Yellow starthistle
Not a thistle, not from here, and fatal to horses. This one is a problem to be managed rather than a plant to admire.
In brief
Centaurea solstitialis, Asteraceae.
- Annual, 30 to 100 cm
- Introduced. Native to the Mediterranean and western Asia
- Flowers June to September
- Fruit a cypsela; a single plant can set tens of thousands
Where it grows
Arrived in California in the 1850s in contaminated alfalfa seed and now covers millions of acres of the state's rangeland. It takes dry, disturbed, overgrazed ground, which describes a great deal of the foothills.
When it blooms
June to September, holding yellow well after the grass has gone brown, which is what makes it easy to spot and easy to map.
Seed and fruit
A cypsela. One plant can produce tens of thousands of seeds, and they stay viable in the soil for years.
Here
Photographed on August 28, 2026 in dry grass. It is a knapweed rather than a true thistle, which is why the flower heads are armed with long straw-colored spines rather than the soft plumes of a real thistle.
It matters here for one specific reason: it is fatal to horses. Sustained eating causes an irreversible brain lesion, chewing disease, for which there is no treatment. Cattle and sheep are unaffected and will graze it.
Pull it before it seeds, take the crown with it, and do not compost the heads.
By Yggdrasil Springs
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