Previous research has found significant differences in flowering time between sympatric, non-hybridizing populations of congeners in the genus Gelsemium (Gelsemium sempervirens and G. rankinii). An experimental approach using a common garden, reciprocal transplant experiments, and observations of natural populations were used to test the hypothesis that soil environmental variables (soil moisture and soil temperature) related to habitat specificity of the two species influence differences in flowering phenology. Replicated pairs of both species were planted into a common garden and into wet and dry habitats that typify the two species (dry for earlier flowering G. sempervirens and wet for later flowering G. rankinii). Two sites with sympatric natural populations of both species were also studied for comparison. The number of open flowers was counted every two weeks in 2007 and 2008 in all sites and in 2009 for the reciprocal transplant experiment. Soil moisture and soil temperature varied by site, date, habitat, and species but was not consistently related to flowering phenology across sites or species. Compared to natural conditions, plants in the common garden and experimental transplant populations did not show significant changes in differences in flowering times, suggesting that flowering phenology is under genetic control and is not influenced by soil environmental conditions.