Many of us received poinsettias at Christmas and would like to keep them thriving so we can watch them bloom again next Christmas season. In The New Fred Wiche Lawn & Garden Almanac, my favorite gardening expert, sadly now deceased, gives some pointers.
Place the poinsettia in a sunny, cool location and water when the soil dries out. It should bloom at least until February. Around mid-March prune the stems from 3 to 6 inches to remove the colored bracts and fertilize. When the weather is warm, move outdoors. Late next September begin to alternate 9 hours of bright light and 15 hours of complete darkness to induce blooming. Continue this until early October. You can place a paper bag over the plant or you can place the plant in a closet. Either way, it must be in total darkness. Even a slight amount of light during the 15 hours can prevent success. Conversely, it must have the bright light for 9 hours.
It usually takes 8 to 10 weeks for the bracts to show red. Then place the poinsettia in a cool place with indirect light and it should bloom again for many weeks.