Unless forced to turn red through carefully controlled exposure to light, poinsettia leaves are naturally green. The poinsettia plant needs 14 hours of darkness per day for eight weeks before it acquires the bright-red leaves traditionally associated with it.
The red color in the leaves of the poinsettia is achieved by exposing it to bright light during the day and total darkness at night. After about four weeks of this treatment, the bracts, or tips of the branches, start to turn red. If the plant is treated this way for eight weeks, the red leaves last through the holiday season.