Poinsettias originate from Mexico, where they bloom during the winter. They prefer bright, sunny windows and evenly moist conditions. But for the colorful bracts to develop, they have to have nights longer than 12 hours.
They begin to set buds and produce flowers as the nights become longer than days. In Georgia, this happens around Sept. 25. If you don't provide artificially long "nights," poinsettias will stay green all winter.
Trigger New Bracts
To trigger new bracts, bring poinsettias indoors in early October and place them in a sunny window. Continue to grow them as you did outdoors, except with less fertilizer.
Beginning that week, keep the plant in complete darkness for 14 continuous hours each night by moving plants into a dark room or placing a large box over them.
If light reaches the plant accidentally during this period, it may revert to its vegetative state and stay green. During the day, allow 6 to 8 hours of bright sunlight. Keep this up for eight to 10 weeks, or until about Nov. 15.