Slenderflower thistle (Carduus tenuiflorus)
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Slenderflower thistle is a winter annual, and sometimes biennial, weed that is native to central Europe and the Mediterranean region. It is a noxious weed in California and is found on the North Coast, the Sierra Nevada foothills, the Central Coast, the San Francisco Bay area, the South Coast Ranges, and the southwestern region up to 1000 meters in elevation. It is also a problem in Oregon and Washington.
Disturbed open areas, roadsides, pastures, annual grasslands, and waste areas that consist of sandy to clay soils.
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 and later leaves are more tapered at the base than the cotyledons.
Leaves of young plants are alternate from each other and form a basal rosette. Leaf margins are irregularly toothed and prickly. The upper surface and lower veins of the young leaves often have long, firm, sparsely distributed hairs. The veins of older leaves are pale.
Mature slenderflower 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 mature plants are elliptical or lance-shaped with prickly, toothed margins (12–20 lobes per leaf). Leaves alternate with one another on the stem. Leaflike extensions (wings), which may be up to 10 mm (0.4 inch) wide, are also present on the stems (between leaves). These wings extend all the way up to the flower heads, with no gaps in between.
Slenderflower thistle blooms from May through July. Flower heads are more or less cylindrical or bell-shaped, and are clustered in groups of 5 to 20 heads at the tops of stems. Flowers are pink to purple. The base of the bracts on the flower heads are nearly hairless or have only sparse hairs.
Fruits are single-seeded (achene), elliptical, curved, and slightly flattened. Slenderflower thistle produces both achenes with feathery bristles and achenes without bristles.
Reproduces via seed, which disperses by wind. Seeds may persist in the soil for up to seven years.
Related or similar plants
- Italian thistle, Carduus pycnocephalus
- Musk thistle, Carduus nutans
- Spiny plumeless thistle, Carduus acanthoides