Yellow Knapweed Centaurea solstitialis

Yellow star thistle is a winter annual in the Asteraceae (daisy) family native to southern Europe and North Africa.  It grows on rangelands, pastures, agricultural areas, along highways or roads, railroad tracks, and other transportation or communication lines. It is dependent on seed production for growth and spread. A distinguishing characteristic is the bracts of the yellow flower heads contain stout, needle-like, straw-colored spines one to two inches long that radiate from the flower head in the shape of a star.  The bracts are light green, variably covered in cobwebby hairs and may become smooth.

Yellow star thistle has a stout taproot and/or pubescent stems. Root growth during the winter and early spring is rapid and can extend well beyond 3 feet in depth. The stem is erect, slender, hairy and branching, and can grow up to three feet tall. 

Leaves are alternate and form a basal rosette of deeply lobed petiolate leaves covered in fine hair when young, up to 6 inches wide tapering to a short stalk.  This rosette will wither by the time the plant flowers.  All leaves are toothless and covered in woolly hairs giving a gray-green cast.  Leaves become much smaller, linear-oblong and undivided as they move further up the stem with the leaf bases extending down the stem, forming “wings." The wings are often wavy and may be broad with jagged or smooth edges.

Prefers well-drained soil of any type and any pH and can grow in nutritionally poor soil as well as very alkaline soils. It can tolerate drought.  It cannot grow in the shade. 

Genus name Centaurea came from the popular name of various plants in the late 14th century, from Medieval Latin centaurea, from Latin centaureum, from Greek kentaureion, meaning "centaur", so called because the plant's medicinal properties were discovered by Chiron the centaur.  Specific epithet solstitialis pertains to the longest day of the year. This is in reference to the ability of yellow star thistle to flower very late into the summer.

Sheep, goats or cattle are effective in reducing Yellow Star Thistle seed production when grazed after plants have bolted but before spines form on the plant. Goats will eat Yellow Star Thistle even in the spiny stage.  When ingested by horses, it destroys the animal’s ability to chew and swallow and death occurs through starvation or dehydration.

Quick ID

• Yellow thistle-like flowers
• Long, sharp spines on the bracts in the shape of a star
• Gray-green hairy foliage and winged stems 

Insects, Diseases, and Other Plant Problems:  It is considered invasive in many of the western United States and upper mid-West.  To manage the plants, in small spaces before bolting, pull the plants by hand and dispose of them.