Yellow Starthistle

Centaurea solstitialis · Asteraceae (aster family)

Winter annual
Broadleaf
Non-native
Toxic / hazard

If a gray, cottony weed in a dry field is covered with bright yellow flowers guarded by long, stiff spines, it is very likely yellow starthistle.

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Yellow Starthistle (Centaurea solstitialis) growing — weed identification photo — Photo: Victor M. Vicente Selvas · CC BY-SA 4.0 · Wikimedia Commons

Safety warning

Toxic to horses. Eating it over weeks causes chewing disease (nigropallidal encephalomalacia), a brain disorder with lip twitching, tongue flicking and involuntary chewing; it is cumulative, untreatable and usually fatal through starvation or dehydration. Poisoning has followed intake of about 50 to 150 percent of a horse's body weight over one to three months. Spines also injure grazing animals' eyes, noses and mouths.

Quick ID

Yellow flower heads ringed by stiff spines up to about an inch long
Winged stems where leaf bases run down them
Gray-green plant with loose cottony hairs
Deeply lobed rosette leaves in winter and spring
Dry, sunny, open ground in the West

Fact sheet

Scientific name: Centaurea solstitialis
Family: Asteraceae (aster family)
Also called: Yellow star-thistle
Life cycle: Winter annual (occasionally biennial)
Native to the US?: No. Native to Eurasia; reached California around 1850 by way of South America.
Height: Anywhere from 6 inches to about 6 feet, depending on moisture and competition.
Leaves: Rosette leaves 2 to 3 inches long and deeply lobed; stem leaves short (about half an inch to an inch) and narrow. Covered in loose, cottony gray hairs.
Flowers: Bright yellow, thistle-like heads ringed by stiff, straw-colored spines roughly 3/4 to 1 inch long. Blooms May to October in California and June into December in the Pacific Northwest.
Seeds / fruit: Two kinds per head: dark outer seeds with no bristles and lighter, glossy inner seeds with a white tuft. Heavy seeds mostly drop near the plant; most sprout within a year and few last past four.
Roots: A deep taproot that passes 3 feet by late spring and can reach 4 feet or more.
Spreads by: Only by seed, carried on vehicles, ATVs, bikes, livestock, boots, clothing, soil and hay.
Where you’ll find it: Dry, sunny grasslands, rangeland, pastures, roadsides, vacant lots, parks, trails and open woodland edges, mainly in the far West.
When you’ll see it: Sprouts after the first soaking fall rains, stays a rosette through winter and spring, bolts in late spring and flowers through summer and fall.
Toxicity: Toxic to horses. Eating it over weeks causes chewing disease (nigropallidal encephalomalacia), a brain disorder with lip twitching, tongue flicking and involuntary chewing; it is cumulative, untreatable and usually fatal through starvation or dehydration. Poisoning has followed intake of about 50 to 150 percent of a horse's body weight over one to three months. Spines also injure grazing animals' eyes, noses and mouths.
Legal status: Not on the federal noxious weed list (checked against the USDA APHIS list). Listed as a noxious weed in Colorado (List A, where fewer than 10 populations are known), Oregon (B) and Washington (Class B). Check your state's list.
Difficulty to control: Hard
Best time to act: Before flowering: herbicides on seedlings and young rosettes in winter and early spring, pulling at bolting, grazing before spines form, and mowing when 2 to 5 percent of heads are blooming.

What is yellow starthistle?

Yellow starthistle is a spiny Eurasian annual that University of California experts rank among the worst weeds in the western United States. Confident yellow starthistle identification comes down to yellow, thistle-like flowers with sharp spines beneath, stems that look winged, and a gray cast from fine cottony hairs. It sprouts with fall rains, builds a deep root through winter and blooms in summer heat.

Dense stands crowd out native plants and forage, offer little to wildlife, make trails impassable and dry into flammable fuel, while the taproot draws down deep soil moisture. It poisons horses, contaminates hay and grain, and its spines injure animals after it flowers. UC estimates more than 14 million infested acres in California alone.

Yellow Starthistle identification: what to look for

Flowers make it easy from early summer on. Before that, look for a flat rosette of deeply lobed, grayish leaves that resemble dandelion leaves. As it bolts, check the stem for green wings running down from each leaf base. Close relatives such as Malta starthistle look nearly the same until they bloom, so confirm with the spines.

