Yellow starthistle (Centaurea solstitialis)

 

Yellow starthistle originated from the Mediterranean area and Asia. It is a winter annual, germinates in the late fall to early winter and lies dormant until the early spring, resuming growth long before most natives are up. It reproduces entirely by seed, as many as 100 per plant, which can remain viable in the soil for up to 10 years, although roughly 97% of the seed will germinate in the first year.

Yellow starthistle is toxic to horses,
causing a fatal neurological disease called "chewing disease." This plant was documented in Nevada as early as the 1920’s, but only recently began to establish large populations. California currently has over 20 million acres infested.