"The Latin term solstitium* refers to the summer solstice or the longest day of the year, and -alis means ‘pertaining to’. Thus, the specific epithet solstitialis means pertaining to the longest day of the year. This is in reference to the ability of Centaurea solstitialis to flower very late into the summer.”* (http://wiki.bugwood.org/Centaurea_solstitialis)
Still, sheep, goats or cattle are effective in reducing C. solstitialis seed production by grazing after plants have bolted (opened prematurely) but before the spines form. Goats will eat star thistle even in the spiny stage.
C. solstitialis is considered a weed even on its native habitat that is believed to be Eurasia. It is native to the Balkans, Asia Minor, the Middle East and southern Europe. Yet, it is kept in check by its natural herbivore enemies and other plants that have co-evolved with it. In many of the non-native regions it has been introduced where the natural biological controls do not exist, it has become an invasive species and noxious weed.
C. solstitialis has been introduced in several parts of the world, including Australia, Argentina, Chile, and the USA. Its introduction in North America is said to have occurred as a seed contaminant in Chilean grown alfalfa seed, also known as Chilean clover. C. solstitialis was first introduced to Chile from Spain in the 1600s and from Chile to California at the time of the gold rush after 1848.
C. solstitialis is actually a winter annual (sometimes biennial) that grows up to a meter high. It can form dense impenetrable ‘stands’ displacing desirable vegetation in natural areas, rangelands (lands grazed by domestic livestock or wild animals), and other places. It is best adapted to open grasslands with deep, well-drained soils and wet winters and dry summers. Plants usually senesce (age and die) in late summer or fall. The majority of seed dispersal occurs soon after dried flowers are detached from the heads. The spines are shed but the chaff (dense fuzzy gray hairs) remains in the involucre (a group of one or more protective whorls of bracts beneath a flower or flower cluster). The plants form dense thatches in this state.
My favorite thistle is not all trouble, though. Not every aspect of Centaurea solstitialis or yellow star thistle is detrimental.
Yellow star thistle is a valuable source of pollen for pollinators. It is a major source of nectar for many butterflies. It is also regarded as an important honey source plant.
Similar to many plants classified as 'weeds', it establishes itself quickly and protects and restores soil that has been left exposed by natural and human-caused disturbances. It populates ground that has been abused; dried up, compacted, or scraped clean. Being a plant with a tap root system, it has a crucial role in restoring soil fertility by bringing up vital micronutrients such as minerals. In a plant with a taproot system, the taproot is the largest, most central, and most dominant root from which other roots sprout laterally. Typically a taproot is somewhat straight and very thick, is tapering in shape, and grows directly downward. It is difficult to remove a taproot from the ground but the system makes up for the inconvenience by breaking up compacted subsoils. “This improves infiltration of moisture into the soil, reduces surface runoff and helps control erosion.” (Rural Development through Carbon Finance, Sebastien M. Scholz-https://books.google.ca/books?isbn=3631592507)
Yellow star thistle is very pretty to look at too.
*The word 'solstice' translates from the Latin solstitium, meaning 'stopped sun,' in reference to the winter and summer solstices where the Sun's daily arc across the sky reaches its extreme southerly and northerly limits.