DiTomaso (2001) reports that Centaurea solstitialis is an erect winter annual (sometimes biennial), which grows mostly to 1m tall (occasionally to 2m tall) with spiny yellow-flowered heads. The first few leaves are typically oblanceolate (shaped like a lance-point reversed, with the tapering point attached to the leafstalk). Subsequent rosette leaves are oblanceolate or entire to pinnate-lobed. Later rosette leaves are 15cm long, typically deeply lobed to midrib. Lobes are mostly acute, with toothed to wavy margins. Stems are stiff, openly branched from near or above the base or sometimes not branched in very small plants. Leaf bases extend down the stems (decurrent) and give stems a winged appearance. Largest stem wings are typically 3mm wide. Foliage is grayish to bluish green, densely covered with fine, white, cottony hairs that hide thick, stiff hairs and glands. Taproots grow vigorously early in the season to soil depths of 1m or more, giving plants access to deep soil moisture during the dry summer and early fall months.
Flowerheads are ovoid, spiny, solitary on stem tips, and consist of numerous, yellow disk flowers. Vigorous individuals of C. Solstitialis may develop flower heads in branch axils. The involucre (phyllaries as a unit) is approximately 12-18mm long. Phyllaries are palmately spined, with one, long central spine and 2 or more pairs of short lateral spines. Phyllaries are more or less densely to sparsely covered with cottony hairs or with patches at the spine bases. The central spines of the main phyllaries are 10-25mm long, stout, and yellowish to straw-coloured throughout. Lateral spines occur typically in 2-3 pairs at the base of the central spine. The corollas are yellow, and mostly 13-20mm long. Flowerheads produce two types of achenes (seeds), both glabrous, approximately 2-3mm long, and with broad bases. Achenes are barrel-shaped, compressed, and laterally notched at the base. Flowers at the periphery of the flowerheads produce dull, dark brown (often speckled with tan) achenes that are darker and have no pappus (the bristly, feathery, or fluffy perianth whorl crowning the ovary). This seed type represents between 10 and 25% of the total seed and often remains in the seedheads until late fall or winter. The central flowers produce glossy, gray, or tan to mottled cream-coloured and tan seeds with a short, stiff, unequal, white pappus (2-5mm long). This represents the majority of seed produced (75-90%).

Over 90% of Centaurea solstitialis seeds are germinable one week after seed dispersal (Benefield et al. 2001, Joley et al. 1997, Roché et al. 1997, Sheley et al. 1983, 1993, in DiTomaso, 2001). Maximum germination of seeds (nearly 100%) occurs when seeds are exposed to moisture, light and temperatures of 10, 15, or 20 Celsius (Joley et al. 1997, Roché et al. 1997, in DiTomaso, 2001). At temperatures above 30 Celsius, germination is dramatically reduced (Joley et al. 1997, Roché et al. 1997, in DiTomaso, 2001). When exposed to light and moisture, germination occurs rapidly (typically by 24 hours) with nearly all seed germinating within 96 hours (Sheley et al. 1983, 1993, in DiTomaso, 2001). DiTomaso (2001) reports that seeds may be relatively short-lived under normal field conditions where seeds are predominantly dispersed on the soil surface. Furthermore, microbial degradation and invertebrate predation of C. solstitialis seeds contribute significantly to the rapid depletion of the soil seedbank.

Zouhar (2002) reports that Centaurea solstitialis is regarded as an important honey source plant in California and other western states. It is used in Turkish folk medicine for the treatment of ulcers. In a laboratory study, aqueous extracts of fresh or dried flowers of C. solstitialis given orally showed significant (p<0.01) antiulcerogenic activity in rats.

DiTomaso (2001) states that Centaurea solstitialis is best adapted to open grasslands with an annual precipitation range of 25 to 150cm per year. It is generally associated with deep, well-drained soils and infestations thrive in disturbed areas with high sunlight exposure. Although populations can occur at elevations at high as 2,400 m, most large infestations are found below 1,500 m. Zouhar (2002) reports that it is uncommon in deserts and moist coastal sites. It invades and dominates annual grasslands by using the deep soil moisture that remains after shallow-rooted annual grasses die in early summer. Seedlings are more likely to establish in soils with deep silt loam and loam with few coarse fragments (Larson and Sheley 1994, in DiTomaso, 2001).

Zouhar (2002) states that the plant is monoecious (male and female flowers on the same plant), pollinator-dependent, and facultatively xenogamous (can cross fertilise with other plants). Most of the plants are self-incompatible. Honeybees play an important role in the pollination of C. solstitialis and can account for 50% of seed set (Barthell et al. 2001, Maddox et al. 1996, in DiTomaso, 2001). Bumblebees are the second most important floral visitor to flowers, but several other insects also contribute to fertilisation of the ovules (Barthell et al. 2001, Harrod and Taylor 1995, in DiTomaso, 2001). Seed production per seedhead ranges from about 35 to over 80 seeds (Benefield et al. 2001, Maddox 1981, in DiTomaso, 2001), depending upon the site. DiTomaso (2001) states that large plants can produce over 100,000 seeds. C. solstitialis infestations can produce 50-100 million seeds per acre (DiTomaso et al. 1999a, Maddox 1981, in DiTomaso, 2001).

