Yellow starthistle (Centaurea solstitialus L.) is usually a summer annual, in protected areas rosettes can appear in the fall. Produces a single stem from the rosette that quickly branches. Rosette leaves are deeply lobed with the end lobe rounder and larger than the side lobes. Stem leaves attach directly to the stem by a wing that usually runs the stem length. Mature plants appear dusty or muted green. Bright yellow flowers begin to form at each stem terminus in early June. The bract tips around the urn shaped seed head develop into stiff sharp spines that will be from 1 to 2 inches long. Variable in height from 6 inches to 3 feet depending on environmental conditions. Two types of seed are produced, one is immediately viable, and the other more numerous must go through a vernalization period. Each seed head can produce 50 seeds with 95% viability. Toxic to horses, causing chewing disease, a chronic incurable and likely fatal condition.

Reproduces only from seed, so stopping that source is the key to control. Don’t underestimate this plant, it can colonize an area very rapidly. It is not particularly competitive and healthy range and pastures are rarely or at least slowly invaded. It is highly sensitive to all herbicides and can be sprayed out quite easily in the early stages of growth.
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Modified from Selected Noxious Weeds of Eastern Oregon, Gary Page, Malheur County Weed Inspector.