Yellow starthistle is an annual plant with yellow flowers with long white spines covering the plant. Under a microscope, the surface of the plant also has thin curly hairs that cover the surface and cause a grey appearance to the stems and leaves. The plant can vary greatly in size from very short, 6 inches, to over 5 feet with stems that appear winged. It has a deep taproot and with many branched stems. Disturbance allows yellow starthistle to rapidly colonize areas. Once established, it is able to rapidly remove the moisture from the soil, preventing other plants from establishing. Ideal habitats are dry and sunny. Flowers are very productive, producing thousands of seeds per plant. Horses should not graze in areas where yellow starthistle is present. It can cause chewing disease, which is fatal for horses that eat the plant.
Distribution of yellow starthistle is broad across the United States. Spreading in our area of Nevada anecdotally follows primarily riverways and weed management work along the Carson river watershed in Carson County has focused on preventing spread further downstream. It has been reported 11 times in Lyon County by EDDMapSWEST and is currently unreported by the USDA PLANTS database. While these reports were in Southern Lyon county, it is likely to spread into Northern Lyon county via waterways or highways. It has been reported to infest between 10 and 15 million acres in California. The photos in this blog were taken in California, where I observed yellow starthistle infesting roadways, fields, irrigation ditches and encroaching into natural areas.
Management of yellow starthistle is most effective when weeds are managed prior to going to seed. Frequent scouting in areas of potential spread, repeated scouting in treated areas and removal of plants prior to seeding. Small patches are best handled with hand pulling ensuring that all flower heads are contained to prevent spread at the disposal location. Larger patches have a variety of control options, but without re-vegetation with desirable plants, yellow starthistle can quickly re-establish. Prevention of establishment relies on effective reporting and treatment of infested areas. Email photos of suspected yellow starthistle for identification confirmation.