THE ISSUE: Yellow starthistle

 Yellow starthistle (Centaurea solstitialis) is a winter annual, native to the Mediterranean region. It is thought that it was introduced into California in the early 1800’s as a contaminate in alfalfa seed. It invades rangelands, pastures, roadsides, and other disturbed sites. Its presence significantly reduces feed quality of forage. It is toxic to horses, causing chewing disease, which can be fatal. It also disrupts native ecology, habitat, and recreational opportunities. 

 Yellow starthistle seeds typically germinate in the fall and then form rosettes in the spring. Rosettes bolt in late spring early summer, stalks are typically 18-24 inches long. Leaves are bluish green in color and are covered in small white hairs.  Yellow starthistle reproduces solely by seed. A single plant can produce 75,000 -150,000 seeds per year if conditions are favorable. About 90 percent of seeds will fall within 2 feet of the parent plant, but seeds can also be transported by wind, water, animals, equipment, and on clothing. 

Integrated Pest Management (IPM) Options: 

• Prevention— Learn to identify this plant. Clean equipment and clothing after leaving invested areas. Always plant clean seed. 

• Mechanical— Hand digging or pulling can be effective on small infestations. Remove as much of the root system as possible. Repeated mowing can be effective, as it can keep this weed from going to seed. 

• Cultural— Establishing a healthy stand of beneficial plants that will compete with yellow starthistle. Utilize proper grazing management practices that favor beneficial plants.

• Biological— Certain weevil and fly species may help control yellow starthistle populations. 

• Chemical—Herbicide applications are most effective when applied to actively growing plants in the seedling and rosette stages. Refer to https://pnwhandbooks.org/weed for specific herbicide recommendations. Type “Starthistle” into the search box to easily find information about controlling yellow starthistle. Always read and follow herbicide label
directions! 

Justin Hatch, University of Idaho Extension Agriculture Educator in Caribou and Bear Lake Counties. 208-547-3205 [email protected]