The poinsettia, or Euphorbia pulcherrima, is a popular plant recognized globally for its association with the holiday season. The vibrant red, pink, or white structures are often mistaken for flowers, but they are actually modified leaves called bracts. The true flowers are small, yellow-green structures, known as cyathia, clustered at the center of the bracts. The vivid color of the bracts gives the plant its ornamental appeal and results from a precise biological mechanism triggered by environmental cues.
The Biological Mechanism Behind Color Change
The color change in poinsettias is driven by light and darkness, a process known as photoperiodism. Poinsettias are classified as short-day plants, meaning they require a sufficient period of uninterrupted darkness to initiate bract coloration. The plant’s photoreceptors detect the ratio of light to dark, signaling the shift from vegetative growth to reproductive development. When the nighttime period exceeds 12 to 14 hours, the plant begins to form its true flowers and the surrounding bracts start to change color.
This transition involves a biochemical change where the production of chlorophyll, the green pigment, decreases in the bracts. Simultaneously, reddish pigments called anthocyanins accumulate. This accumulation of color signals to pollinators that the central flowers are ready for reproduction.
The Natural Timing of Poinsettia Color
In their native habitat of Mexico and Central America, poinsettias begin to color up as the days shorten and nights lengthen in the late fall. An indoor poinsettia will start its color change when daylight hours decrease sufficiently in the Northern Hemisphere, typically beginning in late October or November.
This timing ensures the plant is visually striking during the winter months, leading to its association with the holidays. The entire process of bract coloration requires consistent conditions over eight to ten weeks. If the plant is kept indoors under typical home lighting, the color change may be delayed or prevented because artificial light interrupts the required dark period.
Scheduling the Color Change for the Holidays
To ensure the bracts are fully colored for the December holidays, a strict light manipulation schedule must start in the fall. This process, often called forcing, needs to begin approximately eight to ten weeks before the desired date, usually the last week of September or the first week of October.
The most demanding requirement is providing 12 to 14 consecutive hours of complete, uninterrupted darkness every night. Even a brief flash of light from a streetlamp or reading lamp can disrupt the process. During the day, the plant must receive at least six hours of bright, indirect sunlight to maintain health.
This daily cycle must be maintained until the bracts show a distinct, deep color, typically by early to mid-December. Temperature is also a factor; the plant should be kept in a night temperature range of 60°F to 65°F, with daytime temperatures between 65°F and 75°F. High temperatures, especially at night, can interfere with bract development.
Post-Holiday Care and Summer Maintenance
Once the holidays pass and the colorful bracts fade, the poinsettia enters a resting period. In late winter or early spring (March or April), prune the plant back to four to six inches to encourage bushy growth. Repotting into a slightly larger container with fresh potting mix is recommended if the plant is root-bound.
Throughout spring and summer, keep the plant in a sunny location and fertilize every couple of weeks. When frost danger has passed and nighttime temperatures remain above 50°F, move the plant outdoors to a partially shaded spot. Pinch back new shoots periodically until mid-August to promote a compact shape. Bring the plant back indoors before September’s cool nights to prepare for the renewed light manipulation cycle.