The poinsettia (Euphorbia pulcherrima) is a popular holiday plant known for its vibrant color, which comes from specialized leaves called bracts, not flowers. This coloration is governed by photoperiodism, the plant’s response to the relative length of light and darkness. Manipulating the plant’s light exposure is necessary to “force” the bracts to change color in time for the winter season. This process tricks the plant into believing that winter’s longer nights have arrived, triggering the color change.
Preparing the Poinsettia for Coloration
Achieving successful coloration requires the plant to be healthy after its previous blooming period. After the bracts fade (late winter or early spring), prune the plant back to encourage new, bushy growth. Cut the stems back to four to six inches in height, leaving a few leaves on each stem to shape the plant for the next season.
Following pruning, the plant may enter a rest period where watering should be reduced. Once new growth begins to appear in late spring, repot the plant into a slightly larger container with fresh, well-draining soil. During the summer, the poinsettia needs active growth, benefiting from bright, indirect sunlight and consistent watering.
The summer growth phase requires regular feeding with a balanced, all-purpose fertilizer every two to three weeks. To ensure a full, well-branched plant, pinch back new growth periodically until early September. This year-long care routine ensures the poinsettia has enough stored energy for the light manipulation required for fall coloration.
The Critical Timing for Darkness Initiation
The poinsettia is a short-day plant, requiring a long period of uninterrupted darkness daily to trigger the color-changing process. To ensure the colorful bracts are fully developed by the holiday season, controlled darkness treatment must begin precisely in the fall. The ideal window to start this process is around the autumnal equinox, from late September to early October.
Starting the treatment around September 21st to October 1st gives the plant the necessary eight to ten weeks to complete the coloration cycle before Thanksgiving or Christmas. This timing is critical for holiday display; initiating darkness too early results in colored bracts long before the season begins. Conversely, delaying the start until late October means the bracts may not fully color until January. A consistent start date is the most important factor for seasonal success.
Executing the Daily Darkness Schedule
Forcing coloration requires a strict daily light schedule mimicking winter nights. The plant must receive a minimum of 14 hours of complete, uninterrupted darkness every 24 hours. This dark period is followed by 10 hours of bright, indirect light, allowing the plant to photosynthesize and replenish energy stores.
This routine must be maintained consistently for eight to ten weeks until the bracts show noticeable color. The darkness must be absolute; even a brief flash of light from a streetlight or household lamp can disrupt the delicate hormonal process within the plant. This interruption can reset the plant’s internal clock, delaying or preventing coloration entirely.
Achieving Absolute Darkness
Common methods to achieve black-out conditions include using a dark closet, a spare room, or covering the plant with a light-proof container like a cardboard box. If using a box, sealing seams with tape ensures zero light penetration. During the 10-hour daytime period, place the plant in a sunny location, ideally receiving six to ten hours of bright light.
Maintaining the proper temperature is also important during this time. The environment should remain between 60 and 70 degrees Fahrenheit, with night temperatures ideally in the low 60s. Water the plant only when the top layer of soil feels dry, and reduce or stop fertilization completely while the plant focuses on color change.
Transitioning Back to Normal Light
Once the bracts show significant color (late November or early December), the daily darkness treatments can be discontinued. The plant has completed the photoperiodic process, and the new color will not be inhibited by normal household light. Transition the poinsettia back to a permanent location where it can receive bright, indirect light for display.
After the coloration process is complete, the plant should be kept in a slightly cooler environment to maintain the color and prolong the life of the bracts. An ideal temperature range for the colored plant is between 60 and 65 degrees Fahrenheit. Focus on consistent watering and protecting the plant from cold drafts or excessive heat sources.