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Statewide IPM Program, University of California

Italian thistle  (Carduus pycnocephalus)

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Life stages of Italian Thistle fruits young plant seedling flowers mature plant

Italian thistle is a winter annual, and sometimes biennial, weed. It is native to the Mediterranean region and central Europe. It is a noxious weed in California, and is found mostly in the southern North Coast region, the Sierra Nevada foothills, the Central Coast, the San Francisco Bay Area, and the South Coast Ranges. It is also a common weed in Oregon, Washington, and Idaho. 

Habitat

Disturbed open areas, roadsides, pastures, annual grasslands, and waste areas that consist of sandy to clay soils.

Seedling

The cotyledons (seed leaves) are hairless and dull. They are oval-shaped or oblong, 0.4 to 0.8 inch (10–20 mm) long, and 0.2 to 0.5 inch (6–12 mm) wide. The first two true leaves of Italian thistle seedlings alternate and are more tapered toward the base than the cotyledons.

Young Plant

Leaves of young plants alternate and form a basal rosette. The leaves taper toward the base and are broader and rounder at the tips. Leaf margins are irregularly toothed and prickly. The upper surface and lower veins of young leaves often have long, firm, sparsely distributed hairs. The veins of older leaves are pale.

Mature plant

Mature Italian thistle plants may grow to be up to 6.5 feet (2 meters) tall and are branched at the top. The leaves at the base of the plants are either elliptical or pointed (lance-shaped), with prickly, toothed margins (4–10 lobes per leaf). Leaves on stems are alternated and their petioles extend down the stem. Leaflike wings up to 0.2 inch (5 mm) wide protrude from the stems. These wings tend to be more sparse and are separated by larger gaps near the flower heads, but are much closer together in the lower parts of the plant. 

Flowers

Italian thistle blooms from February through July. The flower heads are more or less cylindrical or bell-shaped, less than 1 inch (1–2 cm) in diameter and are usually clustered in groups of 2 to 5 at the tops of stems. Flowers are pink or purple and are 0.4 to 0.6 inch (10–14 mm) long. Bracts on the flower heads are spiny, linear or pointed, and less than 0.1 inch (no more than 2 mm) wide with a sandpapery texture. Dense patches of wooly hairs are present at the base of the bracts.

Fruits

Fruits are single-seeded (achene), elliptical, curved, and slightly flattened, cream-colored and have about 20 stripes that run lengthwise down the fruit. They may have either no bristles, or many feathery bristles that are about 0.4 to 0.8 inch (10–15 mm) long.

Reproduction

Reproduces via seeds, which are dispersed by wind. Seeds can persist in the soil for up to seven years.

Related or similar plants

  • Slenderflower thistle, Carduus tenuiflorus
  • Musk thistle, Carduus nutans
  • Spiny plumeless thistle, Carduus acanthoides

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