Poinsettias are in full bloom when you buy them, but few ever bloom again. Photo: gpnmag.com
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 (or, actually, when nights are more than 12 hours long). So, flowering starts to be initiated starting at fall solstice (that would be September 22 in the Northern Hemisphere this year). Flowering actually begins after about two months of this short-day treatment, about a month before Christmas. Since the “flowers” (actually brightly colored leaves called bracts) easily last 3 months, your poinsettia can then be the star of your holiday season display for a long, long time!
It all sounds wonderfully simple: as days get shorter, your poinsettia will simply bloom all on its own. 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. Most indoor poinsettias are bought in bloom, stay in bloom for months, but once those pretty bracts fall off, they never appear again.
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 can cause its flowering to abort.
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 p.m. each day and remove it daily, putting it back in the sunlight, at 8 a.m. 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 follow-through). 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 the required daily regime of short days, it will necessarily bloom at Christmas.
Don’t worry that the moonlight the plant receives will throw it off its schedule. Moonlight isn’t strong enough to affect flowering.
A Simple Barrier
You don’t have a room that isn’t used at night? Then place your poinsettia near a sunny window somewhere else indoors and set up a screen of some sort between it and the rest of the room. Even a “wall” of taller houseplants will do, as long as no intense artificial light reaches the poinsettia. And this will give you a beautifully blooming poinsettia with no extra effort.
Do note that, once the central leaves start to take on their Christmas color, even a bit, no further short-day treatment is required. The job is done! You can put the light bulbs back in or remove the barrier. Your plant will continue its maturation until full bloom is achieved.
Other than the short days treatment, continue your usual care through the fall, remembering especially to water when the soil is almost dry. There is no need for special temperatures or extra high humidity … and certainly don’t prune in the fall: you’d be cutting off future flowering stems!
Merry Christmas in advance! 🎄
Article adapted from one first published on September 21, 2015.