A: Poinsettias are named after Joel Roberts Poinsett, the botanist who introduced them into the United States via stem cuttings from southern Mexico in 1828. In their native habitat, poinsettias are a perennial shrub that can grow as tall as 15 feet.
The different colors of poinsettia plants available commercially today represent different varieties of the same species, Euphorbia pulcherrima. Pulcherrima is Latin for beautiful. They are in the Euphorbiaceae or spurge plant family and are related to the noxious weed, leafy spurge; the common houseplant, crown of thorns; and one of my favorite perennial landscape ornamentals for New Mexico, the donkey-tail spurge (Euphorbia myrsinites).
One reason poinsettias are so popular is that the red color (or whatever color you select) lasts a very long time. That’s because the red parts aren’t flowers, they’re modified leaves, also called bracts.
Another plant that seems to bloom forever – but really the beautiful color comes from the bracts – is the bougainvillea. On many other plants, like roses, bracts are green and form the base of the floral structure. The actual flowers on poinsettias are the tiny yellowish structures in the center of an array of bracts. The true flowers do not have a long bloom time, but it is hard to tell because they lack petals completely.
Here are the steps for keeping your poinsettia alive and boosting rebloom next year (warning: it ain’t pretty):
• When leaves, including the colored bracts, begin to fall off the plant in the weeks or months after blooming, cut each of the stems down to only about 4 inches in length. This will look bad for a long while, but encourages a bushier, fuller plant instead of being spindly and top-heavy.
• Allow the soil to dry between light waterings and keep the plant in a shady spot in the house.
• In May, repot the plant with some additional potting soil; move it to a warm, sunny spot (maybe even outside in light shade if all danger of frost is past); and increase water. Soon, new shoots will appear.
• Water regularly with fertilizer and continue pinching back the new shoots, keeping a minimum of two nodes per stem until late August.
• In September, bring the plant inside and keep it in a very sunny location, away from drafts.
• Because poinsettias are true short-day (or long-night) plants, light control is essential to triggering bloom. So, starting at the end of September, keep the whole plant in total darkness for 14 hours of every day (5 p.m. to 7 a.m.). Interrupting the dark period, even by turning on the lights for a second, can interfere with flowering.
Retired NMSU Extension Horticulture Specialist Dr. Curtis Smith says, “Some people recommend moving them to a closet at sunset and returning them to a window after sunrise. A simpler method is to cover with a cardboard box or put them in a black plastic (garbage) bag every night, returning them to the light in the morning. In the garbage bag, they can overheat if left in the sunshine. In general, if you don’t have a greenhouse, it is easier to have beautiful poinsettias by purchasing them each year. The challenge of reblooming them is fun but difficult.”
• Less water is needed at this time, but keep an eye (or finger) on the soil and if the surface is dry, go ahead and drench it without letting the roots sit in standing water.
• Keep up the nighttime/daytime routine for eight weeks and then just leave it in the sunny spot. Now we wait. The flowering mechanisms have already been triggered, and as tiny flowers form, the bracts turn from green to red.
• Continue fertilizing until the end of December and then start all over again! I am going to try it this year. Who’s with me?
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist in the Department of Extension Plant Sciences at the New Mexico State University-Los Lunas Agricultural Science Center. Send questions to desertblooms@ nmsu.edu, or the Desert Blooms Facebook page (@NMDesertBlooms).