Despite holding the official title of most popular potted plant, the familiar poinsettia remains full of surprises. The brightly coloured "flowers" of the poinsettia (Euphorbia pulcherrima) are actually petal-like leaves, called bracts-the true flowers are the tiny beadlike yellow and green parts, called cyathia, at the centre of the bracts. Unlike many flowering plants, which require long, sunny days to induce flowering, poinsettias produce their colourful bracts as the nights lengthen. Once available only in red, poinsettias now come in a range of colors that includes pink, peach, yellow, and white. Some cultivars have marbled or speckled bracts while others have wavy or ruffled bracts that resemble pompoms. For healthy, long-blooming plants, choose poinsettias with tightly grouped cyathia at the tips of the stems and bracts that are fully coloured (bracts that are greenish around the edges will never colour fully). Pick sturdy plants with stiff stems and lush green leaves all the way to the soil.
During their bloom time, keep poinsettias in bright but indirect light (6 to 8 hours of filtered sun daily) and away from drafts and heaters (if you are putting them indoors), which can cause the leaves to drop. Plants thrive at room temperatures during the day (60F-75F) and prefer slightly cooler nights (60F-65F). Poinsettias are very cold sensitive, so keep them away from chilly windows. Water plants only when the soil is dry.
Later, in spring, after the bracts become a muddy green, cut the plants back to about 6 to 8 inches tall; continue watering, and begin fertilizing biweekly with a balanced all-purpose fertilizer for flowering plants. In summer, repot plants in fresh soil-less potting mix in slightly larger pots, and move outdoors if desired to a spot with morning sun. Continue watering and biweekly fertilizing. Prune plants to encourage a bushy, compact habit.
Making poinsettias re-bloom is a challenge. To induce flowering in time for the holidays, continue watering plants but stop fertilizing by early August or so. Beginning on October 1, move the plants to an absolutely dark room or closet each night, or place them under a box so they receive at least 14 hours of uninterrupted darkness per day-even light from a lamp can disrupt the flowering. Colourful bracts should develop in 6 to 10 weeks.