Rachelann C. answered • 09/05/19
Writer, photographer, botanist
If I am understanding the question correctly, you are asking about how so-called day length affects the seasonal timing of flowering in some plant species. If so, the issue is really that the term day-length is a misnomer; plants that take their cues from the relative duration of light and dark durations actually respond seasonally to the length of nighttime, or dark, hours as opposed to daytime, or light, hours. And although it would seem that these would correlate equally in nature, studies where the light/dark cycle is artificially manipulated have found it is actually the night-length that triggers many plants to switch over into flowering. This is most often seen in commercial Poinsettia production. In North America, poinsettias are adapted to bloom in late January, not December when they are most marketable. As a result, producers grow them in greenhouses equipped with "black out" screens so they can gradually and artifically increase the night-length the plants experience by simulating an earlier sundown (but not sunrise) to fool the plants into thinking it is later in the shortening winter days than it actually is.
