Flowering is an important transition from vegetative to reproductive development in the plant life cycle; the determination of flowering time is an important component of flower development. A complicated genetic network involved in the perception of various signals from external environmental cues or internal developmental signs is involved in the complicated developmental process of flower organs. FT gene, encoding protein acting as mobile messenger, is the main integrator in this genetic network. Flowering time is affected by components of signal perception such as CO, GI, FLC and FRI. Flowering time of various plant species has sophisticated expression in the field. Global change, especially global warming, has diverse effects on flowering time for various plant species. Accordance or overlapping of flowering time is important for gene flow among individual plants. Temporal isolation, which originates from the variation in flowering time, could result in the genetic differentiation between parapatric populations or sympatric subpopulations.