The poinsettia (Euphorbia pulcherrima) is recognized globally for its vibrant holiday coloration. Many people mistakenly believe the brilliantly colored structures are the plant’s flowers. Instead, these colorful upper structures are modified leaves called bracts, which surround the small, inconspicuous yellow flowers at the center. The mechanism behind how these bracts change from green to red involves the plant’s chemistry and environmental triggers.
The Role of Bracts and Anthocyanin Pigments
The dramatic shift to red color is a chemical process involving the production of pigments known as anthocyanins. Anthocyanins are water-soluble compounds responsible for the red, purple, and blue hues seen across the plant kingdom. In poinsettias, these pigments are synthesized and stored within the cell vacuoles of the bract tissue, resulting primarily in cyanidin-type anthocyanins.
The accumulation of this pigment is a physiological response. Anthocyanin production coincides with a reduction in the plant’s green photosynthetic pigments, chlorophylls, allowing the red color to dominate. This chemical change is initiated by an environmental cue that signals the plant to begin its reproductive cycle.
Photoperiodism: The Darkness Trigger
The environmental mechanism signaling the color change is photoperiodism, the physiological reaction to the length of day or night. Poinsettias are classified as “short-day” plants, requiring extended, uninterrupted darkness each day to trigger flowering and bract coloration. This requirement synchronizes the plant’s reproductive cycle with the natural shortening of autumn and winter days.
The plant needs a dark period of at least 12 to 14 hours per night, consistently maintained over several weeks. During this long night, the light-sensitive pigment phytochrome changes form, signaling the plant to initiate anthocyanin production. Even a brief flash of light during the dark period instantly interrupts this signal, delaying or preventing the color change entirely. This process must be sustained for approximately eight to ten weeks to achieve full coloration.
Maintaining the Color Change at Home
Successfully re-coloring a poinsettia at home relies on replicating the strict, long-night conditions of photoperiodism. The process should begin around late September or early October, about eight weeks before the desired mid-December display. The plant must be given 13 to 15 hours of total, uninterrupted darkness every night.
This darkness can be achieved by placing the plant in a completely unlit closet, an unused room, or by covering it with an opaque bag from roughly 5:00 PM until 8:00 AM. Even small amounts of artificial light, such as from a streetlamp or adjacent room, will disrupt the process and must be avoided. During the day, the plant needs six to nine hours of bright, indirect light to remain healthy.
Maintaining the correct temperature is also important during this darkening period. Night temperatures should be kept cool, ideally between 60 and 65 degrees Fahrenheit. Allowing the night temperature to climb above 70 degrees Fahrenheit can delay the color change, even if the darkness requirement is met. Once the bracts have fully colored, usually by early December, the plant no longer needs the strict dark treatment.