Experts and BHG readers answer.
Thanks for writing. Poinsettias typically bloom once a year. So if you want yours to rebloom, you'll want to grow it in a bright spot all spring and summer (you can even move it outside to get more light during the warm-weather months). Then in early October or so, bring it indoors to a room that doesn't get any artificial light --- so if you have a bright guest bedroom or other area that no one uses.
Poinsettias are triggered to bloom when the nights get long. Unfortunately one problem is that if you turn on a light in the middle of the night, it interrupts the poinsettia and keeps it from blooming. So it's important to have a room where you don't turn on any lights after dusk. After about 10 weeks of naturally long nights, your poinsettia should bloom.
That's the easy way to do it. You can technically get your poinsettia to rebloom any time of the year if you give it long nights. For example, you could move it to a dark closet every evening at 4.00 p.m. and not take it out until 7.00 or 8.00 a.m. the next morning. Do this every day for about 10 weeks and you'll be rewarded with blooms.
---Justin, Senior Garden Editor, BHG.com