Rosette: Gray-green, 3 to 8 inches across, with deeply lobed leaves 2 to 3 inches long.
Stems: Rigid and spreading, often branching from the base, winged and coated in cottony wool.
Upper leaves: Small, narrow and mostly unlobed, about half an inch to an inch long.
Flower heads: Single bright yellow heads at the branch tips with long yellowish spines radiating like a star.
Seeds: Dark, bristle-free outer seeds and lighter inner seeds with a white tuft in the same head.

Yellow Starthistle (Centaurea solstitialis) — identification photo — Photo: Stefan.lefnaer · CC BY-SA 4.0 · Wikimedia Commons

Where it grows in the United States

The core range runs from Washington and Oregon south through California and east into Idaho, Nevada, Utah and Arizona, with Washington, Oregon, California and Idaho the worst infested. It has been reported in as many as 41 states, but populations in the eastern two-thirds of the country tend to be small and short-lived.

It thrives in a Mediterranean-type climate of cool, wet winters and hot, dry summers, as in California's Central Valley, with about 10 to 60 inches of yearly precipitation, from sea level to roughly 8,600 feet. It needs full sun at ground level and does poorly in shade or maintained turf. Road work, grading and landscaping that disturb soil help it move into grasslands, oak savannas, forest openings and rangeland.

Yellow Starthistle look-alikes

A few plants get mixed up with it:

Malta starthistle (tocalote) (Centaurea melitensis): Nearly identical until bloom, but its spines are shorter and less stiff, with three or four pairs of small spines at the base. It has also been tied to chewing disease in horses.
Diffuse knapweed (Centaurea diffusa): A related weed with small spines at the flower base, but its flowers are white to purple, not yellow.
Spotted knapweed (Centaurea stoebe): Similar lobed, bluish-green leaves, but it is a perennial with a stout taproot and pinkish-purple flowers.

How to get rid of yellow starthistle

Pulling, digging and mowing: Hand pull small patches any time before seed forms; it is easiest at bolting, when plants are recognizable and the soil is moist. Mow when about 2 to 5 percent of heads are blooming, cutting below the lowest branches. Mowing earlier lets light reach new seedlings and plants regrow, and low-branched plants slip under the blade, so string-trim those at ground level. Expect more than one pass, and mow when soil is dry. Dry plants burn easily: mow early in the morning, avoid rocks and windy days, use spark arresters and keep water handy. On bare garden ground, one shallow cultivation as the soil dries in spring, before seed set, kills seedlings.

Cultural control matters most, because the right conditions keep it from coming back.

Graze sheep, goats or cattle hard for a short time in late spring and early summer while plants bolt, before spines form; goats will also eat spiny plants. Never let horses graze it.
Starthistle competes poorly with maintained turf and dense groundcovers; plant perennials closely, mulch between them and reseed range with competitive grasses.
Seed-head insects released by USDA and California agencies, especially the hairy weevil and false peacock fly, cut seed output by about half.
Solarize small bare areas by covering moist soil with clear plastic through the hottest months.

Herbicides: Herbicides work best on seedlings and small rosettes, and because seeds sprout in waves, one treatment rarely finishes the job. UC IPM lists clopyralid and aminopyralid as selective options that spare grasses and keep working in the soil; in California, aminopyralid typically goes on December through March at the seedling to mid-rosette stage, and clopyralid January through March. Both can injure clovers and other legumes, so never use manure, compost or clippings from treated areas in a garden. Triclopyr targets young broadleaf plants, and glyphosate kills all stages but also kills grass, so spot treat. In landscapes, preemergence products with dithiopyr, pendimethalin, prodiamine or trifluralin, where permitted, go down just before the rainy season. Use a product labeled for your site; the label is the law. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Keeping it from coming back

Since it spreads only by seed, keep seed out. Clean tires, mowers, ATVs, boots and clothing after working in infested areas, stay out of flowering stands, and use clean hay and fill soil.

Pull isolated plants first, then work large stands from the edges inward. Plan on at least three straight years of control to drain the seed bank, expect year two to look worse, and coordinate with neighbors along shared fences and roads.

Questions we get about yellow starthistle

Is yellow starthistle poisonous to horses?

Yes. Weeks of eating it can cause chewing disease, an untreatable brain disorder that leaves horses unable to eat. Keep horses off infested ground and out of contaminated hay.

When should I mow yellow starthistle?

When about 2 to 5 percent of the heads are in bloom. Earlier mowing lets plants regrow and flower low.

How long do the seeds last?

Most sprout within a year and few last beyond four, though some survive up to about ten years in ideal soil.

Do bees use it?

Yes. It is an important late-season nectar and pollen source for California honey production, and bees drive much of its seed set.

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