Origin:
Introduced from Europe, and now one of the worst weed problems in rangeland and open areas in the western United States, particularly in California.
Biology:
An annual, fast-growing and very bushy weed that forms massive and thick stands that crowd out all other vegetation. It is an extremely spiny plant when mature, due to the circle of stout spines around the flower head, and is generally unpalatable to livestock. However, if horses are relegated to eating starthistle they can be killed due to lesions in their brain, caused by the weed. Reproduction is from seeds which germinate in winter to early spring. Plants mature after most other annual plants have dried and died back, giving starthistle even less competition for growth.
Identification:
Mature plants can grow as tall as 3 feet, but it is very heavily branched and spreading, and individual plants may occupy an area of several feet in diameter. Branching is from the base as well as along the stems, keeping the base of the plant close to the ground where it covers the soil. Stems are rigid, grayish-green and “winged”, and covered with soft hairs or almost a cottony wool. Basal leaves are large and deeply lobed, but the leaves that form along the upper stems are short and very narrow, and only sparsely placed so that the plant has a very leafless appearance. Flower heads are extremely numerous, forming as solitary heads at the ends of the branches. The round, enclosed bracts below the flowers have numerous long, stiff spines encircling them, and these persist into the winter after the plant has died. Flowers are bright yellow ray and disc flowers.
Characteristicts Important to Control:
Plants tolerate very dry, compacted soils and mature when much of the other vegetation has died back. A very heavy seed production results in massive ground covers of the plants. Chemical controls must be timed for early in the season, prior to production of seeds.
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