Few, if any, plants say Merry Christmas like a bright red Poinsettia. When we buy them around Thanksgiving, they are full of vibrant color. Traditionally the top leaves are a bright red, but modern variants range from white to pink and all manner of variegated patterns. All of them add a bright spot of living color to our seasonal decor.
After Christmas, they tend to shed leaves and loose all of that bright color, returning to a rich green. You may think you've done something wrong or that you just can't keep the plant happy enough to keep it's bright red color. In actuality, this is part of the normal seasonal cycle for the Poinsettia. It flowers during the short days of winter, bringing out all it's colorduring the flowering season. Then resting and preparing for the next flowering season during the spring and summer months.
Even if you keep your Poinsettias happy all year long, you may find they don't ever again show that bright color they had when you first bought them. As a result, many of us run out to buy new colorful Poinsettias every year and dispose of the ones we think we somehow ruined.
The fact of the matter is, you can bring back that bright color by following one simple strategy. Remember, the Poinsettia wants to flower during the shortest days of winter. Indoors, however, your Poinsettia may not know it is winter. You've got to let it know it's time to bring the holiday cheer.
Many plants judge the season by the amount of light they get in a day, or so we've been told. Actually, most plants tell the season by the lenght of the night, not the length of the day. In the absence of daylight, plants stop the process of photosynthesis. Instead they use stored energy for growth and repair. The number of hours of uninterrupted darkness tells the plant what season it is. Long periods of darkness mean it is winter time. Short nights mean summer time.
Indoors, however, we make our own light much later into the winter evening than the sun shines. We have lamps, televisions, even night-lights that all stay on well into the evening or even all night. While most indoor lighting is not as bright as sunlight, it is still bright enough to convince a Poinsettia that it is not dark out. The Poinsettia therefore stays in its summer phase and never tries to go into flowering season when its colors brighten up again.
Even 15 minutes of light during the night is enough to switch the Poinsettias internal processes back into daylight mode and convince it that the night is short. Keeping houselights on past sunset, likewise, tricks the Poinsettia into thinking the day hasn't yet ended and it's still summer.
To convince your Poinsettia that it really is Christmas time, it needs about 14 hours of uninterrupted darkness each day. It takes about 8 weeks of these long nights for the Poinsettia to reach its peak color. They'll start brightening up sooner, but if you start sequestering them in a dark room for 14 hours each day at the beginning of November, you should have brightly colored Poinsettias for Christmas.
This doesn't mean lock them in a closet for eight weeks. They still need light during the day asnd normal watering. Also remember that Poinsettias originated in Central America and Southern Mexico, so they are tropical plants used to warm temperatures. They are easily damaged by cold air and wintery drafts.