The poinsettia, Euphorbia pulcherrima, is a popular ornamental plant with a strong association with the winter holidays. Native to the tropical dry forests of southern Mexico and Central America, it naturally blooms during the short days of winter in its native habitat. This plant became a popular fixture in the United States after its introduction in the 1820s, and its seasonal coloration is widely sought after for festive decoration. The striking red, pink, or white hues are the result of a specific biological process triggered by environmental cues.
Bracts, Not Leaves: The Biological Mechanism of Color
The vibrant, colored structures on the poinsettia are not true flower petals, but modified leaves known as bracts. The actual flowers are the small, yellow-green structures located in the center of the colored bracts, which are called cyathia. These showy bracts attract pollinators to the inconspicuous true flowers, much like petals do on other plants.
The red coloration within the bracts is caused by the accumulation of specific pigments called anthocyanins, which are water-soluble flavonoids. Anthocyanins are responsible for the red, purple, and blue colors found in many fruits and flowers.
These pigments are synthesized and accumulated within the epidermal cells of the bracts during the color change process. The final hue, whether red, pink, or white, is determined by the specific type and concentration of these anthocyanins; white bracts contain few or no detectable anthocyanins.
The Environmental Trigger: The Critical Darkness Requirement
The process of color change is governed by photoperiodism, the plant’s response to changes in the length of night or day. Poinsettias are classified as “short-day plants,” meaning they require long, uninterrupted darkness to initiate bract coloration. This requirement mimics the naturally long nights of late autumn and early winter in their native range.
To trigger the color shift, the plant needs a critical dark period of approximately 12 to 14 hours of continuous darkness every night. This regimen must be maintained for six to eight weeks to ensure full color development. Even a brief flash of light can interrupt the dark period, effectively resetting the process and preventing the bracts from coloring.
The plant perceives the duration of the dark period, not the length of the day, as the signal to begin producing anthocyanins. This strict requirement for uninterrupted darkness is the most challenging aspect of getting a poinsettia to re-color outside of a commercial greenhouse setting. Once the color change is fully initiated, the plant becomes less sensitive to light pollution, and the colored bracts will persist for weeks.
Forcing Color: A Timeline for Home Cultivation
Home growers who want their poinsettias to turn red by the holiday season must begin the darkness regimen in early autumn. Starting this process in late September or early October allows the six to eight weeks required to achieve full coloration by late November or early December. The plant must be moved to a location that provides 12 to 14 hours of complete darkness, and then returned to bright, indirect light for the remaining ten to twelve hours of the day.
Temperature control during this period enhances color saturation. During the day, the plant should be kept in a bright location with temperatures around 65 to 70 degrees Fahrenheit. Nighttime temperatures, particularly during the dark period, should be slightly cooler, ideally remaining in the low 60s Fahrenheit.
The plant should continue to receive regular water, but fertilizer application should be reduced or stopped. Consistency is paramount; missing even a single night of complete darkness can delay or stop the coloring process. Once the bracts begin to show significant color, typically by mid-November, the strict darkness routine can be discontinued, and the plant can be returned to its normal display location.