Poinsettia: The Mexican Christmas Flower That Blooms in the Dark

Poinsettias were cultivated by the Aztecs, and though they didn't grow in the capital city of Tenochtitlan — now Mexico City — Aztec royalty imported the flowers from lower elevations during the winter months for use as a medicine to control fevers and as a reddish-purple fabric dye. The Nahua people of Mexico and Central America call these Aztec favorites cuetlaxochitl, but they go by many other names, too — lobster flower, flame leaf flower, "La Flor de la Nochebuena" (Christmas Eve flower).

But "poinsettia" is probably the weirdest name of all because it's just a shout out to the American diplomat who is credited with being the first to bring them back to the U.S. from Mexico in the 19th century. Joel Roberts Poinsett was the first U.S. minister to Mexico, and as an amateur botanist, is said to have sent some cuttings back to his home in South Carolina from Southern Mexico in 1928, although there is no irrefutable proof of this.

What is known is that the plant was on display in Philadelphia in 1829, associated with Poinsett's name. The plant was immediately popular and was known henceforth as the "poinsettia," although it didn't receive its official Latin name until 1934 when German botanist Karl Willde was given a cutting by a Scottish friend who had seen it in Philadelphia and named it Euphorbia pulcherrima.

In the 1920s the Ecke family of Encinitas, California started farming poinsettias, and they tirelessly pushed them as a symbol of the Christmas season. Today, around 70 percent of the poinsettia plants you buy in the United States come from Ecke Ranch, and poinsettia care is their lifeblood.

Every houseplant — even a hyper-seasonal one — is kept alive somewhere year-round. Poinsettias hail from the mid-elevation regions of Mexico and Central America, where they can grow over 10 feet (3 meters) tall as a perennial winter-flowering shrub with milky sap and branches so long they sometimes look like vines. The big, showy red, white or pink flowers we're used to seeing aren't actually the poinsettia's flowers at all, but modified leaves called bracts. The flower buds are the small yellow buds in the middle of the colorful bracts.

When you buy a poinsettia at the grocery store, it comes already sporting its brightly colored, fancy bracts. You have no idea how hard it was to get them there. Fritz Bahr, the author of "Fritz Bahr's commercial floriculture: a practical manual for the retail grower" (1937), described the delicate and finicky poinsettia thusly: "Perhaps no other plant or flower we handle during Christmas week is more short lived, wilts quicker or is more disappointing to those who receive it; yet, when the next Christmas comes around, there comes again the same demand for poinsettias and the disappointments of a year ago are all forgotten."

Over time, floriculturists overcame some of these problems, but until the mid-1950s, growing poinsettias and getting them into the hands of Christmas revelers in relatively good shape was a real trick. That was, until somebody realized poinsettias need just one thing to turn their green bracts red, pink or white: total darkness.

In order to induce your poinsettia plant to create flower buds and to change the color of its leaves from green in time for Christmas, it must be kept in complete darkness for 16 hours per day. The witholding of light prevents the plant from producing chlorophyll, which is what makes plant parts green. This changes the bracts to red, pink or white, depending on the variety of poinsettia.

So, somewhere around September 21 — right around the fall equinox — pull your poinsettia out of its sunny window and move it into 16 hours of uninterrupted darkness (put the plant under a box if necessary to provide total darkness), alternating with 8 hours of bright light every day. During the dark period, the plant cannot receive even the slightest bit of light at any time. This applies to your year-old poinsettia as well: If you want your plant to produce flower buds again and to change color, it's the daily length of complete darkness, not bright daylight, that matters most. Discontinue this around Thanksgiving.

After Thanksgiving, keep your poinsettia in bright light or the full sun of a sunny window, not keeping the potting soil moist or adding excess water, but watering it when the well-drained soil is dry to the touch. Poinsettias prefer temperatures around or above 65 degrees Fahrenheit (18 degrees Celsius). They will bloom from Christmas until about April — at this point it's a good idea to cut your poinsettia down to a 3- to 8-inch (8- to 20-centimeter) stem and let it regrow, starting the process over again until the next year.

One common urban legend about poinsettias is that they're toxic to people and animals. One Ohio State University study showed that a 50-pound (23-kilogram) child would have to eat over 1 pound (0.45 kilograms) of poinsettia leaves — between 500 and 600 leaves — for toxicity to become a problem. However, they certainly don't taste very good, and the child who ate them would probably get a terrible tummy ache long before they were poisoned.

The milky sap of the poinsettia is another matter. Most members of the Euphorbia family have toxic sap, but the toxin in poinsettias is very mild. However, those with sensitive skin should avoid touching poinsettia sap, just in case.