Yellow Starthistle, Centaurea solstitialis

Everybody loves sunflowers, but nobody loves yellow starthistle. It is a noxious, and obnoxious, cousin of the sunflower. Other relatives include knapweeds, chicory, dandelion, safflower, and artichoke. Starthistle is a European weed that grows on various soil types and usually inhabits rangelands, vineyards, and orchards. Starthistle is also commonly seen along roadsides and in fields. It likes the sun and cannot tolerate shade, which is why it can be found in the wide-open spaces of our Western States. This weed infests about 3 million acres in Idaho, California, Oregon, and Washington.

Yellow starthistle is an annual plant, though it lasts much longer than many other annuals. Often it doesn't even mature until late summer or early fall. As starthistle reaches maturation, it begins to lose its leaves. The yellow flowers fade to light tan as entire fields become filled with skeletonlike silver-grey husks. When the flowers dry sufficiently, the seeds are ready to be scattered by the wind or other means.

Adult plants grow between 2 to 3 feet (60­90 cm) tall. They have very stiff, branching stems covered with a white fuzz. The flowers are bright yellow and located on the ends of branches. There is only one flower per branch. From underneath each flower grow long, straw-colored spines. Seen from above, these spines put the "star" into starthistle. While all this makes for a pretty plant, several States have recognized the danger of this foreign weed and made it illegal to allow yellow starthistle to grow.

Yellow starthistle is a very competitive plant. In the flora Olympics, this noxious weed wins the root-development race against most native perennial grasses every time, even in winter. While cattle, sheep, and goats may graze on the weed in early spring, they won't after the plant develops its star-patterned spines. Yellow starthistle can also present a very serious problem when found in horse pastures. If horses eat it, the starthistle can cause a neurological disorder commonly known as chewing disease (nigropallidal encephalomalacia). Between displacing native forage plants and poisoning horses, yellow starthistle poses a serious threat to U.S. rangelands.