In March or early April, prune the plant back to about 8 inches in height. Three or four green leaves should remain on each shoot. Prune off any remaining colored bracts. (What look like flower petals are actually bracts. The real flowers are the yellow berrylike clusters in the center.) Keep the plant in your sunniest window, water it regularly, and feed it with diluted fish emulsion or kelp extract every two weeks from now until fall.
In June, transplant the poinsettia into a pot 2 to 4 inches bigger than the present one. You can keep it indoors, but the plant will grow more vigorously if you put it outdoors in full sun. Or you can plant it directly into the garden in a sheltered place if you live in an area where nights consistently stay above 50°F year-round. Poinsettias need soil rich in organic matter, such as compost or leaf mold. Every third week or so, pinch the shoots back to two or three fully grown leaves. Do this until mid-August. Then bring the plant indoors before night temperatures drop below 55°F. Put it where it will receive at least six hours of sun and cut back on fertilizing for now.
Now here's the tough part: Poinsettias won't flower unless they are kept in total darkness for 14 continuous hours per night for 8 to 10 weeks. Even momentary exposure to a streetlight or flashlight can delay or inhibit flowering completely. Starting October 1, keep the plant in a closet or an unused room at night or cover it with a box or heavy paper bag. The plant should stay in the dark from 5 p.m. to 8 a.m. Nighttime temperatures shouldn't fall below 60°F. From 8 a.m. to 5 p.m., it will need full sunlight, with daytime temperatures around 70° to 75°F. Keep watering and begin the feeding schedule again. After 8 to 10 weeks, the bracts should be flushed with color. Congratulate yourself on your patience and dedication, and give your rejuvenated poinsettia a place of pride in your holiday decorating!