Restoring a healthy green Poinsettia plant from last year back to its original red color is no problem if you follow a few simple rules. Poinsettias (Euphorbia pulcherrima) need total darkness, for 14 hours each day, starting about eight weeks before you want to display them.
During the day, the plants need bright light, along with the other routine care. However, starting in the evening, the plants must get complete darkness. Even a nightlight can disrupt this process! Depending on where you have the plant (planted outside, or in a pot indoors), will determine how you approach this process. I’ll let you decide that.
The bracts will start to turn color in about four weeks, and continue if you carefully keep up the process. Poinsettias need a humid environment during this time, but be careful not to spray the foliage directly, as you may invite leaf spot, not a desired feature on such a showy leaf! In about eight weeks, the bracts should all be red, if you’ve followed the above guidelines. They’ll stay this way for several weeks, at least until after Christmas.
Eventually the leaves will start to drop off. Once this occurs, cut the stems back to four to six inches. Keep the soil fairly dry, and the plant warm until new growth occurs. You can then replant in the garden in a sunny spot. Add a light amount of fertilizer in the spring and summer. Come next October, start the whole process over again!