Abstract: 【Objective】This research aimed to characterize the morphological differentiation phases of Coreopsis grandiflora ‘Early Sunrise’ and their relationship with vegetative growth.【Method】Two cohorts of seedlings were cultivated separately with long or short photoperiods, respectively. Stages of flower bud differentiation were histologically observed by making paraffin slices. The C. grandiflora growth time(in weeks), plant height, crown diameter, and leaf number were also recorded for flower groups in each bud differentiation phase.【Result】The C. grandiflora displays eight distinct phases of flower bud differentiation. The C. grandiflora exposed to long and short photoperiod had flower buds in inflorescence primordium and vegetative growth prophase in week 17, respectively(i.e., the plants began to respond to the long photoperiods from this week). We found significant linear relationships between height, branch number, crown diameter, and the bud differentiation phases of C. grandiflora. We also showed that increasing the photoperiod sensitivity index strengthened the correlation with the flower bud differentiation phase. The results of these studies indicate that the “inflorescence primordium differentiation phase” to the “bract primordium differentiation phase” is the main period of C. grandiflora flowering induction under the long photoperiod.【Conclusion】According to the quantitative indicator t_DP = 3 of flower bud differentiation, flowering can be induced by long days until the plant height reaches 20.60 cm, crown diameter is 26.01 cm, and branch number is 13. Under these conditions, ‘Early Sunrise’ blooms earlier, with higher flower quality.