Flower bud formation and flowering in chrysanthemum occur under short day conditions (SD), but the molecular basis for the switch to reproductive growth is less well understood than in model plants. Here, a spontaneous mutant able to flower under long days is described. In an attempt to reveal the pathway(s) involved in the formation of flower buds under contrasting daylengths, transcriptome sequencing was carried out in plants grown both under SD and under long day conditions (LD). A number of differentially transcribed genes involved in the various known flowering pathways were identified. Both circadian clock genes and CmFTL3 were up-regulated under SD, thereby inducing floral bud formation and flowering. The gibberellin (GA) signalling pathway-related genes GA20ox and GID1 were up-regulated in the mutant under LD, while the catabolic gene GA2ox and GAI was down-regulated, thereby inducing the transcription of CmFTL1, SOC1 and LFY. The GA content of the leaf was higher in the mutant than in the wild type under LD. When treated with GA, the mutant flowered earlier under both SD and LD, but there was no other detectable phenotype difference between the two lines. The indication was that the photoperiod pathway majorly regulates flower bud formation and flowering time in chrysanthemum under SD. GA signalling pathway only plays a subsidiary role for flowering. However, the GA signalling pathway predominated for flowering under LD.