Carduus pycnocephalus, with common names including Italian thistle, Italian plumeless thistle, and Plymouth thistle, is a species of thistle. It is native to: the Mediterranean region in southern Europe, North Africa, and Western Asia; East Europe and the Caucasus; and the Indian Subcontinent. The plant has become an introduced species in other regions, and on other continents, often becoming a noxious weed or invasive species.
A winter annual, Carduus pycnocephalus stems range from 8 inches (20 cm) to 6.6 feet (2.0 m), and are glabrous to slightly wooly. The multiple stems are winged with spines.
The plant grows in a rosettes of 10–14 inches (25–36 cm) in diameter, with four to ten lobed basal leaves that are 4–6 inches (10–15 cm) long. Cauline leaves are tomentose on the underside and contain spines on the lobe tips.
Flower heads are 2-5 per cluster, densely matted with cobwebby hairs at the base of the phyllaries and spiny towards the tips. Corollas are pink to purple, approx. .4-.6 in (1-1.4 cm) long, and the fruits are brown to gold, with a bristly, minutely barbed pappus.
It is a C-listed weed by the California Department of Agriculture and a Moderate Invasive Plant rating by the California Invasive Plant Council. It favors grasslands, woodlands, and chaparral vegetation types, but is especially prevalent in oak woodlands in and around the Central Valley. It is found in disturbed areas, often with basaltic soils, fertile soils, or soils with a relatively high pH (> 6.5).
Italian thistle can grow densely, crowding out other vegetation with dense rosette 'colonies' in the winter, thereby preventing establishment of native plants. Its spiny leaves, stems, and phyllaries prevent animals from grazing on it and nearby forage. Its tendency to grow under the canopy of oaks increases the risk of wildfire damage to the trees, as fire can more easily spread to the canopy.