Conservancy staff hand-pull yellow starthistle.

December 7, 2022

By Jackson Brooke, Restoration Manager

Referred to as one of the most serious rangeland weeds in the state, yellow starthistle (Centaurea solstitialis) favors drought conditions and invades grassland, coastal prairie, and oak savanna habitats across California, opportunistically taking over heavily disturbed areas like roadsides and working lands. What makes it so invasive? It proliferates quickly, dispersing nearly 75,000 seeds per plant, and forms dense thickets that crowd out native species.   

Originating in Southern Europe, yellow starthistle was accidentally introduced with alfalfa imports in the mid-1800s. This weed not only displaces native plants and reduces forage for wildlife, it is toxic and causes chewing disease in horses. To identify it, look for a bright yellow flower surrounded by sharp spines and grey-green leaves covered in fine cottony hairs. 

Yellow starthistle leaves, stems, and flower heads. Photo courtesy of Neal Kramer / CAL-IPC.

How the Conservancy Manages Yellow Starthistle

Since 2019, The Conservancy has been managing yellow starthistle using a combination of hand removal and spot herbicide application. We use a targeted spray method, coating individual plants only in high-density patches where hand-pulling would be infeasible. A couple of months after spraying, we do a follow-up hand pulling treatment to make sure that we remove any plants we missed while spraying.

Despite its wide distribution on the West Coast, yellow starthisle is present in just four areas of The Preserve. Each area has undergone a different level of grazing intensity since the Conservancy re-introduced cattle to the landscape in 2013 to address thatch buildup and the resulting decline of grassland health. 

Penon Peak is grazed every 18 months as part of our standard cattle rotation throughout The Preserve and has a small enough yellow starthistle population that it is managed purely by hand removal.

Our largest and most dense thistle patch is on Black Mountain and has been grazed every 12 months for the past two years. This site has been both sprayed and hand-pulled for three consecutive years.

Long Ridge was grazed in May of 2022 for the first time since cattle were removed when The Preserve was established on the former ranch in the early 1990s. We spot-sprayed yellow starthistle at Long Ridge in 2019 and it has been managed by hand removal annually since. 

Our final site, Pronghorn, has never been grazed. We removed patches of yellow starthistle by hand in 2019 and 2022 and sprayed in 2020 and 2021. 

Using hours spent working on each site as an indicator of starthistle population size and density, we can measure the impact of our treatments year over year. So far, Penon Peak has been the most successful site. Our restoration staff spent 75 hours removing yellow starthistle from Penon Peak in 2019. This year, it took just five hours to hand-pull the remaining patch. 

Black Mountain, on the other hand, has required additional labor to tackle the weeds each year. Due to staffing shortages at the end of the 2022 field season, hand-pulling was not completed at this site and we anticipate more time will be needed to treat the area in 2023. 

There are a variety of factors influencing the trends we’re seeing in yellow starthistle management and the timing of cattle grazing may play a role. To explore the relationship between cattle grazing and yellow starthistle management success, we set up long-term monitoring plots within the invaded areas this year. 

Hours spent treating Yellow Starthistle at each of our four priority areas on The Preserve. Graphs by Jackson Brooke. 

What to do if you find Yellow Starthistle

Yellow starthistle is on the Conservancy’s priority invasive weed list. If you find it anywhere on The Preserve, please contact the Conservancy immediately so we can properly manage it.

If you don’t reside on The Preserve, we recommend working with a landscaper or removing yellow starthistle yourself. Please note that effective management will likely require the use of multiple methods over the course of several years. Yellow starthistle begins growth with low clusters of leaves that can be effectively dug or pulled in early spring. Hand removal is effective for control throughout the plant’s life cycle, just watch out for those long spines if it’s flowering! Be sure to remove the entire root or cut 4” below the surface. We recommend planting or seeding native vegetation after you hand-pull to compete with the next season’s starthistle. 

Treating the weeds with herbicide is effective at the rosette stage from February to April. Non-Preserve landowners interested in using herbicide for starthistle treatment should consult with a Pest Control Advisor. Always read herbicide labels for recommended use before applying. 

Mowing can provide control if timed correctly, but should only be used if all other treatment methods have been exhausted or are not feasible due to patch size. If you are mowing to control Yellow starthistle, mow immediately once plants flower to prevent the flowers from maturing and dropping seeds. Yellow starthistle resprouts quickly and often flowers again closer to the ground following a mow, so multiple mows in a single season may be required to achieve control. 

For questions about invasive plant removal and native habitat restoration, please refer to our weed management guide or contact Restoration Manager Jackson Brooke at jbrooke@slconservancy.org