The poinsettia, scientifically known as Euphorbia pulcherrima, is a plant species native to Mexico and Central America that has become a recognized symbol of the winter holidays. Its visual appeal comes from the vibrant, often red, color it displays during the short days of winter. The timing of this color transition is a direct response to seasonal environmental shifts, a process which can be replicated in a home setting.
Identifying the Coloring Parts
The most common misconception about the poinsettia is that its bright red structures are the flowers themselves. These colorful, leaf-like parts are actually specialized, modified leaves known as bracts. The true flowers are small, unassuming, yellow structures called cyathia, found clustered in the center of the bract arrangement. The bracts’ visual function is to attract pollinators to these inconspicuous cyathia. Poinsettia varieties are not limited to red; they have been cultivated to display colors including pink, white, marbled, and even orange.
The Biological Requirement for Color Change
The mechanism behind the poinsettia’s transformation is photoperiodism, a physiological process dictating a plant’s response to the relative length of day and night. Poinsettias are classified as “short-day plants,” meaning they require a specific duration of uninterrupted darkness to trigger bract pigmentation. To develop its characteristic color, the plant needs a long daily period of darkness, typically 12 to 14 hours. This requirement signals the arrival of autumn and initiates winter coloring.
This process must begin in early fall, usually around late September or early October, to ensure full color development by the December holidays. The plant must undergo this cycle consistently for eight to ten weeks for the green leaves to fully develop the red pigment. The bracts gain their color through the accumulation of anthocyanin pigments, produced in response to the lengthening night. If the necessary darkness is interrupted, even briefly, the plant will not complete the color change process.
Inducing Color at Home
Successfully encouraging a poinsettia to turn red requires the strict, daily application of the correct light regimen. The darkness treatment must be absolute and uninterrupted for 12 to 14 hours every night. This means placing the plant in a completely dark space, such as a closet or under a light-proof box, from approximately 5:00 p.m. until 7:00 or 8:00 a.m. the following morning. Exposure to even a small amount of artificial light can disrupt the chemical signal that initiates coloration.
During the daytime, the poinsettia must be returned to a location where it receives six to eight hours of bright, indirect sunlight. Temperature management is also a factor during this period. The plant thrives when daytime temperatures are maintained between 60 and 70 degrees Fahrenheit, and night temperatures should never drop below 50 degrees Fahrenheit, as the poinsettia is sensitive to cold. This precise schedule must be adhered to for about two months until the bracts show their desired color.