As sessile organisms, plants cannot physically escape during inclement weather, but flowering time variation allows them to match their reproductive timing to suitable environmental conditions. Altered flowering time is one of the most common responses to harsh environmental conditions, and it often involves selection on genetic variation in critical photoperiod, the daylength required to cue flowering. In order to understand the origin, mechanism, and consequences of critical photoperiod adaptation, it is necessary to map the genetic basis of this important trait. Adaptive changes in critical photoperiod have been studied genetically between sister species and within a species across its geographic range. However, less is known about the process of flowering time adaptation across narrow spatial scales. My research seeks to characterize and map the genetic basis of flowering time diversity in the yellow monkeyflower (Mimulus guttatus) across Yellowstone National Park’s geothermal mosaic. In Yellowstone, M. guttatus grows both on and off geothermally heated soils. Thermal M. guttatus flower under the short days of early spring when local thermal inputs melt the snow, generally self-fertilize, and are annual, senescing in July when soils become extremely hot and dry. In contrast, nonthermal M. guttatus are perennial, flower in late summer, and are bee pollinated. Despite these heritable differences in life history, mating system, and flowering time, we have found evidence of extensive gene flow between thermal and nonthermal populations: rather than incipient speciation, this is a case of adaptive divergence in the face of gene flow. Through growth chamber experiments under varying daylengths, we have discovered that variation in critical photoperiod in Yellowstone M. guttatus is comparable to range-wide variation in the species complex: the most extreme thermal populations only require a 12-hour daylength to flower, intermediate populations have a 13-14-hour requirement, and nonthermal populations require the 15-hour days of Wyoming summers. Using complimentary approaches of F2 mapping, pooled sequencing, linkage disequilibrium mapping, and whole genome sequencing, I have identified a novel locus that underlies the extreme 12-hour critical photoperiod variation in Yellowstone M. guttatus on chromosome 6. This short-day locus on chromosome 6 (or sd6) is distinct and independent from previously discovered flowering time loci in the M. guttatus species complex. Furthermore, sd6 is packaged very close on the genome to the locus governing life history in Yellowstone M. guttatus. This type of genomic packaging can help overcome the homogenizing effects of gene flow and maintain advantageous combinations of traits when there is divergent selection across narrow spatial scales. Broadly, understanding the genetics of flowering time adaptation has applications for both agriculture and conservation. Although daylength is a common cue for flowering across the plant kingdom, daylength has become increasingly decoupled from suitable temperature and moisture conditions for flowering due to anthropogenic change. Exploring the genetics of flowering time adaptation in response to geothermal warming can help us build genetic models to understand how plant populations cope with climate change.