When many people think of flowers, they think of spring and summer. However, some plants flower only in the fall or even in winter, as is the case with poinsettias, and always flower at the same time of year. Flowers aren't responding to temperature or weather, much less the calendar, but the amount of light they receive each day, a phenomenon called "photoperiodism."
Flowers contain the reproductive structures of plants. Sexual reproduction occurs when pollen from one plant transfers to the female structures of another. For this reason, it is to the plant's benefit to have its flowers open at the same time as related plants with which it is able to reproduce. Because weather may vary during a season or even in adjacent regions, regulating flowering by daylight, which remains constant from year to year and in nearby areas, assures that all flowers open at the same time.
The term "photoperiod" refers to the amount of light during a 24-hour period, according to the University of Massachusetts Extension. For example, a 10-hour photoperiod includes 10 hours of light and 14 hours of darkness. Daylight occurs between sunrise and sunset, while civil twilight describes the time period when the sun is just over the horizon. Although light is dim, it is often enough to cause a photoperiodic response in plants. Short-day plants flower when they receive little daylight, while long-day plants flower during the long days of summer.
The way botanists describe photoperiods reflects an old misunderstanding that attributed light-dependent flowering on the amount of light the plant received each day. In fact, photoperiodism is controlled by the amount of darkness that the plant receives each day. For example, spinach is a long-day plant; it flowers only when it receives brief periods of darkness every day. (See References 1)
According to retired biology professor John W. Kimball, photoperiodism occurs because of a relationship between two types of biochemicals called phytochromes. One type of phytochrome converts to a second type, called PR, in the darkness. PR controls flowering, so when plants receive enough darkness, adequate PR is produced to start the development of flowers.
Most gardeners observe photoperiodism as anticipated seasonal changes. For example, chrysanthemums flower in the fall. Chrysanthemums are short-day plants, requiring long nights to flower. When gardeners want a plant to flower outside of its usual season, according to the University of Massachusetts Extension, the trick is to manage the amount of darkness that the plant receives to match its photoperiod, covering it to extend darkness or providing artificial light to shorten it.