That is an excellent question! Ecologists have been trying to answer this question for centuries. Flower blooming is often related to a number of different factors including genetics, water availability, sunlight, temperature and even herbivory by animals!

Flower creation is tied to various chemical signals that pass through the plant that determine when a plant will bloom. Sometimes reducing water will make a flower bloom faster. Other times flowers bloom based on the hours of sunlight it receives each day. Here is a more thorough answer to your question :)

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Good question! It depends on the flower. Most I think use temperature in some way.

Flowers know when to bloom since a gene called Apetala 1 triggers the development of a plant and tells it when is the time for the plant to bloom. Apetala 1, a single gene, can make the plant to produce flowers.