starthistle is native to the Mediterranean region of Europe. It
is thought to have been transported to California in hay seed. This
plant is considered a noxious weed and is included on the State
Noxious Weed list. Yellow starthistle is a winter annual, which
means that it completes its lifecycle within 12 months, from winter
to late spring. The bright-yellow flowerhead consists of many individual
flowers and bracts. Each bract is armed with a stout, straw-colored
spine 1 to 2 inches long. The stem is white-wooly and branches from
the base and above to form a bushy plant, 1 to 2 feet tall. The
leaves are white-wooly, and are deeply lobed, 2 to 3 inches long.
starthistle depends on seed production for population survival.
A single yellow starthistle plant has the potential to produce up
to 150,000 seeds. This plant produces two types of seeds: those
with parachute-like plumes, and those without plumes. Studies of
yellow starthistle seed dispersal suggest that more than 90% of
the seeds fall within 2 feet of the parent plant, creating a slow
invasion front. Yellow starthistle is a serious problem on rangeland,
pastures, roadsides, and wastelands.
This species displaces native plant communities and reduces plant
diversity. Yellow starthistle forms solid stands that drastically
reduce forage production for livestock and wildlife. This plant
is poisonous to horses, causing a nervous disorder called "chewing
disease". Livestock can be injured if they are forced to feed on
the spiny portions of yellow starthistle.
starthistle has infested approximately 10 to 15 million acres in
California. Within the past 3 years, patches have been discovered
in Colorado in Mesa, Boulder, Broomfield, and Larimer Counties.
So far, Adams County has no reported infestations of yellow starthistle.