The tallgrass prairie of North America was once a vast continuous habitat but is now fragmented into small, isolated remnants.  Fragmentation can affect successful pollination by limiting the number of nearby synchronously flowering conspecifics and by changing the abundance or foraging behavior of pollinators. Many studies have shown effects of spatial isolation on fragmented populations; however, few studies have examined how flowering phenology can increase temporal isolation in a fragmented landscape.  Here we examine how flowering phenology influences reproductive success of individual plants. We studied flowering of Echinacea angustifolia, a widespread prairie perennial in a common garden experiment with 3000 plants--224 flowering in 2005. We quantified the flowering intensity as the number of anthers shedding pollen on each day during the 67 day flowering season and flowering density of all flowering plants at multiple spatial scales.  We then determined the rate of seed set for each flowering head.  We found that flowering phenology directly influences seed set in Echinacea.  There is little reproductive cost for flowering before the peak intensity but there is reduced seed set for plants that flower even slightly after peak.  Very low seed set occurred in plants that were spatially isolated and flowered past peak. This study thus quantifies how temporal and spatial isolation interact in an experimental plot.  Our findings suggest that temporal isolation likely reduces reproductive success in a fragmented landscape.