Other than the trademark pine tree, there is no greater plant that embellishes and reflects Christmas like the poinsettia. Also known as Christmas flower, Mexican flame leaf, flor de pascua and flores de Noche Buena (flowers of the Holy Night, or Christmas Eve), the plant splashed with festive color decorates household rooms, hotels, shopping centers and business offices during the holidays.

But when the season's over, unlike cut trees that must be discarded, the living poinsettia continues to grow and reflower. So don't throw out that plant just because we're halfway through January.

Reflowering a poinsettia (Euphorbia pulcherrima) is rewarding but challenging. The poinsettia is a photoperiodic plant, or short-day plant, meaning that flower buds develop as the days get progressively shorter and the nights correspondingly longer. Exposure to artificial light of any kind can disrupt this natural seasonal cycle. Manipulation of light is required to induce blooming. Plants grown outdoors must be placed where they receive no light at night from street lamps, yard spotlights, building windows or other artificial light sources. If so exposed, the natural blooming cycle is interrupted and the plant will flower late, poorly or not at all. Starting Oct. 1, indoor plants must be placed each night on a set schedule — from 5 p.m. to 8 a.m., they should be put in a closet, or enclosure, completely shut off from light. This regimen must be done for eight to 10 weeks. Any light received by the plants will delay or inhibit flowering.

Another crucial element to reflowering your plant is night temperature. Poinsettias require night temperatures of 60 to 64 degrees and 11 to 12 hours of darkness to stimulate flower- bud growth. If night temperatures are higher, the dark period must be longer. Nurseries where night temperatures are warmer may compensate by using blackout cloth to protect plants from all light, with dark periods as much as 15 hours.

Growing poinsettias in the outdoor garden requires a sunny exposure, porous, well-drained earth with low fertility and a slight acidic pH, and shelter from salt spray.

During the summer growing period, plants should be watered regularly and fertilized two or three times with a general fertilizer. After establishment, plants should be hard pruned in late February or March, by cutting down to the woody stems, leaving stubs with two or three dormant buds. Prune plants again in August or September to encourage side branch development, which will terminate in flower heads. Stems removed during pruning may be trimmed into 4- to 6-inch lengths, set in the ground where plants are desired, and kept moist until shoots appear. Or cuttings with their lower leaves removed may be rooted in pots with a sterile medium such as perlite, vermiculite, or a 1:1 blend, plus peat moss. Cuttings root within two weeks if treated with a rooting hormone.

Pests include root knot nematode, poinsettia scab, scale, spider mites, whiteflies and thrips. Use commercial application for specific pest.

When poinsettias are brought indoors, plants should be kept evenly damp, in a well-lighted, ventilated area that offers protection from strong drafts (open doors, windows, air-conditioning vents) and not exposed to fluctuating temperatures. Fertilization is not necessary indoors.

So don't throw, but regrow your poinsettia. The holiday reward of witnessing your plant develop its first shades of color on its flower bracts will bring great personal gratification to your green thumb and additional joy to your holiday season.