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 Christmasit 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.
Otherwise, 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.