Bringing poinsettias back into bloom for Christmas is all about the timing. Poinsettias are “day-length” sensitive, which defined means that they must have short days and long nights (just the opposite of African Violets). The reason that poinsettias can be grown naturally is because short days start happening on the first day of fall. From that day forth, day-length gets shorter and nighttime is longer.
Now we have all heard the old saying, “put your poinsettia in the closet” but that is not the best way to bring your poinsettia back into bloom. Yes, it can work but you must ask yourself, “When should I bring it back out of the closet?” You may be surprised to hear that leaving it in the closet is only for 15 hours, which then means that you must move it out into full sun each day for 9 hours. This is too labor intensive so try placing your poinsettia into a spare bedroom in front of a sunny window (south or southwest).
Poinsettias are extremely “light-sensitive” which means that any amount of light (including a street light outside of the window) will prevent it from coming back into bloom. Any lights turned on in the house after dark will bother your poinsettia so make sure that the door is closed in that spare room and try to keep the temperature a little cooler rather than warmer-ideally 65 to 68 degrees. Water and fertilize regularly and it will grow and bloom on time for Christmas.
One other trick you could use if you do not have a spare bedroom is to get a box larger than your poinsettia and simply cover your plant around 5:00PM then uncover at 8:00AM the following morning. This is a little labor intensive but the good news is that you only need to do this until mid-November. It should be showing color by Thanksgiving.