A poinsettia’s red, pink, or white flowers are really bracts, or modified leaves, surrounding its true, nublike, yellow-green flowers.
During the winter flowering season, keep a poinsettia in bright filtered light, at normal room temperatures (above 55°1′ at night): avoid drafts. Water well, letting the soil dry out slightly between waterings.
To bring a poinsettia into flower the next season, water sparingly after the bracts drop and the leaves shrivel. In May, cut back the stems to about 6 inches. Remove the plant from its pot and pick off most of the old potting mix; repot in as small a pot as will comfortably accommodate the roots (use a mix of 2 parts sterilized fibrous soil, 1 part sphagnum moss, and 1 part perlite). Resume regular watering. Once the weather warms and settles (temperatures above 58°F at night), move the plant to a partially shaded area, outdoors if possible. When it becomes pot-bound, transfer it to a pot 2 to 3 inches wider. When this pot fills with roots, start applying standard liquid fertilizer every 2 weeks until October.
In early September, place the poinsettia in bright filtered light indoors. For 6 to 8 weeks, from late September to mid-November, give it 14 hours daily of complete and uninterrupted darkness by placing it at night in a dark closet or under a cardboard box. The poinsettia should develop colorful bracts 2 to 3 weeks after this short-day treatment.