What makes a plant bloom at the right time of year?

All flowering plants have a mechanism to make sure that their blooms develop at just the right time of year, whether in the springtime or autumn. Most respond to the amount of sunlight, and can distinguish between 16 hours of light and 8 hours of light. Some flower only when days are long and nights are short, such as radishes, asters, petunias, and beets. Others, such as chrysanthemums, goldenrods, and poinsettias, flower only when nights are long and days are short.