Poinsettias (Euphorbia pulcherrima) are popular winter houseplants because they flower in mid-winter, and their beauty comes from colored leaves or bracts, instead of flowers, making their attractiveness long-lasting:
Some gardeners are not satisfied with this long season of indoor beauty and attempt to save the plants to re-bloom the following winter.
Poinsettias can be kept year after year, and they will bloom each year if you give them proper care.
When the leaves begin to yellow or when the plant is no longer desired as an ornamental, gradually withhold water, advises Curtis W. Smith, Extension horticulturalist at New Mexico State University.
The leaves will pale and fall off. The bracts — colorful leaves just below the true flowers — will be the last to go.
After all the leaves have fallen, store the plant, in its pot, in a cool (50 to 60 degreesF), dry, dark area.
Keep the plant somewhat on the dry side; water only enough to keep the stems from withering.
In April or May, bring the plant out of storage.
Cut the main stems six inches above soil level, Smith says. Remove the plant from the pot and gently wash the old soil from the roots.
Repot the plant in fresh potting soil that has good drainage.
Poinsettias are susceptible to stem and root diseases if the soil is heavy and retains excess moisture.
Soak the soil well, and then allow all excess water to drain away.
Place the plant in a warm, sunny spot for renewed growth. Keep the humidity high to encourage rapid new growth.
Once the plant is growing actively, apply a weak fertilizer solution (one tablespoon of a soluble fertilizer, such as 20-20-20 or its equivalent, per gallon of water) at monthly intervals.
After frost danger is past, sink the pot into a protected and sunny bed.
Light shade is ideal during the hottest part of the day. Lift the pot occasionally to prevent root growth into surrounding soil. If the plant becomes root bound, repot it into a larger pot. Watch for insects and control them promptly.
Keep the poinsettia plant actively growing all summer by watering and fertilizing regularly.
When the top of the soil feels dry, liberally apply water to moisten the soil completely, and allow the excess to drain away. Add no more water until the top of the soil is dry again.
To obtain a bushy plant, pinch the tips of new shoots back leaving at least two nodes on each new shoot.
Continue pinching new shoots until late August. Remove weak stems completely, so only a few of the stronger ones develop.

—Courtesy New Mexico State University