Yellow starthistle (Centaurea solstitialis)
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Yellow starthistle is a long-lived winter annual, and occasionally, a biennial broadleaf plant. It is highly competitive and often develops impenetrable stands, displacing desirable vegetation. In the western United States, yellow starthistle is considered one of the most serious rangeland weeds. It is found throughout most of central California and northward, typically to about 5900 feet (1800 m), but has been found at higher elevations. It is common in the Sacramento Valley, San Joaquin Valley, Sierra Nevada foothills, Cascade Range, Klamath Ranges, eastern North Coast Ranges, and the central-western region. It is less common in Southern California and uncommon in the desert regions, moist coastal regions, and east of the Sierra Nevada. Yellow starthistle inhabits agricultural land and other disturbed sites.
Although several natural enemies of yellow starthistle have established in California, these insects have yet to provide a significant reduction in plant populations in most areas.
In addition to being a serious rangeland weed, yellow starthistle is sometimes problematic in grain fields where it can contaminate grain harvest, lowering grain value and quality. The plant also contains an unidentified compound that can cause a nervous system disorder in horses. A positive quality of yellow starthistle is that bees produce flavorful, high quality honey when they forage on yellow starthistle.
Grassland, cultivated fields, pastures, roadsides, and open sites on hillsides and in woodlands.
Seedlings are dull green. Cotyledons (seed leaves) are oblong to spatula shaped, round at the tip, wedge shaped at the base, and are about 1/4 to 2/5 of an inch (6–9 mm) long. The first few leaves are usually lance shaped with the widest part of the leaf above the middle. Subsequent leaves form a rosette. The early rosette leaves are highly variable, often lance shaped with the widest part above the middle, and have simple to lobed edges. Later rosette leaves can grow to a length of 15 inches (38 cm) and are typically deeply lobed almost to the midvein and frequently appear ruffled. The lobes mostly taper to a point and have toothed to wavy edges. Rosette leaves that develop in low light conditions are usually larger and more erect. Generally, the upper and lower leaf surfaces are densely covered with stiff thick hairs and fine cottony hairs (visible with magnification).
Yellow starthistle is a gray-green to blue-green plant that ranges from 6 inches to 6-1/2 feet (0.15–2 m) tall. Stems are stiff, wiry and single in small plants and openly branch near the base or above in larger plants. Rosette leaves are typically deeply lobed, often appear ruffled, have toothed to wavy edges, and usually wither by bloom. Stem leaves are mostly linear to narrowly oblong, or lance shaped with the widest part above the middle. Leaf bases extend down the stems giving stems a winged appearance. Lower stem leaves are sometimes deeply lobed. All leaves are densely covered with fine, white, cottony hairs that hide most of the stiff, thick hairs and glandular dots also present on the leaves.
Flowering takes place from June through December, but can be delayed by mowing and grazing. Many small and narrow, bright yellow, tubular flowers (disk flowers) cluster into a round to egg-shaped flower head. Flower heads form singly at both the stem tip and sometimes where branches meet the main stem (axils). At the base of each flower head are long stiff spines.
Yellow starthistle produces two kinds of fruit. Both are hairless, mostly about 1/12 to 1/8 of an inch (2-3 mm) long, and generally barrel shaped. However, one type is glossy, gray or tan to a mottled cream color and tan, and ends in a tuft of slender, stiff, white bristles. The other type is dull dark brown, often with tan speckles, and is tuftless.
Reproduces by seed.