by Devii Rao , University of California Cooperative Extension
Yellow starthistle is an invasive rangeland weed that takes over pastures, reducing quality and quantity of livestock forage. Ranchers continue to struggle with controlling this species. Weed Control in Natural Areas in the Western United States (DiTomasso, Kyser, et al. 2013) is an excellent book with information on how to control yellow starthistle and many other invasive species that occur in California. The yellow starthistle chapter is available at:
This article summarizes the yellow starthistle chapter from this book.
Three of the main yellow starthistle control strategies are herbicide, grazing, and mowing.
Several herbicides are effective on yellow starthistle, but two of the best options are Aminopyralid (Milestone) and Clopyralid (Transline). The best time to spray Aminopyralid is from the seedling stage to the rosette stage. The best time to spray Clopyralid is when the plant is in the late rosette stage. Grasses are not harmed by either herbicide.
Grazing: Cattle, sheep, and goats can all be used to graze yellow starthistle. The best time to graze is from the bolting stage to right before the spiny heads emerge. Bolting is the stage after the stem comes up out of the rosette, but before the flower head begins to emerge. Protein content during this stage is relatively high: 8% to 14%. Once the spines come out yellow starthistle becomes less attractive to cattle and sheep and they start avoiding it. Goats, on the other hand, are not dissuaded by the spines so they are often used in yellow starthistle targeted grazing programs. Short-duration, high-intensity grazing is the most effective grazing strategy to control yellow starthistle.
Mowing: Mowing can be an effective control option if you have an area that’s flat enough for a mower to be safe and not tip over and doesn’t have too many big rocks to get in the way of the mower. It will take several years of mowing to control yellow starthistle. Control will be even better if mowing is used in conjunction with other control methods. The best time to mow is when 2%-5% of all the yellow starthistle plants are in bloom. Timing of mowing is critical. If you mow too early, yellow starthistle can grow back and produce even more seed than if you had not mowed. Mowing too early will also eliminate grasses and other existing plants that were competing with yellow starthistle. Yellow starthistle doesn’t do as well if there’s a thick mat of other vegetation to compete with. So, removing these plants releases yellow starthistle from competition, allowing it to grow better. If you mow too late, you’ll spread the seed. Researchers found that they got the best results when they mowed twice: once during the early flowering stage and then a second time 4-6 weeks later after the plants had regrown and had produced flower buds.
Effectiveness of mowing also depends on how your yellow starthistle plants are growing. If you have plants that are tall, and the branches are high up on the plants, you may only have to mow once during the early flowering stage. But, if you have plants that are spread out at the base and the branches are lower on the plant, you may not be able to control it very well because the mower can’t get low enough to cut the branches.
DiTomaso, J.M., G.B. Kyser et al. 2013. Weed Control in Natural Areas in the Western United States. Weed Research and Information Center, University of California. 544 pp.