A poinsettia left from last Christmas needs uninterrupted darkness at night from now until early December, in order to bloom. It is a "photoperiod sensitive" plant, which means it does not initiate flower buds until days are short and nights are long.
If it is kept in a room where electric lights are turned on at night it will not bloom. If it is on a window sill and gets light from a street light nearby it will not bloom.
Put the plant in a room that will be continuously dark from sunset until sunrise. If a dark room (or closet) is not available, cover the plant with a cardboard box or black plastic from 5 p.m. until 8 the next morning.
The short-day treatment may be discontinued after the bracts begin to show color (usually in early December). The plant should be in full bloom for Christmas.
It is important to give the plant as much light as possible during the day. Full daylight is best, near a window is ideal. Dim light or darkness will reduce its attractiveness.
Some plants need watering every day, and others not so often. When the soil feels dry, water until water comes out through drainage holes in the bottom of the pot. Use room temperature water, neither hot nor cold. Wait about 30 minutes and empty excess water from the saucer. Poinsettias do not like wet feet.
Ideal temperatures are 72 daytime and 65 at night. Poinsettias really do best at 60 degrees F. with high humidity, but they can adjust.
Temperatures above 75 with low humidity are detrimental to the poinsettia. Aovid hot or dry air, drafts or sudden changes. The top of the TV is a disaster area because of the heat that is generated by the set. The same is true of raditors with the heat turned on.
The night temperature during the dark period should be from 60 to 65 degrees. If the temperature is too high, the development of buds may be delayed or halted.
Even if you do everything right, your poinsettia may or may not come into bloom. If there is no color showing by late November, do not be too disappointed. Just enjoy your green foliage plant, and try again next year with a new plant.