and was first introduced into the United States in 1873 by Russian immigrants as a contaminant in flax seed in South Dakota. After its introduction, it spread by contaminated seed, threshing crews, railroad cars (especially livestock cars), and by its windblown pattern of seed dissemination. In 1895 Russian thistle moved to the Pacific Coast in contaminated railroad cars that transported cattle to Lancaster in California’s Antelope Valley. Today it is common throughout the western United States—having invaded about 100 million acres. It is particularly well adapted to California’s climate of winter rainfall and summer drought.
Russian thistle is primarily a weed in sites where the soil has been disturbed, such as along highways and fencelines. It is also prevalent in vacant lots and other noncrop areas, in field and vegetable crops, and in poorly tended landscapes. It is rarely a problem in well-managed gardens or turfgrass.
Russian thistle is a bushy summer annual with numerous slender ascending stems that become quite woody at maturity. Stems vary from 8 to 36 inches in length and usually have reddish to purplish stripes. Seedlings have very finely dissected leaves that almost look like pine needles. Leaves of young plants are fleshy, dark green, narrow, and about 1 inch in length. Young plants are suitable for livestock forage and are sometimes grazed. As the plant matures in July through October, the older leaves become short and stiff with a sharp-pointed tip. The single, inconspicuous flowers lack petals and are borne above a pair of small spine-tipped bracts (a small modified leaf at the base of the flower) in most leaf axils (where the narrow leaves meet the stem). The bracts and spiny leaves prevent predation by herbivores as the plant nears maturity. The overall shape of the plant becomes oval to round and at maturity can attain a diameter of 18 inches to 6 feet or more under favorable soil moisture and fertility conditions. After the plant dries, the base of the stem becomes brittle and breaks off at soil level in fall and early winter. These round, spiny plants are capable of dispersing seed for miles as they tumble along in the wind. This dispersal characteristic has led to the more commonly used name of tumbleweed. (source)