Invasive Plant of the Month: February
Yellow starthistle (Centaurea solstitialis)
Also Known As
Golden starthistle, St. Barnaby's thistle, yellow cockspur
General Description
Yellow starthistle is an erect annual, sometimes biennial herb that is native to Europe. It can form impenetrable stands that displace native species and degrade wildlife habitat. Its flowers have sharp spines which make it inedible for animals and it can be toxic to horses. Yellow starthistle reproduces by seed and is considered one of the most serious rangeland weeds in the western US.
Growth Habit
Plants are grayish or bluish-green with rigid stems that branch from the base. Stems are covered with cottony grayish hairs. Yellow starthistle grows 1.5–5 feet tall and forms a deep, stout taproot.
Leaves
Lower leaves are bluish-green, 2–6 inches long, divided, and densely covered with fine hairs. Upper leaves become progressively shorter toward the top of the plant and are alternate and narrow, with smooth margins and sharp points.
Flowers
Bright yellow flowers resemble dandelions and occur individually on branch tips. Floral bracts have a small cluster of sharp, stiff, straw-colored spines that radiate outward, with a stout central spine that grows 0.4–1 inch long. Yellow starthistle flowers from May to December.
Seeds and Fruit
Each seed head produces 35–80 seeds. After seeds are dispersed, a whitish, cottony ball remains on the stem end.
Habitat
Yellow starthistle thrives in areas with full sunlight and well-drained or rocky soil. It can be found in disturbed areas such as pastures, rangeland, and along ditches and roadsides. It does not tolerate shade.
Additional Resources
Species Identification and Characteristics
Alberta Invasive Plants Council Fact Sheet (PDF)BugwoodWiki
CIPM Yellow Starthistle Weed Model
CIPM Yellow Starthistle Weed ID Card
Colorado Department of Agriculture Fact Sheet (PDF) Identification of Knapweeds and Starthistles in the Pacific Northwest: Pacific Northwest Extension (PDF)
Invasive.org Images and Overview
Montana State University Extension Fact Sheet (PDF)
Montana Weed Control Association Fact Sheet
Plant Conservation Alliance Fact Sheet
Texas Invasives Fact Sheet
USDA Forest Service Fact Sheet (PDF)
Management and Control Resources
Identification, Biology, and Integrated Management of Yellow Starthistle: Montana State University Extension (PDF)Selected Knapweeds of Washington: Washington State Noxious Weed Control Board (PDF)
USDA Forest Service Species Information
Yellow Starthistle Management Guide: California Invasive Plant Council (PDF)