Polyploidy can impact response to variation in the biotic and abiotic environment through alteration of biochemistry, physiology, and morphology. Although progress has been made in understanding the functional effects of polyploidy, the implications of such changes on population-level processes, like adaptation, are not well understood. To address this gap, we are studying the role of mixed-ploidy in local adaptation of Andropogon gerardi Vitman, the dominant grass species in endangered North American tallgrass prairies. A. gerardi is a mixed-ploidy species composed of hexaploids (6x) and enneaploids (9x), which are equally common but not equally distributed. Previous research has found 9x are more common in regions with reduced summer precipitation and increased variation in temperature range but it is unclear why this distribution occurs. Genotyping data shows that 9x genotypes have similar, low relatedness to all other 9x genotypes, which is consistent with each 9x genotype being a neopolyploid. In a common garden experiment, we found 9x genotypes have a higher relative growth rate, larger stomata, and lower stomatal density. Our results suggest the distribution of the 9x cytotype may be explained by beneficial ecophysiological differences related to whole genome duplication. Together with previous research documenting that 9x have low reproductive viability, our results indicate the 9x cytotype is an adaptive ‘dead-end’. Consequently, populations with high percentages of the 9x cytotype may be at elevated risk to climate change as their adaptive potential is low. More broadly, our results suggest polyploidy may be an important genetic component to consider in conservation and restoration decisions.