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Measuring pollen limitation, or reduced seed production caused by inadequate pollen receipt, is an important component in understanding why self-fertilization and early flowering often co-evolve in flowering plants. The reproductive assurance hypothesis suggests that selfing evolves because plants need to reproduce when pollinators are absent or unreliable. In the rapidly drying Southern Sierra Nevada foothills, selfing and early flowering would benefit a species if pollinators are absent early in the flowering season when conditions may more favorable for reproduction. The annual wildflower genus Clarkia contains several pairs of outcrossing and selfing sister taxa. In Clarkia, selfing and early flowering have co-evolved several times, making this group an excellent candidate for our study. To test the hypothesis that poor pollinator service promotes the evolution of selfing in Clarkia, we studied pollen limitation in Clarkia xantiana ssp. xantiana, a predominantly outcrossing species that often occurs in sympatry with its selfing relative C. xantiana ssp. parviflora. In 2010, we visited three populations and conducted a series of manipulative field experiments. We compared seed set (the proportion of ovules that become full seeds) between flowers that were open-pollinated and flowers that were provided with supplemental pollen (via hand-pollinations). To determine whether the magnitude of pollen limitation differed for early or late flowering plants, we administered the experimental treatments twice (once at the start of the flowering season and once near the end of the season). With this approach, we have not found statistical evidence showing that pollen limitation occurs; open-pollinated fruits exhibited similar seed set to that of hand-pollinated fruits. Early-flowering fruits, however, consistently exhibited greater seed set than late-flowering ones. These findings suggest that selfing may not have evolved in response to poor pollinator service, and that other factors such as drought avoidance or herbivore avoidance could contribute to the joint evolution of selfing and early flowering.