Italian thistle (Carduus pycnocephalus) is native to the Mediterranean region and was introduced to California in the 1930s. It is a common weed in disturbed sites in the foothills of the Sierra Nevada and elsewhere in California. It forms dense populations in moist grassy areas and along stream banks. Its tendency to crowd out native plants makes it a species of concern in Sequoia and Kings Canyon National Parks.
Italian thistle is an annual (sometimes biennial) thistle. It can grow as tall as two meters. The leaves have spiny margins and the lower leaves are lobed. The stems usually have spiny wings. The flowering heads are generally clustered into groups of two to five heads. The individual heads are narrow (1-2 centimeters or 0.4-0.8 inches) when compared with other local thistles. The flower clusters are pink to purple.
There are species of native thistles in Sequoia and Kings Canyon that could be confused with Italian thistle. Western thistle (Cirsium occidentale) is the most common thistle encountered in the foothills of Sequoia National Park. Western thistle is generally more white-hairy on the stems and leaves than Italian thistle. Western thistle also lacks the prominent spiny wings along the stem that are obvious on Italian thistle. The flowering heads are generally much wider (1.5-5 centimeters or 0.6-2 inches) than on Italian thistle. Most western thistles in Sequoia National Park have white-to-light-pink flowers, although it is possible for western thistle to have pink-to-rose flowers.
Photo by Brother Alfred Brousseau, St. Mary's College
Italian thistle is an annual plant and therefore it reproduces entirely by seed. Wind is the primary means of dispersal for Italian thistle. Its seeds germinate very easily and do not need a long dormancy period. The seedlings grow most easily on bare and disturbed soils, and they can remain viable in the soil for up to eight years.
Italian thistle is a common weed in disturbed sites below 1500 meters (5,000 feet) in Sequoia National Park. It is most common within and adjacent to roads and campgrounds. It is also scattered throughout the less-disturbed blue oak woodland, gaps in chaparral stands, and riparian areas.
In 2002 Sequoia and Kings Canyon Natural Resource Management personnel began actively eradicating Italian thistle with the goal of containing the species to below 850 meters (2,800 feet). The species is being mapped to determine the extent of invasion in the Kaweah River Drainage. Control is by manual removal, mowing, and application of approved glyphosate or clopyralid herbicide. Follow-up surveys and eradication efforts will be undertaken in future years to exhaust the stored seed bank of this annual species.
In 2003, a few individuals of this species were detected in the Cedar Grove area of Kings Canyon National Park. Italian thistle was previously not known to be present in Kings Canyon. Keeping this species from spreading in Kings Canyon is a high priority. If you find a plant in Kings Canyon that you suspect is Italian thistle, please report it immediately. Remember, do not remove any plants in the park--non-native plants can easily be mistaken for valuable natives.