What causes some plants to flower in spring, while others bloom in summer, and still others in fall?
The season a plant flowers in has much to do with the length of the day. There are short day plants, those that flower in late summer, fall, or early winter; and there are long day plants, those that flower in late spring and summer. There are also day neutral plants, plants that will flower at any time of year.
What makes a plant a short-day plant or a long-day plant?
Well, flowering is largely triggered by communication between a plant's internal circadian clock and light receptors called phytochromes and cryptochromes. Circadian simply means "about a day." Plants, like a lot of other organisms, are internally set to a 24- hour cycle.
Though the clock is internal, external signals such as light reset the clock on a daily basis. The intricacies of this relationship between the circadian clock and the light receptors are still being discovered. It does appear though that flowering responses are tuned to the length of the nights and that photoreceptors then re-set the clock each day. What makes a plant a short-day plant or a long-day plant appears to be largely a matter of how the circadian clock operates in the plant.