Yellow star thistle is really not a thistle, but instead is a species of knapweed. It is a weedy annual or biennial, ranging from 20-80 cm in height. The stems are thinly covered with woolly hairs and appear winged as a result of decurrent leaf bases. The basal leaves, especially those of young seedlings, are deeply lobed or pinnatifid, and may look like those of the common dandelion and range to 20 cm long and 5 cm wide. The stem leaves are reduced in size upwards, becoming linear and entire margined.
The flower heads are few to numerous, located at the ends of the branched stems, with the involucre 10-15 mm tall, and the middle and outer bracts tipped with long spines from 11-22 mm long (note photos). The spines are a straw color. The flowers are yellow.
Yellow star thistle is a bothersome weed of fields, waste areas, and roadsides.
Native to the Mediterranean region, yellowstar thistle is found over much of the western United States on the east side of the Cascades from Washington to California. It is also becoming established in the upper Rogue Valley of southwestern Oregon.
