The answer lies within the winter before. In some species, such as model organism Arabidopsis sp. the promotion of flowering in the spring can only occur after prolonged exposure to the cold temperatures of winter. This response to stimuli is known as vernalisation and it affects many other species in many different ways. Some may commence flowering right after the winter ends, and others can take more then half a year post vernalisation. The effect of the cold treatment can also depend on the stage of the plants life cycle, some will show no response until they reach a certain height, and others can be verbalized as seeds.
The molecular process behind vernalisation is still not completely understood and is still the focus of many current studies. Progress so far has uncovered that the ecological stimulus of cold temperatures are directly related to the increased production of the FLC gene (flowering locus C), which has identified it as a floral inhibitor. As winter comes to pass and warmer temperatures return, the expression of FLC is downregulated, and the plant is allowed to proceed to flower.
Primary Reference:
Michaels S. D., Amasino R. M. (2000) Memories of winter: Vernalization and the competence to flower. Plant, Cell and Environment 23 (11), 1145–1153.
Supporting article:
Dimech A. (2004) Vernalisation: Cold Temperature Exposure
