Organismal developmental traits, such as the timing of life history transitions, are often the traits that are most responsive to changing environmental conditions. To clarify the effect of selection and environment on the phenology of germination and reproductive commencement, we performed a quantitative genetic study of phenotypic selection on Chamaecrista fasciculata (Fabaceae). An experimental population was planted into a restored prairie in each of two years. The earliest dates of germination and flowering were tracked for paternal families over two parental generations and one progeny generation. Selection for advancement in reproductive stage was detected, as well as a response of 2.2 days earlier flowering in the second generation. Germination phenology varied more between years than between generations. The effect of the environment was the strongest contributor to differences in phenology. Response to selection became apparent when accounting for the effect of the environment on phenotypic differences, highlighting the importance of controlling for year-to-year environmental variation in quantitative genetic studies. The experimental garden was hosted by the Shakopee Mdewakanton Sioux Community and research was conducted by myself (Choctaw) as well as Anishinaabe and Lakota/Dakota students.