Thanks again to Bill of San Jose, California (bbum@Flickr) for today’s photograph (BPotD Flickr Group Pool | original image 1 | original image 2). I’ve pointed out Bill’s eclectic weblog before, too: bbum’s weblog-o-mat.
Native to Mediterranean Eurasia, yellow starthistle was introduced into California sometime in the mid-1800s via a batch of Chilean-grown alfalfa seed. (the species had already been established in South America for decades, if not centuries, prior). It is estimated that it is now spread over 15 million acres (6 million ha) in the state of California alone. Yes, it is highly invasive.
A site dedicated to yellow starthistle from the University of California in Davis provides the biology and ecology of the species. It also includes suggestions on pest control for this weedy species.