Summer brings a wide assortment of unwanted weeds to Lake County. There are a number of invasive aquatic weeds in Clear Lake and the open fields around the county are loaded with various types of weeds. Most of these invasive weeds come from other countries and have become established throughout the United States.
One of the most abundant and undesirable weeds is the yellow star thistle. It is a native of Eurasia and it is believed to have arrived in American in shipments of alfalfa seed and introduced into western North America in the mid-1800s.
California has more star thistle than any other state with about 15 million acres infested. There are thousands of acres in Lake County alone that are covered with star thistle, and the weed continues to spread. Star thistle seeds are often transported by vehicle tires. In fact, star thistle can even be found in the more remote sections of the Mendocino National Forest such as the Snow Mountain Wilderness Area. It got to these remote areas on the hooves of pack horses. Seeds have even been found on the soles of hiking boots.
The mature plant is gray-green in color and grows to a height of 1-5 feet with a deep root system. It produces bright, thistle-like yellow flowers with sharp spines surrounding the base. The stems of mature plants are rigid, spreading and typically branching from the base in open areas.
Stems and leaves are covered with a loose, cottony wool that gives them a whitish appearance. Stems appear winged because of leaf bases that extend beyond the nodes. Basal leaves are 2-3 inches long and deeply lobed. Upper leaves are short (half-inch to an inch long) and narrow with few lobes.
Yellow star thistle is an invasive competitor to desirable plants in ranges and pastures. It can crowd out grasses where soil moisture is limited or where forage has been weakened by grazing. Where this weed is well established its sharp spines may exclude livestock from grazing on any grasses growing beneath.
Yellow star thistle produces a toxin that causes death in horses through an illness called “Chewing Disease,” which makes it impossible for the animal to swallow. The plant has little or no nutritional value to wildlife. Actually, star thistle will crowd out native forbes and other plants that deer and other wild animals eat.
Yellow star thistle seeds germinate from fall through spring, which corresponds to the normal rainy season in California. After germinating, the plant initially allocates most of its resources to root growth. By late spring, roots can extend over 3 feet into the soil profile although the portion above ground is a relatively small basal rosette. This allows yellow star thistle to out-compete shallow-rooted annual species during the drier summer months when moisture availability is limited near the soil surface. It also helps explain why yellow star thistle survives well into the summer, long after other annual species have dried up, and why it can regrow after top removal from mowing or grazing.
There are methods used to control star thistle. Mowing can be used provided it is well timed and used on plants with a high-branching pattern. Mowing early growth stages results in increased light penetration and rapid regrowth of the weed. If plants branch from near the base, regrowth will occur from recovering branches. Repeated mowing of plants too early in their life cycles or when branches are below the mowing height will not prevent seed production as flowers will develop below the mower cutting height.
Plants with a high-branching pattern are easier to control as recovery will be greatly reduced. Even plants with this growth pattern must be mowed in the late spiny or early flowering stage to be successful. An additional mowing may be necessary in some cases.
Beetles were recently imported from the Eurasia that eat the flowers and buds of star thistle. The introduction has been successful in some areas and there is hope that the success rate will improve. If you own more than an acre of land in Lake County it’s almost certain you have star thistle.
