New weeds to watch for in Douglas County: Yellow Starthistle

By Aaron Rosenblum
Foster Creek Conservation District

This is the third in a series of articles that introduces readers to new weed invaders in Douglas County. The weed species featured are known from only a few locations in the county, but have the potential to become wide-spread and problematic. The goal of these articles is to inform and educate Douglas County residents, so that you can help us with the early detection and rapid treatment of small weed populations before they become major issues throughout the county.

 

Yellow starthistle is a highly aggressive and invasive plant. (Provided photo/Washington State Noxious Weed Control Board)

This week’s weed to watch for is yellow starthistle (Centaurea solstitialis). Yellow starthistle is a highly aggressive, invasive plant that is native to the Mediterranean region of the world. It was introduced to the United States in the 1800s and has become a widespread problem throughout the western states where it now occupies more than 18 million acres.

Yellow starthistle is a member of the sunflower family (Asteraceae) and is closely related to spotted knapweed, last week’s “New Weed to Watch for in Douglas County,” as well as all other knapweeds.

It is a winter annual, meaning seeds germinate in the fall following rain events, and the plant over-winters as a rosette (a circle of leaves close to the ground lacking a stem) with deeply lobed leaves. In the spring and early summer, stems emerge with multiple branches, each generally reaching between 1-3 feet tall prior to flower formation. The entire plant is covered in dense cobwebby hairs, giving the yellow starthistle it’s characteristic gray-green color. Each branch produces a single, terminal, flower head with yellow flowers. The lower portion of the flower heads are composed of structures known as bracts. Yellow starthistle’s bracts have pointy, straw colored spines up to one-inch long. Each plant can produce up to 6,000 seeds under favorable conditions that can remain viable in the soil up to 10 years.

Yellow starthistle over-winters as a rosette (a circle of leaves close to the ground lacking a stem) with deeply lobed leaves. (Provided photo/Washington State Noxious Weed Control Board)

Yellow starthistle thrives in disturbed locations, but it can invade undisturbed areas as well. It prefers full sun, relatively dry soil, and is commonly found on roadsides, rangeland, pastures, shrub steppe, abandoned cropland, or any other disturbed location.

Yellow starthistle is highly competitive and will quickly outcompete native plants. Part of this advantage is due to the species’ ability to rapidly grow extensive roots. These large roots aggressively utilize soil moisture before native plants are able to. Without intervention, this can result in large, dense infestations of yellow starthistle.

Reduced habitat, forage and property values are all results of yellow starthistle invasion. In addition to replacing native plants and pasture grasses, the long spines on the flower heads interfere with grazing and recreation. Additionally, ingestion of yellow starthistle by horses can cause chewing disease, which can be fatal.

Foster Creek Conservation District is leading the development of the Douglas County Cooperative Weed Management Area and has funding to assist landowners with the control of yellow starthistle as well as the other weeds featured in this series of articles. If you believe you have yellow starthistle on your land, please contact me at 423-5990 or arosenblum@fostercreekcd.org; or Dale Whaley with the WSU Extension at 745-8531, ext. 6352.

 

Aaron Rosenblum is the natural resource specialist for the Foster Creek Conservation District.