Reproductive biology of yellow starthistle: maximizing late-season control

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Abstract
Field studies at three sites and growth chamber experiments were conducted to determine the reproductive potential, flower phenology, seed viability and germination, and overall seedbank longevity of yellow starthistle in the Central Valley of California. At the three study sites, seedheads contained an average of between 65 and 83 achenes. Overall, 85% of the achenes were the interior pappus-bearing type, and the remaining 15% were the outer nonpappus-bearing type. Germinable seed did not initially develop until the late corolla senescence stage 8 d after flower initiation. Seed germination and viability 1 wk after dispersal were similar (86 and 91%, respectively). Comparison in flower phenology in 1996 and 1997 indicated that development from initial anthesis to achene dispersal more closely corresponded to days, rather than thermal units. In the field, germinable seed was produced when more than 2% of the total seedheads had initiated anthesis. To minimize seed production with late-season control methods, such as prescribed burning, mowing, or herbicide treatment, management strategies should be timed before the plant population has advanced beyond the 2% flower initiation stage. Over 84% of the seed germinated under growth chamber conditions 1 wk after seedheads reached the dispersal stage. This indicates that most yellow starthistle seed had little or no after-ripening requirements. In a field experiment, yellow starthistle seed germination corresponded to seasonal rainfall. A total of 44 and 39% of the pappus-bearing and nonpappus-bearing seed, respectively, germinated after one growing season. Of seed recovered from the soil after the first growing season, 88 and 81% of the pappus-bearing and nonpappus-bearing seed, respectively, was either damaged or degraded. From projected values based on recovered and germinated seed, it was estimated that over 97% of the total seed was removed from the soil seedbank after two growing seasons. These findings should assist land managers in developing long-term yellow starthistle management strategies.