The poinsettia, Euphorbia pulcherrima, is a popular holiday plant known for its striking, vibrant color. What people commonly mistake for flowers are actually specialized leaves called bracts. To successfully encourage these leaves to change from green to their festive color for the holiday season, you must artificially create the specific environmental conditions the plant needs. This process involves a strict, daily routine of providing a long, uninterrupted period of darkness.
Why Poinsettias Need Darkness
Poinsettias are classified as “short-day” plants, meaning their development is regulated by the duration of light and dark they receive. The plant requires a long, continuous night period to trigger the hormonal shift necessary for the bracts to change color. This process is known as photoperiodism.
The color change signals the plant is ready to enter its reproductive cycle, which naturally occurs during the shorter days of late fall and early winter. Even a minimal amount of light exposure during the required dark period can interrupt this delicate cycle. A brief flash from a street light, a hallway lamp, or a crack of light under a door is enough to “reset” the plant’s internal clock and prevent the bracts from coloring.
The Critical Timing for Color Setting
When to Start
To have your poinsettia fully colored and ready for display by the Christmas season, begin the darkness routine approximately eight to ten weeks before your target date. This means the process must typically start around the last week of September or the first week of October. Starting the cycle too late will result in the bracts only partially changing color by the time the holidays arrive.
Duration and Development
The total duration of this controlled light cycle needs to be consistently maintained for about eight weeks until the bracts show significant color. The plant needs a sufficient number of these inductive cycles to fully develop the color and the tiny true flowers, called cyathia, at the center of the bracts. Cultivars can vary in their “response time,” but many modern varieties require at least 56 days of the darkness treatment.
Temperature Requirements
Temperature is another factor that influences the timing of the color change during this period. For the fastest and most reliable color development, the plant should be kept in a night temperature no lower than 60°F and an ideal daytime temperature between 65–70°F. Allowing temperatures to drop below 50°F can cause damage and leaf drop, while temperatures significantly above 70°F can delay the process.
Maintaining the Daily Darkness Routine
The routine requires providing 13 to 14 continuous hours of complete, pitch-black darkness nightly. This typically starts around 5:00 PM and ends the next morning at 7:00 AM or 8:00 AM. To achieve total darkness, place the plant inside a light-proof cabinet, a dark closet, or cover it completely with a large cardboard box or thick black plastic bag.
During the remaining ten to eleven hours of the day, the poinsettia requires bright, direct sunlight to store the energy needed for the color change. A south-facing window that receives several hours of sun is an ideal location. The plant must be returned to its dark location promptly every evening to maintain the strict schedule.
Watering should continue as normal; check the soil daily and only water when the surface feels dry to the touch. Avoid letting the plant wilt from dryness, but also avoid overwatering, which can lead to root problems. Once the bracts have fully colored, the daily darkness routine can stop, and the plant can be moved to its permanent holiday display location.