San Luis Obispo County Demonstration Garden
- Author: Tami Reece
Published on: March 5, 2012
By Tami Reece Master Gardener
Never give up, never give up, never give up. Winston Churchill
Yellow starthistle is the most widespread invasive weed in California. It is a gray-green to blue-green plant with a deep, vigorous taproot. It produces bright, thistlelike yellow flowers with sharp spines surrounding the base. Yellow starthistle seeds germinate from fall through spring. After germinating, the plant initially puts all its energy into root growth. By late spring, roots can extend over 3 feet into the soil, although the portion above ground is a small green plant. Control of yellow starthistle cannot be accomplished with a single treatment or in a single year. Effective management requires control of the current plants and the stopping of seed production, combined with the planting of competitive, desirable vegetation. Both postemergent and preemergent herbicides are needed to control yellow starthistle. Preemergent herbicides must be applied before seeds germinate to be effective and the long germination period of yellow starthistle requires that a preemergent material may need to be combined with a postemergent herbicide. Make applications of preemergents before a rain, so the water will soak the herbicide into the soil. Postemergent herbicide treatments generally work best on seedlings, but the long germination period of yellow starthistle makes control with a single application almost impossible. Control practices, such as herbicides, are capable of reducing yellow starthistle populations, but in the absence of competition, starthistle will often reestablish. Desirable plantings of wildflowers and perennial grasses are great choices to help with effective management. Other control methods can include grazing by goats, sheep or cattle. Mowing once the flower spikes are up but before the flowers open, is optimal time. If you see one flower and it goes to seed, you will have a seed source for several years. The website www.ipm.ucdavis.edu is a great resource for eradication choices and herbicide suggestions. If you own horses beware, as yellow starthistle is poisonous to horses and they should not be allowed to graze in pastures where yellow starthistle is present. With patience, hard work and tenacity, starthistle can be eradicated on your property.
Yellow Starthistle Management Guide is available through California Invasive Plant Council.