Yellow starthistle (Centaurea solstitialis) is among the most invasive plant species in the western United States. The long-term management of yellow starthistle should include an integrated approach that incorporates establishment of competitive vegetation. In this study, conducted in two locations at Fort Hunter Liggett in Monterey County, California, we evaluated the combination of prescribed burning, the herbicide aminopyralid, and reseeding of native broadleaf and grass species on both yellow starthistle control and native plant restoration. Both study sites were burned in late October 2009. Over the following season, aminopyralid was applied at three timings and native plant species were seeded at three timings, using both a drill-seeder and broadcast spreader. Evaluations over the next 3 yr showed that aminopyralid provided complete to nearly complete control of yellow starthistle when applied between January and March, and this level of control was maintained for two seasons. Native plants failed to establish when broadcast seeded, regardless of the timing. December and January drill seeding timings were the most successful in establishing native species. There was a strong herbicide and drill seed timing interaction effect on native grass cover at both study sites. Over the course of the study the native perennial grass Stipa cernua was the most successful seeded species to establish, but establishment was slow and required 3 yr. Our results indicate that a January or March aminopyralid treatment integrated with a native perennial grass drill seeding program in January offers the greatest probability of both successful yellow starthistle control and perennial grass establishment.
Nomenclature: Aminopyralid, yellow starthistle, Centaurea solstitialis L., Stipa cernua Stebbins & Love.
Management Implications: In previous studies we showed that a properly timed summer burn followed by application of the herbicide clopyralid in winter gave excellent control of yellow starthistle for two seasons. However, following management of a severe yellow starthistle infestation, resident vegetation often does not recover sufficiently to resist reinvasion, and it is often difficult to reseed native species in a Mediterranean climate characterized by seasonal rainfall and drought. In this study we evaluated the effect of prescribed burning followed by application of the similar herbicide aminopyralid at three timings (November, January, and March), crossed with drill or broadcast seeding of native species at three timings (December, January, and March). We assessed both yellow starthistle management and native plant restoration. Prescribed fire followed by January to March aminopyralid treatment gave 2 yr of near complete control of yellow starthistle. The November treatment did not provide season-long control of late-germinating starthistle. Broadcast seeding of native plants failed to establish any species. Of the three drill seed timings, only December and January resulted in successful establishment of just one native perennial grass species, even with above-average rainfall in the first 2 yr of the study. Neither a native annual grass nor several broadleaf species were able to establish or sustain their populations over time. Over the three seasons of the study, Stipa cernua established most successfully. There was a strong interaction effect for herbicide and drill seed timing, with January being the optimal timing for both herbicide application and perennial grass seeding. This timing not only offers the greatest probability of both successful yellow starthistle control and perennial grass establishment, but also can be used in a single-entry program that reduces costs by consolidating activities to one timing.