Q: Are poinsettias annuals or can I keep them year-round? I never see them sold any other time of year, which makes me think they’re short-lived. If they are perennial, when would they bloom again and how do I care for them?
A: The reason they’re so fleeting at nurseries is because they’ve become so strongly associated with the Christmas season, and their season of color is limited so that they wouldn’t offer any special aesthetic interest the rest of the year. (Nurseries need that greenhouse space to devote to other plants, so they don’t keep restocking poinsettias throughout the winter.) The good news, though, is that you can indeed carry them over from year to year if you are up to a challenge.
Dec. 12 is National Poinsettia Day, so let’s take a moment to consider this North American native plant that’s undergone an amazing two centuries of selective breeding to become the holiday décor we all know and love. Poinsettias are actually shrubs or small trees in the wild, and are native to Mexico and Guatemala, in tropical forests along the Pacific coast. They only occur from low- to midlevel elevations on those mountain slopes, so aren’t adapted to cold; best to keep them above 50℉.
They can bloom every year, but they are short-day plants, meaning that they bloom while the day length is short and the nights are long. Since we can’t keep them outside to experience naturally short days here in Maryland, we need to take this into consideration when growing them indoors. Even though typical room lighting is generally not bright enough to support sufficient photosynthesis, it is enough for the plant to detect and respond to in terms of flowering.
The colorful parts we view as petals are actually bracts, which are specialized leaves that act like petals to draw attention to the tinier true flowers. Once these flowers are spent, the bracts gradually lose their color. After flowering the entire plant might go dormant, and since they are deciduous in nature in their seasonally-dry forest habitats, this means you may be left with bare stems after a period of leaf drop. Don’t give up on the plant at this point, though, since it might not be dead; water just enough to keep the stems from withering and see if it starts regrowing leaves once we enter spring.
Once our nights stay warm enough in late spring, you can treat a poinsettia just like you would any indoor plant that spends a summer outside. Being mindful of cold snaps, put it in a shady location (gradually introducing it to part sun or even full sun over a week or two). Repot if it needs a larger container, and periodically fertilize.
Around Memorial Day, and again around the Fourth of July, you can pinch the stems back to promote branching for bushiness. Remember these plants can reach nearly one story high in the wild, though for various reasons cultivated plants probably aren’t capable of attaining those heights. Still, you’ll need to find enough room for the plant if you successfully keep one going for several years, though annual trimming does help to keep them manageable.
When nights get cool again in autumn, bring it back inside. Starting around the fall equinox (late September) when our nights start to get longer than our days, make sure the plant receives 14-16 hours of uninterrupted darkness. (No peeking, no flashlights, no reading lamp in the opposite corner of the room…dark!) For the remaining hours of daylight it needs, give it as bright an environment as you can, like a very sunny window. Ideal temperatures are highs 65-70 and lows in the upper 50s to low 60s.
Around Thanksgiving or a bit earlier (give the darkness treatment at least four weeks), you can discontinue the overnight light exclusion. The bracts will gradually color-up again and last for about two months. If exposed to warm drafts from a fireplace or heat vent, the bloom cycle will be briefer.
If you forget to start the nighttime regimen “on time,” the plant should still bloom, but it might be delayed based on when you begin the light exclusion. I suspect you could have blooms lasting as late as Valentine’s Day if you began light management late and were able to keep plants humid but cool to maximize flower longevity. Bright light, humidified air, and cool-ish temperatures would make them great overwintering companions for citrus, cyclamen, and Norfolk Island pine.