The following information is provided by Ecke Ranch, the premier poinsettia breeder and propagator in the world. With some additional notes from Bemis Farms Nursery that are in Bold italic.
When the poinsettia’s bracts age and lose their aesthetic appeal, there’s no reason to throw it out. With proper care, dedication and a certain amount of luck, you too can re-bloom your poinsettia!
By late March or early April, cut your poinsettia back to about 8″ in height. Continue a regular watering program, and fertilize your plant with a good, balanced all-purpose fertilizer. By the end of May, you should see vigorous new growth.
Place your plants outdoors, where they can bask in the warmth of spring and summer, after all chance of frost has passed and night temperatures average 55° F or above. Continue regular watering during the growth period, and fertilize every 2 to 3 weeks.
Pruning may be required during the summer to keep plants bushy and compact. Late June or early July is a good time for this step, but be sure not to prune your plant later than September 1. Keep the plants in indirect sun and water regularly.
Around June 1, you may transplant your poinsettia into a larger pot. Select a pot no more than 4 inches larger than the original pot. A soil mix with a considerable amount of organic matter, such as peat moss or leaf mold, is highly recommended. In milder climates, you may transplant the plant into a well-prepared garden bed. Be sure the planting bed is rich in organic material and has good drainage.
The poinsettia is a photoperiodic plant, meaning that it sets bud and produces flowers as the Autumn nights lengthen. Poinsettias will naturally come into bloom during November or December, depending on the flowering response time of the individual cultivar. Timing to produce blooms for the Christmas holiday can be difficult outside of the controlled environment of a greenhouse. Stray light of any kind, such as from a street light or household lamps, could delay or entirely halt the re-flowering process.
Starting October 1, the plants must be kept in complete darkness for 14 continuous hours each night. Accomplish this by moving the plants to a totally dark room, or by covering them overnight with a large box. During October, November and early December, poinsettias require 6 – 8 hours of bright sunlight daily, with night temperatures between 60 – 70° F. Temperatures outside of this range could also delay flowering.
Please note: the plants need 14 hours of UNINTERRUPTED darkness. This means not even light from a streetlight or that 10 second refrigerator raid in the middle of the night. From your poinsettias perspective, ten seconds of light turns the 14 hours into two seven hour nights, with a 10 second day in between! Also it is important to understand that it NEEDS bright light during the day during these periods. An unused bedroom with a bright sunny window is perfect. While October 1 is the day you should theoretically start, each time you forget and turn on the light, the poinsettias bloom date gets set back by a day. So start a little early, just in case.
Continue the normal watering and fertilizer program. Carefully following this regime for 8 to 10 weeks should result in a colorful display of blooms for the holiday season!