Yellow Starthistle (Centaurea solstitialis)
Yellow starthistle is a tap-rooted annual forb known for its bright yellow flowers and sharp, straw-colored spines. It is an aggressive invader of rangelands, roadsides, pastures, and disturbed areas.
​
Plants germinate in fall or early spring and form a basal rosette of deeply lobed leaves. As the plant matures, it produces upright, branching stems that typically grow 1 to 3 feet tall.
​
Basal leaves are deeply lobed and gray-green. Upper leaves are smaller, narrower, and sharply pointed, becoming progressively reduced in size up the stem. Leaves and stems are covered with a cottony, wool-like fiber, giving the plant a grayish appearance.
​
Flowering occurs from late spring through summer. Flowers are solitary and bright yellow, borne at the ends of stems. Each flower head is surrounded by stiff, sharp spines up to 1 inch long that radiate outward from the involucre — a key identifying feature.
​
The plant develops a strong taproot that allows it to survive drought conditions. Seeds are produced in the flower heads and can remain viable in the soil for several years.
Quick ID Guide
Life Cycle:
• Annual
• Germinates in fall or early spring
Height:
• 1–3 feet tall
Roots:
• Deep taproot
Leaves:
• Basal leaves deeply lobed
• Upper leaves are small and pointed
• Gray-green
• Cottony, woolly texture
Flowers (Late Spring–Summer):
• Bright yellow
• Solitary flower heads
• 1-inch sharp spines around involucre
Seeds:
• Produced in spiny flower heads
• Viable for multiple years
Management Options
Prevention and early detection are critical. Clean equipment, vehicles, and livestock before moving from infested areas, and use certified weed-free forage and seed to prevent spread.
​
Small infestations can be managed by hand-pulling before seed production. Plants must be removed before flowering and seed set. Mowing can reduce seed production if timed during early flowering, but mowing too early may stimulate regrowth, and mowing after seed formation may spread seeds.
​
Targeted grazing with sheep or goats can reduce seed production when properly timed.
​
Larger infestations can be controlled with herbicides. Applications are most effective when made to rosettes in late fall, winter, or early spring before bolting. Treatment during bolting or early flowering may also reduce seed production.
​
Always read and follow herbicide label instructions before use. Contact Sublette County Weed & Pest for guidance on herbicide selection, application rates, and timing.
