Poinsettias don't actually bloom flowers. The red "flowers" of the Christmas standard are simply leaves that turn color in the winter--similar to leaves on trees outside--if the proper light/dark routine has been put in place to stimulate the color process.
Poinsettias, a long-held holiday tradition, add color and texture to Christmas décor. In recent years, there have been new hybrid varieties introduced that have white, pink or even yellow leaves, but red is still the staple. Although many people dispose of the plant after the season is over and the leaves begin to wilt, with proper year-round care you can have subsequent "blooming" periods.
After the Holidays
The leaves will likely begin to wilt by New Year's, but continue to water the plant as it goes dormant. Poinsettias like moist soil, but should never be left sitting in water. In May, cut back the poinsettia significantly, to just a few inches above the soil line, and plant in a larger pot to give the roots room to grow.
After the last danger of frost the poinsettia can go outdoors. Begin a monthly fertilizing regimen with a soluble fertilizer with a nutrient analysis of 20-10-20. Bring plant inside by late September before night temperatures go below 60 degrees and place the plant in an area that will receive as much natural light as possible.
Acheiving the Color
Since it takes about 12 weeks to produce the color on poinsettias, by the end of September, (October first at the latest) begin a light/dark routine making sure the plant has no more than 10 hours of daylight and at least 14 hours of darkness each day. You must keep this up daily for at least six weeks, and then you may shorten the dark periods by a couple hours for another four to six weeks, usually until December 15. This process is not difficult, but it does take patience.
You must eliminate all light in the dark period for if any light is introduced at all, it will delay the color process. For this reason, it is a good idea to put the plant in a seldom-used closet and tape the door cracks closed, or put the plant in a box at night and cover the box with a heavy blanket. It might be a good idea to set an alarm for each evening and morning, usually 5 p.m. and 7 a.m., to remind you to move the poinsettia.
In contrast to the ultimate darkness at night, the poinsettias require as much bright light as possible during the day in order to absorb energy for color production. After five to six weeks of the light/dark rotation you should start to see the color emerging, a process called photoperiodism, which is when the bracts, or the flower-like leaves, change from dark green to red.
Even if you go through all of the steps, it is unlikely that any at-home care will result in the brilliance and color that greenhouse-grown plants display. Even the smallest light during the dark period will increase the length of time it takes for the color to appear, and missing a day altogether will likely mean that no color will appear at all.
Because of how time-intensive the process is to turn your poinsettias red, and because of how inexpensive the plants are to buy at the store, many gardening experts will simply recommend that you compost your plants at the end of the holiday season and buy new ones the next year.