The poinsettia (Euphorbia pulcherrima) you bought for Christmas last year is probably now a small green shrub… and will remain a small green shrub if you don’t do something about it.
You see, the poinsettia is a short-day plant, that is to say, it only blooms when days are less than 12 hours long. So its flowering starts to be initiated starting about September 22 in the Northern Hemisphere… and usually actually begins to occur about two months later, well in time for Christmas.
It all sounds wonderful: as days get shorter, the poinsettia should simply bloom naturally, right? Well, that may work in the plant’s native Mexico, or in other tropical countries where it grows outdoors, but it won’t work in the average home.
You see, we light our homes at night, extending the number of hours of daylight to 16, 17, or 18 hours a day. Yet what the plant really requires is no light at all from the end of the afternoon until the following morning. Even a single ray of light at the wrong time and it may not bloom.
So what’s a gardener to do?
The Hard Way
When I first started gardening, I was told you had to put your poinsettia in a closed box or a closet at 4 pm each day and remove it daily, putting it back in the sunlight, at 8 am. And that does work… but what a job! It means you have to be home at the right time each day (forget job considerations, or taking a weekend trip), plus you have to remember to do it every single day, without fail (not my strength: I’m good on resolutions, but weak in followthrough). If you forget even once, the plant won’t bloom. I’d be surprised even one person in 10 gets their poinsettia to bloom that way, yet check out most websites and books: that’s still the usual advice!
The Laidback Way
Here’s how I get my poinsettias (note the plural: I have all kinds, in lots of different colors) to rebloom. It works every time and requires no daily effort.
Place the plant in a room that you don’t usually use at night, but that is at least moderately sunny during the day: a guest room, for example. Now unscrew all the light bulbs in the room. Next, place the poinsettia near the window. Since you removed the light bulbs, even if you enter the room in the evening and try to turn the light on by accident (forgetting that is temporarily forbidden), you simply can’t. Whatever you were looking for in that room, you’ll just have to search for in the dark or wait until daylight to retrieve. And because your poinsettia has had a daily regime of short days, it will necessarily bloom at Christmas.
You don’t have a room that is not used at night? Then place your poinsettia near a sunny window somewhere else indoors and set up a panel of some sort between it and the rest of the room. Even a “wall” of taller houseplants will do, as long as no artificial light reaches the poinsettia. And this will give you a beautifully blooming poinsettia with no extra effort.
Otherwise, continue your usual care through the fall, remembering especially to water when the soil is almost dry and adding a bit of fertilizer. There is no need for special temperatures or extra high humidity… and certainly don’t prune (you’d be cutting off future flowering stems).
Merry Christmas in advance!