However, if you've kept a poinsettia throughout the year, you would notice that during the summer it is a white plant, and it's only when the days become shorter that it develops it's redness.
|The Poinsettia flowers red at Christmastime (C) bourgeoisbee|
The poinsettia is a short-day plant, requiring a long period of darkness to flower. A lot of the experiments to understand when plants flower have been done on long day plants such as Arabidopsis. These plants need a long period of light in the day to flower (usually >12 hours) and a short night.
The chemical that absorbs light is called phytochrome, this can exist in two forms. The "red" form, PR, which absorbs light during the day, and the "far-red" form, PFR, that is converted from PR during the night when there is no daylight.
|Phytochrome is the chemical in the plant that absorbs light which converts it from the Red to the Far Red form|
During the day PR is produced by daylight. During the night all the PR is converted into PFR, this only takes 2 hours to complete. If you expose a long-day plant to pulses of light during the night, it can trick it to thinking the day is long and it's time to flower. If the plant were merely counting the number of hours in the day, this wouldn't work.
The longer hours of light allow for the accumulation of PR alongside the protein Constans (CO), which initiates flowering. CO is controlled by the plants internal clock, so it has low levels during the day, and only rise during the late afternoon if there is enough light and PR. In order to flower the Arabidopsis requires both daylight to be detected by phytochrome, and the circadian rhythm of CO.
Understanding how short day plants work was a bit more tricky. But genetic studies on the short day plant rice are beginning to reveal that the clock in short day plants effect the same genes, but they have opposite effects on the plant . So in a long day plant genes that activate flowering in a short day plant inhibit flowering.
Other short day plants include the tobacco plant, strawberries, and chrysanthemums.
I hope you enjoy celebrating National Poinsettia Day with a greater appreciation of the seasonal colouring in these short day flowering plants.
 "Genetic Control of Flowering Time in Rice, a Short-Day Plant" Yano, M et al., Plant Physiology, vol. 127 no. 4 1425-1429, 2001