A wash of prickly, pale green stems is scattered across a dry field. Yellow flowers are surrounded by a mean halo of long narrow thorns. Here is yellow star-thistle (Centaurea solistalis) one of the nastiest invasive plants around. It’s a big problem on farmland and in wild places along the west coast. It interferes with grazing, and eventually leads to permanent brain damage in horses that eat it.
Introduced in the 1850s, this thistle is now the most widespread invasive plant in California, infesting between 10 and 15 million acres in the state. A native to southern Europe, it probably first came to the US indirectly, along with alfalfa seed imported from Chile. By the early 1900s it was a serious weed in the Sacramento Valley and was spreading quickly along roads, railways, trails and streams, according to the CalIPC. “It is a thousand times as common as ten years ago, and perhaps even six years ago,” observed Willis Jepson in 1919.
Things have only gone downhill from there! And it’s not surprising why. This deeply taprooted annual invades summer-dry grasslands across most of the US. A single large plant can produce nearly 75,000 seeds, and blooms from late spring through fall. It’s largely pollinated by honeybees but doesn’t have much in the way of predators, and so it continues to spread.