Flowers of the coyote tobacco (Nicotiana attenuata) exhibit a distinct diurnal rhythm: they open at night, close the following morning, and repeat this movement over two days, synchronized with nighttime scent emission. While circadian clock components like LATE ELONGATED HYPOCOTYL (LHY) and ZEITLUPE (ZTL) are known to regulate floral rhythms in N. attenuata, the molecular mechanism underlying these processes remains largely unknown. Here, we identify the transcription factor CONSTANS-LIKE 5 (COL5) as a key regulator of flower opening and scent emission in N. attenuata and the large white petunia (Petunia axillaris). We screened for genes in N. attenuata corolla displaying (1) diurnal rhythms, (2) altered expression in ZTL mutants, and (3) co-regulation with EMISSION OF BENZENOIDS II. Using this multifaceted strategy, we found that col5 mutants exhibited incomplete flower opening and lacked emission of the major floral scent, benzylacetone, without affecting overall plant growth and floral development. Silencing the orthologous gene in P. axillaris produced similar phenotypes, confirming a conserved role for COL5 in facilitating flower opening and scent emission. These results demonstrate that COL5 coordinates circadian-regulated floral rhythms, advancing our understanding of the molecular basis underlying floral rhythms.