The nutritional component of yellow starthistle leaves is high during the growing season (Callihan et al. 1995), However, its nutrient value declines as the plants mature. Yellow starthistle in the pre-spiny stage contains between 8 to 14% protein (Thomsen et al. 1990). However, an analysis of the nutritional status of cattle manure in the fall indicated that yellow starthistle-infested pastures contain considerably less crude protein and total digestible nutrients compared to uninfested pastures (Barry 1995).

Its introduction in North America probably occurred sometime after 1849 as a seed contaminant in Chilean-grown alfalfa seed (DiTomaso, 2001).The movement of contaminated hay and uncertified seed are also important long distance transportation mechanisms (DiTomaso, 2001).Seed is transported in large amounts on the undercarriage of vehicles (DiTomaso, 2001).

DiTomaso (2001) states that due to the spiny nature of the plant, livestock and wildlife avoid grazing in heavily infested areas. Thus, infestations can greatly increase the cost of managing livestock. In addition to rangeland, pastures and grasslands, C. solstitialis is also an important weed problem along roadsides and an occasional problem in dryland cereals, orchards, vineyards, cultivated crops, and wastelands (Maddox et al. 1985, in DiTomaso, 2001). It can also reduce land value and reduce access to recreational areas (DiTomaso et al. 1998, Roché and Roché 1988, in DiTomaso, 2001). In addition, C. solstitialis infestations can reduce wildlife habitat, displace native plants, and decrease native plant and animal diversity (Sheley and Larson 1994, in DiTomaso, 2001). Because of its high water usage, C. solstitialis threatens both human economic interests as well as native plant ecosystems (Dudley 2000, in DiTomaso, 2001). Although no economic assessments have been conducted for C. solstitialis, millions of dollars in losses probably occur from interference with livestock grazing and forage harvesting procedures, producing lower yield and forage quality of rangelands (Callihan et al. 1982, Roché and Roché 1988, in DiTomaso, 2001). DiTomaso (2001) states that when ingested by horses, it causes a neurological disorder of the brain called nigropallidal encephalomalacia or \"chewing disease.\" Continued feeding results in brain lesions and ulcers in the mouth (Kingsbury 1964, in DiTomaso, 2001). In most cases, poisoning destroys the animal’s ability to chew and swallow and death occurs through starvation or dehydration (Panter 1991, in DiTomaso, 2001). DiTomaso (2001) states that only horses are affected by ingesting C. solstitialis. Other animals, including mules and burros, are not susceptible to the toxic effect of the weed.

DiTomaso (2001) states that mechanical, cultural, biological, and chemical control options are available for management of C. solstitialis. Most often a single method is not effective in the sustainable control of C. solstitialis and other range weeds. A successful long-term management program should be designed to include combinations of mechanical, cultural, biological, and chemical control techniques.

\r\nPhysical: Mowing can be used as a mechanical option for control provided it is well timed and used on plants with a high branching pattern. Cultural control options include grazing, prescribed burning, and re-vegetation with competitive species. Burning should be timed to coincide with the very early C. solstitialis flowering stage; at this time, it has yet to produce viable seed, whereas seeds of most desirable species have dispersed and grasses have dried to provide adequate fuel. Fire has little, if any, impact on seeds in the soil. In addition to controlling C. solstitialis, burning will reduce the thatch layer, expose the soil, and recycle nutrients held in the dried vegetation. Re-vegetation programs using perennial grasses or legumes can be effective for management of C. solstitialis, but establishment may be difficult in areas without summer rainfall.

\r\nChemical: Clopyralid and picloram (not registered in California) are the most effective herbicides for full season control of the weed. Unlike most post-emergence herbicides, they provide both foliar and soil activity. The best timing for application is when C. solstitialis is in the early rosette stage. Clopyralid gives one season of control and is generally used at 110gm a.e./ha; 290gm product/ha. Picloram has longer soil residual activity than clopyralid (two to three years) and is applied at 0.28kg and 0.42kg a.e./ha. Glyphosate is a non-selective herbicide that is also effective on C. solstitialis. It will control bolted plants at 1.1kg a.e./ha; 9.4 liters product/ha or 1% solution and can be used as a late season spot treatment to small infestations or escaped plants.

\r\nBiological: Sheep, goats, or cattle are effective in reducing C. solstitialis seed production when grazed after plants have bolted but before spines form on the plant. Goats will eat the plant even in the spiny stage. Six biological control agents of C. solstitialis have been imported from Europe and are well established in the western United States. Of these, most effective are the hairy weevil (Eustenopus villosus) and the false peacock fly (Chaetorellia succinea). These insects attack the flower/seed head, and directly or indirectly reduce seed production by 43 to 76%. They do not, by themselves, provide sustainable management of C. solstitialis but can be an important component of an integrated approach. The most widely studied pathogen for C. solstitialis control is the Mediterranean rust fungus, Puccinia jaceae. It can attack the leaves and stem of C. solstitialis, causing enough stress to reduce flowerhead and seed production. The organism is currently under investigation and has not been released for use.