Various facets of the pollination biology affecting achene production of Centaurea solstitialis L. were examined. Production of viable achenes commenced in capitula bagged or excised in the late flowering stage (F-2 midstage). In all studies in which capitula were prevented from crossing, only one plant had a high level of achene production, all others producing either a few achenes or none at all. Achene production in plants allowed to cross was high and significantly greater than in plants prevented from crossing. Persistent low levels of self-fertilization in most plants indicated that there was selection against inbreeding. The pollen presentation mechanism for C. solstitialis was similar to that of other Centaurea spp. described previously. The most commonly observed insect visitors of yellow starthistle capitula were Hymenoptera spp., especially the European honeybee. Keywords: Centaurea, pollination, yellow starthistle.

Studies were conducted to compare changes in germinability and/or viability of yellow starthistle achenes buried (in packets) in soil or stored dry in the laboratory, and to determine the rate of achene depletion from a soil seed bank. In one study, after 72 mo, 0 to 96% germinable achenes remained in packets buried 5 cm deep, and a mean above 99% after dry storage. In another study, total live (germinable plus viable) achenes declined from 100% at harvest to 77.6% after 24 mo of burial. Germinable pappus-bearing achenes remaining in packets increased with depth of burial. After 12 mo of burial, maximum survival of achenes occurred at depths of 5 cm or more. The density of yellow starthistle achenes and seedlings in a natural soil seed bank declined, when achene rain was prevented, to 3.9 and 1.1% of initial density, respectively, after 36 mo. Decline was attributed primarily to seedling emergence and achene death. Achene density in the upper 2.5 cm of soil appeared to be a good predictor of seedling emergence after autumn rains.

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