With a vibrant yellow crown of petals atop an array of one-inch purplish spines, yellow starthistle (Centaurea solstitialis) is the most common exotic plant in California, covering some 15 million acres (Pitcairn et.al. 2006). During the dry summer months, its colors fade and the plant becomes dangerously combustible — a fuel for wildfires. Able to thrive in the disturbed soils of agricultural lands, this invasive releases a toxin that deters native plant growth and has deep tap roots that rob other plants of water. It also spreads very rapidly.
At Fort Hunter Ligget, an Army training facility in Jolon, California, a small patch of yellow starthistle spread to 20,000 acres in just five years (Department of Defense 2011). The plant’s sharp spines tore through trainees’ parachutes at the cost of several thousand dollars apiece. It also clogged vehicle air filters and created a wildfire threat during munitions training. At significant cost, the Army in 1999 was able to cut the infestation by 4,800 acres, but budget limitations currently prevent further action.
Statewide, it costs about $12.5 million annually to combat yellow starthistle on just half a million acres of agricultural, range, and wildlands — and this is just a fraction of the total acreage affected (California Invasive Plant Council 2006). The biodiversity costs are harder to measure. Research suggests that the plant soaks up roughly 15 billion gallons of water a year, amounting to $16 million to $75 million in water conservation costs in the Sacramento River watershed alone (Gerlach 2004). In addition, cattle and sheep can’t feed on the plant after its spines emerge, so a pasture covered with 20 percent yellow starthistle will reduce grazing capacity by 10 percent. Horses that feed on large amounts of the plant will suffer from “chewing disease,” a neurodegenerative disease that is usually fatal.
To combat yellow starthistle, the California Department of Food and Agriculture, the California Invasive Plant Council, and other organizations recommend early detection and eradication. For now, herbicides and introduced plant-eating insects from Europe—including two weevils (Bangasternus orientalis and Eustenopus villosus) and two flies (Urophora sirunaseva and Chaetorellia succinea)—are barely keeping the plants at bay. But such efforts may become pointless. “One of the big concerns in California is that it is invading higher elevations in the Sierra Nevada mountains that were once thought to be too cold,” says Elizabeth Brusati, Science Program manager at the California Invasive Plant Council. As the climate warms, the plant will spread, creating what may be an insurmountable problem for managers.