Yellow starthistle is an extremely invasive, fast-spreading member of the knapweed family. California has reported more than 15 million acres infested and yellow starthistle has displaced leafy spurge as the most invasive weed found in Idaho. It is also been reported in Montana, South Dakota and Minnesota. Most infestations in North Dakota have been on CRP lands where the grass seeds were produced in areas with yellow starthistle.
Yellow starthistle is an annual that often grows 3 feet or more tall and is branched with winged stems. Each stem terminates in bright yellow flowers with needlike straw-colored bracts often up to 2 inches long. Both stems and leaves are covered with pubescent hairs that give the plant a grayish appearance. Yellow starthistle reproduces and spreads only by seed. A single plant can produce as many as 150,000 seeds, of which 90 percent or more are viable, and can remain dormant in the soil for a few years.
Livestock and wildlife will not graze where yellow starthistle grows because of the sharp spines around the flower. It is adapted to a wide variety of environments and will out-compete most native plants, reducing both native wildlife and plant diversity.
Control on non-crop lands:
Prevention is the best method to keep yellow starthistle from invading North Dakota cropland, rangeland and wildlands. Yellow starthistle is most likely to be found in recently seeded pastures or CRP fields; along highways, railroad tracks and other transportation or communication lines.
Chemical: The most effective herbicides are Milestone (aminopyralid), Tordon (picloram) and dicamba (various). Treat an additional 10 to 15 feet around the infestation area to control seedlings. A careful follow-up program is necessary to control missed plants and seedlings.
Cultural: Grazing, mowing, burning and maintaining competitive forages can be used in conjunction with herbicides to keep yellow starthistle from establishing. Hand pulling is also effective.
Biological: Biological control is in the research and implementation stage in states with large acreage of yellow starthistle infestations.