It is not impossible to get a poinsettia to rebloom in your home, but it does require special lighting conditions and constant attention. You will have to subject it to a strict regimen if you want it to rebloom for Christmas. And don't expect your plant to be as compact or showy as one from a commercial greenhouse.
The poinsettia is a so-called short-day plant. In fall, the naturally shorter days initiate the formation of new flower buds and cause the bracts to become coloured. This phenomenon usually begins in mid-September, sometimes earlier for certain cultivars. Often, in a home, the naturally shorter daylength is not enough. To get a poinsettia to rebloom in time for the holiday season, it needs a period of total darkness every day.
From mid-September onward, give the plant a minimum of 14 consecutive hours of complete darkness every single day of the week, from 6 p.m. to 8 a.m., for eight to ten weeks or until the bracts start showing colour. Different cultivars may respond somewhat differently to this treatment. If the plant isn't given total darkness, the bracts may be malformed or the flower buds may drop off. Even stray light from outdoors can interfere with flower initiation. Some people cover their plants with a black sheet. You can also put your poinsettia inside a cardboard box or simply move it to a closet or a dark, windowless room. Just remember that the plant needs bright light in daytime.
Before starting this treatment (in mid-September), remove any weak stems, keeping only the most attractive ones.
Temperature and fertilizing
While the flower buds are forming and the bracts are changing colour (mid-September to mid-November), it is also important to keep the temperature at 20°C to encourage good-sized bracts to form. Once the bracts are fully coloured, you can lower the temperature to 15-18°C to intensify the colour.
Fertilize with a 20-20-20 product every two weeks. Starting in November, fertilizer with more potassium, like 15-15-30, will help promote flower formation. After the darkness treatment, care for your plant as described earlier in this leaflet (while in bloom).