When fall officially starts, it’s time to start preparing your poinsettia (Euphorbia pulcherrima) for its Christmas bloom. Its flowering is stimulated by short days, that is days less than 12 hours long. (Actually, it’s the nights longer than 12 hours that stimulate bloom, but let’s not quibble over details, as the results are the same.) So anything that reduces day length in the fall, while still maintaining viable conditions (warm temperatures, occasional waterings, etc.), will probably give good results. It really will bloom again for Christmas, year after year, if you just give it a chance.
However, anything that artificially extends day length, such as growing it in a room you keep lit at night, will prevent it from blooming.
The “Put It Outdoors” Method
Some Internet sites suggest that the best way to get a poinsettia to rebloom is to put it outside in the fall (or to leave it outside if you had already put your plant out for the summer) and to leave it there until the bracts start to change color. And that certainly sounds easy enough to do. After all, it will automatically receive short days outdoors from September 22 on (thank you Mother Nature!). So unless you place it near a lighted window, street lamp or other source of night illumination that will artificially extend day length, it really ought to start changing color in November… and once the color change is initiated, it will continue even if the days are no longer short. So you can bring your just-starting-to-bloom poinsettia indoors where it will complete its blooming cycle.
There is a major flaw with this technique, though. Poinsettias are all right at fairly cool temperatures, but dislike cold (less than 45?F/7?C) and frost will kill them back to the base, if indeed it doesn’t kill them outright. So if you live in an area where temperatures can dip below 50?F (10?C) in October or November, and that would cover all of pretty much all of North America north of Florida as well as all but the Mediterranean region of Europe, leaving poinsettias outside in the fall is inherently risky.
But it’s not just the risk of cold or frost you have to consider.
Poinsettia plants that acclimate to cool fall temperatures outdoors tend to shed their leaves massively when you do bring them back indoors. They find the transition from cool conditions to warm ones a severe shock. So yes, your plants will bloom, but sans most of their leaves. Not too charming an effect!
The Easy Way
To get a poinsettia to bloom with a full cohort of leaves, first bring it indoors early in the fall, while night temperatures indoors and out are pretty much equal. That way the plant will already be acclimated and won’t lose leaves. Then all you have to do is to give it short days indoors (and keep watering it, of course)… and that’s a snap. To learn how, see Blooming a Poinsettia the Laidback Way). Try it: it really works!